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llvm-project/lld/ELF/Writer.cpp
Fangrui Song 7b6a89f346
[ELF] Detect convergence of output section addresses 
Some linker scripts don't converge. https://reviews.llvm.org/D66279
("[ELF] Make LinkerScript::assignAddresses iterative") detected
convergence of symbol assignments.

This patch detects convergence of output section addresses. While input
sections might also have convergence issues, they are less common as
expressions that could cause convergence issues typically involve output
sections and symbol assignments.

GNU ld has an error `non constant or forward reference address expression for section` that
correctly rejects
```
SECTIONS {
  .text ADDR(.data)+0x1000 : { *(.text) }
  .data : { *(.data) }
}
```

but not the following variant:
```
SECTIONS {
  .text foo : { *(.text) }
  .data : { *(.data) }
  foo = ADDR(.data)+0x1000;
}
```

Our approach consistently rejects both cases.

Link: https://discourse.llvm.org/t/lld-and-layout-convergence/79232

Pull Request: https://github.com/llvm/llvm-project/pull/93888
2024-05-31 09:31:15 -07:00

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//===- Writer.cpp ---------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "AArch64ErrataFix.h"
#include "ARMErrataFix.h"
#include "CallGraphSort.h"
#include "Config.h"
#include "InputFiles.h"
#include "LinkerScript.h"
#include "MapFile.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "lld/Common/Arrays.h"
#include "lld/Common/CommonLinkerContext.h"
#include "lld/Common/Filesystem.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/BLAKE3.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/RandomNumberGenerator.h"
#include "llvm/Support/TimeProfiler.h"
#include "llvm/Support/xxhash.h"
#include <climits>
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
namespace {
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
LLVM_ELF_IMPORT_TYPES_ELFT(ELFT)
Writer() : buffer(errorHandler().outputBuffer) {}
void run();
private:
void addSectionSymbols();
void sortSections();
void resolveShfLinkOrder();
void finalizeAddressDependentContent();
void optimizeBasicBlockJumps();
void sortInputSections();
void sortOrphanSections();
void finalizeSections();
void checkExecuteOnly();
void setReservedSymbolSections();
SmallVector<PhdrEntry *, 0> createPhdrs(Partition &part);
void addPhdrForSection(Partition &part, unsigned shType, unsigned pType,
unsigned pFlags);
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs(Partition &part);
void checkSections();
void fixSectionAlignments();
void openFile();
void writeTrapInstr();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> &buffer;
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSection &osec);
uint64_t fileSize;
uint64_t sectionHeaderOff;
};
} // anonymous namespace
template <class ELFT> void elf::writeResult() {
Writer<ELFT>().run();
}
static void removeEmptyPTLoad(SmallVector<PhdrEntry *, 0> &phdrs) {
auto it = std::stable_partition(
phdrs.begin(), phdrs.end(), [&](const PhdrEntry *p) {
if (p->p_type != PT_LOAD)
return true;
if (!p->firstSec)
return false;
uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr;
return size != 0;
});
// Clear OutputSection::ptLoad for sections contained in removed
// segments.
DenseSet<PhdrEntry *> removed(it, phdrs.end());
for (OutputSection *sec : outputSections)
if (removed.count(sec->ptLoad))
sec->ptLoad = nullptr;
phdrs.erase(it, phdrs.end());
}
void elf::copySectionsIntoPartitions() {
SmallVector<InputSectionBase *, 0> newSections;
const size_t ehSize = ctx.ehInputSections.size();
for (unsigned part = 2; part != partitions.size() + 1; ++part) {
for (InputSectionBase *s : ctx.inputSections) {
if (!(s->flags & SHF_ALLOC) || !s->isLive() || s->type != SHT_NOTE)
continue;
auto *copy = make<InputSection>(cast<InputSection>(*s));
copy->partition = part;
newSections.push_back(copy);
}
for (size_t i = 0; i != ehSize; ++i) {
assert(ctx.ehInputSections[i]->isLive());
auto *copy = make<EhInputSection>(*ctx.ehInputSections[i]);
copy->partition = part;
ctx.ehInputSections.push_back(copy);
}
}
ctx.inputSections.insert(ctx.inputSections.end(), newSections.begin(),
newSections.end());
}
static Defined *addOptionalRegular(StringRef name, SectionBase *sec,
uint64_t val, uint8_t stOther = STV_HIDDEN) {
Symbol *s = symtab.find(name);
if (!s || s->isDefined() || s->isCommon())
return nullptr;
s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, stOther,
STT_NOTYPE, val,
/*size=*/0, sec});
s->isUsedInRegularObj = true;
return cast<Defined>(s);
}
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
void elf::addReservedSymbols() {
if (config->emachine == EM_MIPS) {
auto addAbsolute = [](StringRef name) {
Symbol *sym =
symtab.addSymbol(Defined{ctx.internalFile, name, STB_GLOBAL,
STV_HIDDEN, STT_NOTYPE, 0, 0, nullptr});
sym->isUsedInRegularObj = true;
return cast<Defined>(sym);
};
// Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
// so that it points to an absolute address which by default is relative
// to GOT. Default offset is 0x7ff0.
// See "Global Data Symbols" in Chapter 6 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
ElfSym::mipsGp = addAbsolute("_gp");
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT.
if (symtab.find("_gp_disp"))
ElfSym::mipsGpDisp = addAbsolute("_gp_disp");
// The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
// pointer. This symbol is used in the code generated by .cpload pseudo-op
// in case of using -mno-shared option.
// https://sourceware.org/ml/binutils/2004-12/msg00094.html
if (symtab.find("__gnu_local_gp"))
ElfSym::mipsLocalGp = addAbsolute("__gnu_local_gp");
} else if (config->emachine == EM_PPC) {
// glibc *crt1.o has a undefined reference to _SDA_BASE_. Since we don't
// support Small Data Area, define it arbitrarily as 0.
addOptionalRegular("_SDA_BASE_", nullptr, 0, STV_HIDDEN);
} else if (config->emachine == EM_PPC64) {
addPPC64SaveRestore();
}
// The Power Architecture 64-bit v2 ABI defines a TableOfContents (TOC) which
// combines the typical ELF GOT with the small data sections. It commonly
// includes .got .toc .sdata .sbss. The .TOC. symbol replaces both
// _GLOBAL_OFFSET_TABLE_ and _SDA_BASE_ from the 32-bit ABI. It is used to
// represent the TOC base which is offset by 0x8000 bytes from the start of
// the .got section.
// We do not allow _GLOBAL_OFFSET_TABLE_ to be defined by input objects as the
// correctness of some relocations depends on its value.
StringRef gotSymName =
(config->emachine == EM_PPC64) ? ".TOC." : "_GLOBAL_OFFSET_TABLE_";
if (Symbol *s = symtab.find(gotSymName)) {
if (s->isDefined()) {
error(toString(s->file) + " cannot redefine linker defined symbol '" +
gotSymName + "'");
return;
}
uint64_t gotOff = 0;
if (config->emachine == EM_PPC64)
gotOff = 0x8000;
s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, STV_HIDDEN,
STT_NOTYPE, gotOff, /*size=*/0, Out::elfHeader});
ElfSym::globalOffsetTable = cast<Defined>(s);
}
// __ehdr_start is the location of ELF file headers. Note that we define
// this symbol unconditionally even when using a linker script, which
// differs from the behavior implemented by GNU linker which only define
// this symbol if ELF headers are in the memory mapped segment.
addOptionalRegular("__ehdr_start", Out::elfHeader, 0, STV_HIDDEN);
// __executable_start is not documented, but the expectation of at
// least the Android libc is that it points to the ELF header.
addOptionalRegular("__executable_start", Out::elfHeader, 0, STV_HIDDEN);
// __dso_handle symbol is passed to cxa_finalize as a marker to identify
// each DSO. The address of the symbol doesn't matter as long as they are
// different in different DSOs, so we chose the start address of the DSO.
addOptionalRegular("__dso_handle", Out::elfHeader, 0, STV_HIDDEN);
// If linker script do layout we do not need to create any standard symbols.
if (script->hasSectionsCommand)
return;
auto add = [](StringRef s, int64_t pos) {
return addOptionalRegular(s, Out::elfHeader, pos, STV_DEFAULT);
};
ElfSym::bss = add("__bss_start", 0);
ElfSym::end1 = add("end", -1);
ElfSym::end2 = add("_end", -1);
ElfSym::etext1 = add("etext", -1);
ElfSym::etext2 = add("_etext", -1);
ElfSym::edata1 = add("edata", -1);
ElfSym::edata2 = add("_edata", -1);
}
static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
if (map.empty())
for (auto [i, sec] : llvm::enumerate(sym.file->getSections()))
map.try_emplace(sec, i);
// Change WEAK to GLOBAL so that if a scanned relocation references sym,
// maybeReportUndefined will report an error.
uint8_t binding = sym.isWeak() ? uint8_t(STB_GLOBAL) : sym.binding;
Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
/*discardedSecIdx=*/map.lookup(sym.section))
.overwrite(sym);
// Eliminate from the symbol table, otherwise we would leave an undefined
// symbol if the symbol is unreferenced in the absence of GC.
sym.isUsedInRegularObj = false;
}
// If all references to a DSO happen to be weak, the DSO is not added to
// DT_NEEDED. If that happens, replace ShardSymbol with Undefined to avoid
// dangling references to an unneeded DSO. Use a weak binding to avoid
// --no-allow-shlib-undefined diagnostics. Similarly, demote lazy symbols.
//
// In addition, demote symbols defined in discarded sections, so that
// references to /DISCARD/ discarded symbols will lead to errors.
static void demoteSymbolsAndComputeIsPreemptible() {
llvm::TimeTraceScope timeScope("Demote symbols");
DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
for (Symbol *sym : symtab.getSymbols()) {
if (auto *d = dyn_cast<Defined>(sym)) {
if (d->section && !d->section->isLive())
demoteDefined(*d, sectionIndexMap[d->file]);
} else {
auto *s = dyn_cast<SharedSymbol>(sym);
if (sym->isLazy() || (s && !cast<SharedFile>(s->file)->isNeeded)) {
uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK);
Undefined(ctx.internalFile, sym->getName(), binding, sym->stOther,
sym->type)
.overwrite(*sym);
sym->versionId = VER_NDX_GLOBAL;
}
}
if (config->hasDynSymTab)
sym->isPreemptible = computeIsPreemptible(*sym);
}
}
static OutputSection *findSection(StringRef name, unsigned partition = 1) {
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
if (osd->osec.name == name && osd->osec.partition == partition)
return &osd->osec;
return nullptr;
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
// Now that we have a complete set of output sections. This function
// completes section contents. For example, we need to add strings
// to the string table, and add entries to .got and .plt.
// finalizeSections does that.
finalizeSections();
checkExecuteOnly();
// If --compressed-debug-sections is specified, compress .debug_* sections.
// Do it right now because it changes the size of output sections.
for (OutputSection *sec : outputSections)
sec->maybeCompress<ELFT>();
if (script->hasSectionsCommand)
script->allocateHeaders(mainPart->phdrs);
// Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
// 0 sized region. This has to be done late since only after assignAddresses
// we know the size of the sections.
for (Partition &part : partitions)
removeEmptyPTLoad(part.phdrs);
if (!config->oFormatBinary)
assignFileOffsets();
else
assignFileOffsetsBinary();
for (Partition &part : partitions)
setPhdrs(part);
// Handle --print-map(-M)/--Map and --cref. Dump them before checkSections()
// because the files may be useful in case checkSections() or openFile()
// fails, for example, due to an erroneous file size.
writeMapAndCref();
// Handle --print-memory-usage option.
if (config->printMemoryUsage)
script->printMemoryUsage(lld::outs());
if (config->checkSections)
checkSections();
// It does not make sense try to open the file if we have error already.
if (errorCount())
return;
{
llvm::TimeTraceScope timeScope("Write output file");
// Write the result down to a file.
openFile();
if (errorCount())
return;
if (!config->oFormatBinary) {
if (config->zSeparate != SeparateSegmentKind::None)
writeTrapInstr();
writeHeader();
writeSections();
} else {
writeSectionsBinary();
}
// Backfill .note.gnu.build-id section content. This is done at last
// because the content is usually a hash value of the entire output file.
writeBuildId();
if (errorCount())
return;
if (auto e = buffer->commit())
fatal("failed to write output '" + buffer->getPath() +
"': " + toString(std::move(e)));
if (!config->cmseOutputLib.empty())
writeARMCmseImportLib<ELFT>();
}
}
template <class ELFT, class RelTy>
static void markUsedLocalSymbolsImpl(ObjFile<ELFT> *file,
llvm::ArrayRef<RelTy> rels) {
for (const RelTy &rel : rels) {
Symbol &sym = file->getRelocTargetSym(rel);
if (sym.isLocal())
sym.used = true;
}
}
// The function ensures that the "used" field of local symbols reflects the fact
// that the symbol is used in a relocation from a live section.
template <class ELFT> static void markUsedLocalSymbols() {
// With --gc-sections, the field is already filled.
// See MarkLive<ELFT>::resolveReloc().
if (config->gcSections)
return;
for (ELFFileBase *file : ctx.objectFiles) {
ObjFile<ELFT> *f = cast<ObjFile<ELFT>>(file);
for (InputSectionBase *s : f->getSections()) {
InputSection *isec = dyn_cast_or_null<InputSection>(s);
if (!isec)
continue;
if (isec->type == SHT_REL)
markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rel>());
else if (isec->type == SHT_RELA)
markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rela>());
}
}
}
static bool shouldKeepInSymtab(const Defined &sym) {
if (sym.isSection())
return false;
// If --emit-reloc or -r is given, preserve symbols referenced by relocations
// from live sections.
if (sym.used && config->copyRelocs)
return true;
// Exclude local symbols pointing to .ARM.exidx sections.
// They are probably mapping symbols "$d", which are optional for these
// sections. After merging the .ARM.exidx sections, some of these symbols
// may become dangling. The easiest way to avoid the issue is not to add
// them to the symbol table from the beginning.
if (config->emachine == EM_ARM && sym.section &&
sym.section->type == SHT_ARM_EXIDX)
return false;
if (config->discard == DiscardPolicy::None)
return true;
if (config->discard == DiscardPolicy::All)
return false;
// In ELF assembly .L symbols are normally discarded by the assembler.
// If the assembler fails to do so, the linker discards them if
// * --discard-locals is used.
// * The symbol is in a SHF_MERGE section, which is normally the reason for
// the assembler keeping the .L symbol.
if (sym.getName().starts_with(".L") &&
(config->discard == DiscardPolicy::Locals ||
(sym.section && (sym.section->flags & SHF_MERGE))))
return false;
return true;
}
bool lld::elf::includeInSymtab(const Symbol &b) {
if (auto *d = dyn_cast<Defined>(&b)) {
// Always include absolute symbols.
SectionBase *sec = d->section;
if (!sec)
return true;
assert(sec->isLive());
if (auto *s = dyn_cast<MergeInputSection>(sec))
return s->getSectionPiece(d->value).live;
return true;
}
return b.used || !config->gcSections;
}
// Scan local symbols to:
//
// - demote symbols defined relative to /DISCARD/ discarded input sections so
// that relocations referencing them will lead to errors.
// - copy eligible symbols to .symTab
static void demoteAndCopyLocalSymbols() {
llvm::TimeTraceScope timeScope("Add local symbols");
for (ELFFileBase *file : ctx.objectFiles) {
DenseMap<SectionBase *, size_t> sectionIndexMap;
for (Symbol *b : file->getLocalSymbols()) {
assert(b->isLocal() && "should have been caught in initializeSymbols()");
auto *dr = dyn_cast<Defined>(b);
if (!dr)
continue;
if (dr->section && !dr->section->isLive())
demoteDefined(*dr, sectionIndexMap);
else if (in.symTab && includeInSymtab(*b) && shouldKeepInSymtab(*dr))
in.symTab->addSymbol(b);
}
}
}
// Create a section symbol for each output section so that we can represent
// relocations that point to the section. If we know that no relocation is
// referring to a section (that happens if the section is a synthetic one), we
// don't create a section symbol for that section.
template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
for (SectionCommand *cmd : script->sectionCommands) {
auto *osd = dyn_cast<OutputDesc>(cmd);
if (!osd)
continue;
OutputSection &osec = osd->osec;
InputSectionBase *isec = nullptr;
// Iterate over all input sections and add a STT_SECTION symbol if any input
// section may be a relocation target.
for (SectionCommand *cmd : osec.commands) {
auto *isd = dyn_cast<InputSectionDescription>(cmd);
if (!isd)
continue;
for (InputSectionBase *s : isd->sections) {
// Relocations are not using REL[A] section symbols.
if (isStaticRelSecType(s->type))
continue;
// Unlike other synthetic sections, mergeable output sections contain
// data copied from input sections, and there may be a relocation
// pointing to its contents if -r or --emit-reloc is given.
if (isa<SyntheticSection>(s) && !(s->flags & SHF_MERGE))
continue;
isec = s;
break;
}
}
if (!isec)
continue;
// Set the symbol to be relative to the output section so that its st_value
// equals the output section address. Note, there may be a gap between the
// start of the output section and isec.
in.symTab->addSymbol(makeDefined(isec->file, "", STB_LOCAL, /*stOther=*/0,
STT_SECTION,
/*value=*/0, /*size=*/0, &osec));
}
}
// Today's loaders have a feature to make segments read-only after
// processing dynamic relocations to enhance security. PT_GNU_RELRO
// is defined for that.
//
// This function returns true if a section needs to be put into a
// PT_GNU_RELRO segment.
static bool isRelroSection(const OutputSection *sec) {
if (!config->zRelro)
return false;
if (sec->relro)
return true;
uint64_t flags = sec->flags;
// Non-allocatable or non-writable sections don't need RELRO because
// they are not writable or not even mapped to memory in the first place.
// RELRO is for sections that are essentially read-only but need to
// be writable only at process startup to allow dynamic linker to
// apply relocations.
if (!(flags & SHF_ALLOC) || !(flags & SHF_WRITE))
return false;
// Once initialized, TLS data segments are used as data templates
// for a thread-local storage. For each new thread, runtime
// allocates memory for a TLS and copy templates there. No thread
// are supposed to use templates directly. Thus, it can be in RELRO.
if (flags & SHF_TLS)
return true;
// .init_array, .preinit_array and .fini_array contain pointers to
// functions that are executed on process startup or exit. These
// pointers are set by the static linker, and they are not expected
// to change at runtime. But if you are an attacker, you could do
// interesting things by manipulating pointers in .fini_array, for
// example. So they are put into RELRO.
uint32_t type = sec->type;
if (type == SHT_INIT_ARRAY || type == SHT_FINI_ARRAY ||
type == SHT_PREINIT_ARRAY)
return true;
// .got contains pointers to external symbols. They are resolved by
// the dynamic linker when a module is loaded into memory, and after
// that they are not expected to change. So, it can be in RELRO.
if (in.got && sec == in.got->getParent())
return true;
// .toc is a GOT-ish section for PowerPC64. Their contents are accessed
// through r2 register, which is reserved for that purpose. Since r2 is used
// for accessing .got as well, .got and .toc need to be close enough in the
// virtual address space. Usually, .toc comes just after .got. Since we place
// .got into RELRO, .toc needs to be placed into RELRO too.
if (sec->name == ".toc")
return true;
// .got.plt contains pointers to external function symbols. They are
// by default resolved lazily, so we usually cannot put it into RELRO.
// However, if "-z now" is given, the lazy symbol resolution is
// disabled, which enables us to put it into RELRO.
if (sec == in.gotPlt->getParent())
return config->zNow;
if (in.relroPadding && sec == in.relroPadding->getParent())
return true;
// .dynamic section contains data for the dynamic linker, and
// there's no need to write to it at runtime, so it's better to put
// it into RELRO.
if (sec->name == ".dynamic")
return true;
// Sections with some special names are put into RELRO. This is a
// bit unfortunate because section names shouldn't be significant in
// ELF in spirit. But in reality many linker features depend on
// magic section names.
StringRef s = sec->name;
return s == ".data.rel.ro" || s == ".bss.rel.ro" || s == ".ctors" ||
s == ".dtors" || s == ".jcr" || s == ".eh_frame" ||
s == ".fini_array" || s == ".init_array" ||
s == ".openbsd.randomdata" || s == ".preinit_array";
}
// We compute a rank for each section. The rank indicates where the
// section should be placed in the file. Instead of using simple
// numbers (0,1,2...), we use a series of flags. One for each decision
// point when placing the section.
// Using flags has two key properties:
// * It is easy to check if a give branch was taken.
// * It is easy two see how similar two ranks are (see getRankProximity).
enum RankFlags {
RF_NOT_ADDR_SET = 1 << 27,
RF_NOT_ALLOC = 1 << 26,
RF_PARTITION = 1 << 18, // Partition number (8 bits)
RF_LARGE_ALT = 1 << 15,
RF_WRITE = 1 << 14,
RF_EXEC_WRITE = 1 << 13,
RF_EXEC = 1 << 12,
RF_RODATA = 1 << 11,
RF_LARGE = 1 << 10,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
};
unsigned elf::getSectionRank(OutputSection &osec) {
unsigned rank = osec.partition * RF_PARTITION;
// We want to put section specified by -T option first, so we
// can start assigning VA starting from them later.
if (config->sectionStartMap.count(osec.name))
return rank;
rank |= RF_NOT_ADDR_SET;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
if (!(osec.flags & SHF_ALLOC))
return rank | RF_NOT_ALLOC;
// Sort sections based on their access permission in the following
// order: R, RX, RXW, RW(RELRO), RW(non-RELRO).
//
// Read-only sections come first such that they go in the PT_LOAD covering the
// program headers at the start of the file.
//
// The layout for writable sections is PT_LOAD(PT_GNU_RELRO(.data.rel.ro
// .bss.rel.ro) | .data .bss), where | marks where page alignment happens.
// An alternative ordering is PT_LOAD(.data | PT_GNU_RELRO( .data.rel.ro
// .bss.rel.ro) | .bss), but it may waste more bytes due to 2 alignment
// places.
bool isExec = osec.flags & SHF_EXECINSTR;
bool isWrite = osec.flags & SHF_WRITE;
if (!isWrite && !isExec) {
// Among PROGBITS sections, place .lrodata further from .text.
// For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
// layout has one extra PT_LOAD, but alleviates relocation overflow
// pressure for absolute relocations referencing small data from -fno-pic
// relocatable files.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
rank |= config->zLrodataAfterBss ? RF_LARGE_ALT : 0;
else
rank |= config->zLrodataAfterBss ? 0 : RF_LARGE;
if (osec.type == SHT_LLVM_PART_EHDR)
;
else if (osec.type == SHT_LLVM_PART_PHDR)
rank |= 1;
else if (osec.name == ".interp")
rank |= 2;
// Put .note sections at the beginning so that they are likely to be
// included in a truncate core file. In particular, .note.gnu.build-id, if
// available, can identify the object file.
else if (osec.type == SHT_NOTE)
rank |= 3;
// Make PROGBITS sections (e.g .rodata .eh_frame) closer to .text to
// alleviate relocation overflow pressure. Large special sections such as
// .dynstr and .dynsym can be away from .text.
else if (osec.type != SHT_PROGBITS)
rank |= 4;
else
rank |= RF_RODATA;
} else if (isExec) {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
} else {
rank |= RF_WRITE;
// The TLS initialization block needs to be a single contiguous block. Place
// TLS sections directly before the other RELRO sections.
if (!(osec.flags & SHF_TLS))
rank |= RF_NOT_TLS;
if (isRelroSection(&osec))
osec.relro = true;
else
rank |= RF_NOT_RELRO;
// Place .ldata and .lbss after .bss. Making .bss closer to .text
// alleviates relocation overflow pressure.
// For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
// .bss/.lbss being adjacent reuses the NOBITS size optimization.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) {
rank |= config->zLrodataAfterBss
? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
: RF_LARGE;
}
}
// Within TLS sections, or within other RelRo sections, or within non-RelRo
// sections, place non-NOBITS sections first.
if (osec.type == SHT_NOBITS)
rank |= RF_BSS;
// Some architectures have additional ordering restrictions for sections
// within the same PT_LOAD.
if (config->emachine == EM_PPC64) {
// PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
// that we would like to make sure appear is a specific order to maximize
// their coverage by a single signed 16-bit offset from the TOC base
// pointer.
StringRef name = osec.name;
if (name == ".got")
rank |= 1;
else if (name == ".toc")
rank |= 2;
}
if (config->emachine == EM_MIPS) {
if (osec.name != ".got")
rank |= 1;
// All sections with SHF_MIPS_GPREL flag should be grouped together
// because data in these sections is addressable with a gp relative address.
if (osec.flags & SHF_MIPS_GPREL)
rank |= 2;
}
if (config->emachine == EM_RISCV) {
// .sdata and .sbss are placed closer to make GP relaxation more profitable
// and match GNU ld.
StringRef name = osec.name;
if (name == ".sdata" || (osec.type == SHT_NOBITS && name != ".sbss"))
rank |= 1;
}
return rank;
}
static bool compareSections(const SectionCommand *aCmd,
const SectionCommand *bCmd) {
const OutputSection *a = &cast<OutputDesc>(aCmd)->osec;
const OutputSection *b = &cast<OutputDesc>(bCmd)->osec;
if (a->sortRank != b->sortRank)
return a->sortRank < b->sortRank;
if (!(a->sortRank & RF_NOT_ADDR_SET))
return config->sectionStartMap.lookup(a->name) <
config->sectionStartMap.lookup(b->name);
return false;
}
void PhdrEntry::add(OutputSection *sec) {
lastSec = sec;
if (!firstSec)
firstSec = sec;
p_align = std::max(p_align, sec->addralign);
if (p_type == PT_LOAD)
sec->ptLoad = this;
}
// A statically linked position-dependent executable should only contain
// IRELATIVE relocations and no other dynamic relocations. Encapsulation symbols
// __rel[a]_iplt_{start,end} will be defined for .rel[a].dyn, to be
// processed by the libc runtime. Other executables or DSOs use dynamic tags
// instead.
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (config->isPic)
return;
// __rela_iplt_{start,end} are initially defined relative to dummy section 0.
// We'll override Out::elfHeader with relaDyn later when we are sure that
// .rela.dyn will be present in the output.
ElfSym::relaIpltStart = addOptionalRegular(
config->isRela ? "__rela_iplt_start" : "__rel_iplt_start",
Out::elfHeader, 0, STV_HIDDEN);
ElfSym::relaIpltEnd = addOptionalRegular(
config->isRela ? "__rela_iplt_end" : "__rel_iplt_end",
Out::elfHeader, 0, STV_HIDDEN);
}
// This function generates assignments for predefined symbols (e.g. _end or
// _etext) and inserts them into the commands sequence to be processed at the
// appropriate time. This ensures that the value is going to be correct by the
// time any references to these symbols are processed and is equivalent to
// defining these symbols explicitly in the linker script.
template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
if (ElfSym::globalOffsetTable) {
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
// to the start of the .got or .got.plt section.
InputSection *sec = in.gotPlt.get();
if (!target->gotBaseSymInGotPlt)
sec = in.mipsGot ? cast<InputSection>(in.mipsGot.get())
: cast<InputSection>(in.got.get());
ElfSym::globalOffsetTable->section = sec;
}
// .rela_iplt_{start,end} mark the start and the end of .rel[a].dyn.
if (ElfSym::relaIpltStart && mainPart->relaDyn->isNeeded()) {
ElfSym::relaIpltStart->section = mainPart->relaDyn.get();
ElfSym::relaIpltEnd->section = mainPart->relaDyn.get();
ElfSym::relaIpltEnd->value = mainPart->relaDyn->getSize();
}
PhdrEntry *last = nullptr;
OutputSection *lastRO = nullptr;
auto isLarge = [](OutputSection *osec) {
return config->emachine == EM_X86_64 && osec->flags & SHF_X86_64_LARGE;
};
for (Partition &part : partitions) {
for (PhdrEntry *p : part.phdrs) {
if (p->p_type != PT_LOAD)
continue;
last = p;
if (!(p->p_flags & PF_W) && p->lastSec && !isLarge(p->lastSec))
lastRO = p->lastSec;
}
}
if (lastRO) {
// _etext is the first location after the last read-only loadable segment
// that does not contain large sections.
if (ElfSym::etext1)
ElfSym::etext1->section = lastRO;
if (ElfSym::etext2)
ElfSym::etext2->section = lastRO;
}
if (last) {
// _edata points to the end of the last non-large mapped initialized
// section.
OutputSection *edata = nullptr;
for (OutputSection *os : outputSections) {
if (os->type != SHT_NOBITS && !isLarge(os))
edata = os;
if (os == last->lastSec)
break;
}
if (ElfSym::edata1)
ElfSym::edata1->section = edata;
if (ElfSym::edata2)
ElfSym::edata2->section = edata;
// _end is the first location after the uninitialized data region.
if (ElfSym::end1)
ElfSym::end1->section = last->lastSec;
if (ElfSym::end2)
ElfSym::end2->section = last->lastSec;
}
if (ElfSym::bss) {
// On RISC-V, set __bss_start to the start of .sbss if present.
OutputSection *sbss =
config->emachine == EM_RISCV ? findSection(".sbss") : nullptr;
ElfSym::bss->section = sbss ? sbss : findSection(".bss");
}
// Setup MIPS _gp_disp/__gnu_local_gp symbols which should
// be equal to the _gp symbol's value.
if (ElfSym::mipsGp) {
// Find GP-relative section with the lowest address
// and use this address to calculate default _gp value.
for (OutputSection *os : outputSections) {
if (os->flags & SHF_MIPS_GPREL) {
ElfSym::mipsGp->section = os;
ElfSym::mipsGp->value = 0x7ff0;
break;
}
}
}
}
// We want to find how similar two ranks are.
// The more branches in getSectionRank that match, the more similar they are.
// Since each branch corresponds to a bit flag, we can just use
// countLeadingZeros.
static int getRankProximity(OutputSection *a, SectionCommand *b) {
auto *osd = dyn_cast<OutputDesc>(b);
return (osd && osd->osec.hasInputSections)
? llvm::countl_zero(a->sortRank ^ osd->osec.sortRank)
: -1;
}
// When placing orphan sections, we want to place them after symbol assignments
// so that an orphan after
// begin_foo = .;
// foo : { *(foo) }
// end_foo = .;
// doesn't break the intended meaning of the begin/end symbols.
// We don't want to go over sections since findOrphanPos is the
// one in charge of deciding the order of the sections.
// We don't want to go over changes to '.', since doing so in
// rx_sec : { *(rx_sec) }
// . = ALIGN(0x1000);
// /* The RW PT_LOAD starts here*/
// rw_sec : { *(rw_sec) }
// would mean that the RW PT_LOAD would become unaligned.
static bool shouldSkip(SectionCommand *cmd) {
if (auto *assign = dyn_cast<SymbolAssignment>(cmd))
return assign->name != ".";
return false;
}
// We want to place orphan sections so that they share as much
// characteristics with their neighbors as possible. For example, if
// both are rw, or both are tls.
static SmallVectorImpl<SectionCommand *>::iterator
findOrphanPos(SmallVectorImpl<SectionCommand *>::iterator b,
SmallVectorImpl<SectionCommand *>::iterator e) {
OutputSection *sec = &cast<OutputDesc>(*e)->osec;
// As a special case, place .relro_padding before the SymbolAssignment using
// DATA_SEGMENT_RELRO_END, if present.
if (in.relroPadding && sec == in.relroPadding->getParent()) {
auto i = std::find_if(b, e, [=](SectionCommand *a) {
if (auto *assign = dyn_cast<SymbolAssignment>(a))
return assign->dataSegmentRelroEnd;
return false;
});
if (i != e)
return i;
}
// Find the first element that has as close a rank as possible.
auto i = std::max_element(b, e, [=](SectionCommand *a, SectionCommand *b) {
return getRankProximity(sec, a) < getRankProximity(sec, b);
});
if (i == e)
return e;
if (!isa<OutputDesc>(*i))
return e;
auto foundSec = &cast<OutputDesc>(*i)->osec;
// Consider all existing sections with the same proximity.
int proximity = getRankProximity(sec, *i);
unsigned sortRank = sec->sortRank;
if (script->hasPhdrsCommands() || !script->memoryRegions.empty())
// Prevent the orphan section to be placed before the found section. If
// custom program headers are defined, that helps to avoid adding it to a
// previous segment and changing flags of that segment, for example, making
// a read-only segment writable. If memory regions are defined, an orphan
// section should continue the same region as the found section to better
// resemble the behavior of GNU ld.
sortRank = std::max(sortRank, foundSec->sortRank);
for (; i != e; ++i) {
auto *curSecDesc = dyn_cast<OutputDesc>(*i);
if (!curSecDesc || !curSecDesc->osec.hasInputSections)
continue;
if (getRankProximity(sec, curSecDesc) != proximity ||
sortRank < curSecDesc->osec.sortRank)
break;
}
auto isOutputSecWithInputSections = [](SectionCommand *cmd) {
auto *osd = dyn_cast<OutputDesc>(cmd);
return osd && osd->osec.hasInputSections;
};
auto j =
std::find_if(std::make_reverse_iterator(i), std::make_reverse_iterator(b),
isOutputSecWithInputSections);
i = j.base();
// As a special case, if the orphan section is the last section, put
// it at the very end, past any other commands.
// This matches bfd's behavior and is convenient when the linker script fully
// specifies the start of the file, but doesn't care about the end (the non
// alloc sections for example).
auto nextSec = std::find_if(i, e, isOutputSecWithInputSections);
if (nextSec == e)
return e;
while (i != e && shouldSkip(*i))
++i;
return i;
}
// Adds random priorities to sections not already in the map.
static void maybeShuffle(DenseMap<const InputSectionBase *, int> &order) {
if (config->shuffleSections.empty())
return;
SmallVector<InputSectionBase *, 0> matched, sections = ctx.inputSections;
matched.reserve(sections.size());
for (const auto &patAndSeed : config->shuffleSections) {
matched.clear();
for (InputSectionBase *sec : sections)
if (patAndSeed.first.match(sec->name))
matched.push_back(sec);
const uint32_t seed = patAndSeed.second;
if (seed == UINT32_MAX) {
// If --shuffle-sections <section-glob>=-1, reverse the section order. The
// section order is stable even if the number of sections changes. This is
// useful to catch issues like static initialization order fiasco
// reliably.
std::reverse(matched.begin(), matched.end());
} else {
std::mt19937 g(seed ? seed : std::random_device()());
llvm::shuffle(matched.begin(), matched.end(), g);
}
size_t i = 0;
for (InputSectionBase *&sec : sections)
if (patAndSeed.first.match(sec->name))
sec = matched[i++];
}
// Existing priorities are < 0, so use priorities >= 0 for the missing
// sections.
int prio = 0;
for (InputSectionBase *sec : sections) {
if (order.try_emplace(sec, prio).second)
++prio;
}
}
// Builds section order for handling --symbol-ordering-file.
static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
DenseMap<const InputSectionBase *, int> sectionOrder;
// Use the rarely used option --call-graph-ordering-file to sort sections.
if (!config->callGraphProfile.empty())
return computeCallGraphProfileOrder();
if (config->symbolOrderingFile.empty())
return sectionOrder;
struct SymbolOrderEntry {
int priority;
bool present;
};
// Build a map from symbols to their priorities. Symbols that didn't
// appear in the symbol ordering file have the lowest priority 0.
// All explicitly mentioned symbols have negative (higher) priorities.
DenseMap<CachedHashStringRef, SymbolOrderEntry> symbolOrder;
int priority = -config->symbolOrderingFile.size();
for (StringRef s : config->symbolOrderingFile)
symbolOrder.insert({CachedHashStringRef(s), {priority++, false}});
// Build a map from sections to their priorities.
auto addSym = [&](Symbol &sym) {
auto it = symbolOrder.find(CachedHashStringRef(sym.getName()));
if (it == symbolOrder.end())
return;
SymbolOrderEntry &ent = it->second;
ent.present = true;
maybeWarnUnorderableSymbol(&sym);
if (auto *d = dyn_cast<Defined>(&sym)) {
if (auto *sec = dyn_cast_or_null<InputSectionBase>(d->section)) {
int &priority = sectionOrder[cast<InputSectionBase>(sec)];
priority = std::min(priority, ent.priority);
}
}
};
// We want both global and local symbols. We get the global ones from the
// symbol table and iterate the object files for the local ones.
for (Symbol *sym : symtab.getSymbols())
addSym(*sym);
for (ELFFileBase *file : ctx.objectFiles)
for (Symbol *sym : file->getLocalSymbols())
addSym(*sym);
if (config->warnSymbolOrdering)
for (auto orderEntry : symbolOrder)
if (!orderEntry.second.present)
warn("symbol ordering file: no such symbol: " + orderEntry.first.val());
return sectionOrder;
}
// Sorts the sections in ISD according to the provided section order.
static void
sortISDBySectionOrder(InputSectionDescription *isd,
const DenseMap<const InputSectionBase *, int> &order,
bool executableOutputSection) {
SmallVector<InputSection *, 0> unorderedSections;
SmallVector<std::pair<InputSection *, int>, 0> orderedSections;
uint64_t unorderedSize = 0;
uint64_t totalSize = 0;
for (InputSection *isec : isd->sections) {
if (executableOutputSection)
totalSize += isec->getSize();
auto i = order.find(isec);
if (i == order.end()) {
unorderedSections.push_back(isec);
unorderedSize += isec->getSize();
continue;
}
orderedSections.push_back({isec, i->second});
}
llvm::sort(orderedSections, llvm::less_second());
// Find an insertion point for the ordered section list in the unordered
// section list. On targets with limited-range branches, this is the mid-point
// of the unordered section list. This decreases the likelihood that a range
// extension thunk will be needed to enter or exit the ordered region. If the
// ordered section list is a list of hot functions, we can generally expect
// the ordered functions to be called more often than the unordered functions,
// making it more likely that any particular call will be within range, and
// therefore reducing the number of thunks required.
//
// For example, imagine that you have 8MB of hot code and 32MB of cold code.
// If the layout is:
//
// 8MB hot
// 32MB cold
//
// only the first 8-16MB of the cold code (depending on which hot function it
// is actually calling) can call the hot code without a range extension thunk.
// However, if we use this layout:
//
// 16MB cold
// 8MB hot
// 16MB cold
//
// both the last 8-16MB of the first block of cold code and the first 8-16MB
// of the second block of cold code can call the hot code without a thunk. So
// we effectively double the amount of code that could potentially call into
// the hot code without a thunk.
//
// The above is not necessary if total size of input sections in this "isd"
// is small. Note that we assume all input sections are executable if the
// output section is executable (which is not always true but supposed to
// cover most cases).
size_t insPt = 0;
if (executableOutputSection && !orderedSections.empty() &&
target->getThunkSectionSpacing() &&
totalSize >= target->getThunkSectionSpacing()) {
uint64_t unorderedPos = 0;
for (; insPt != unorderedSections.size(); ++insPt) {
unorderedPos += unorderedSections[insPt]->getSize();
if (unorderedPos > unorderedSize / 2)
break;
}
}
isd->sections.clear();
for (InputSection *isec : ArrayRef(unorderedSections).slice(0, insPt))
isd->sections.push_back(isec);
for (std::pair<InputSection *, int> p : orderedSections)
isd->sections.push_back(p.first);
for (InputSection *isec : ArrayRef(unorderedSections).slice(insPt))
isd->sections.push_back(isec);
}
static void sortSection(OutputSection &osec,
const DenseMap<const InputSectionBase *, int> &order) {
StringRef name = osec.name;
// Never sort these.
if (name == ".init" || name == ".fini")
return;
// Sort input sections by priority using the list provided by
// --symbol-ordering-file or --shuffle-sections=. This is a least significant
// digit radix sort. The sections may be sorted stably again by a more
// significant key.
if (!order.empty())
for (SectionCommand *b : osec.commands)
if (auto *isd = dyn_cast<InputSectionDescription>(b))
sortISDBySectionOrder(isd, order, osec.flags & SHF_EXECINSTR);
if (script->hasSectionsCommand)
return;
if (name == ".init_array" || name == ".fini_array") {
osec.sortInitFini();
} else if (name == ".ctors" || name == ".dtors") {
osec.sortCtorsDtors();
} else if (config->emachine == EM_PPC64 && name == ".toc") {
// .toc is allocated just after .got and is accessed using GOT-relative
// relocations. Object files compiled with small code model have an
// addressable range of [.got, .got + 0xFFFC] for GOT-relative relocations.
// To reduce the risk of relocation overflow, .toc contents are sorted so
// that sections having smaller relocation offsets are at beginning of .toc
assert(osec.commands.size() == 1);
auto *isd = cast<InputSectionDescription>(osec.commands[0]);
llvm::stable_sort(isd->sections,
[](const InputSection *a, const InputSection *b) -> bool {
return a->file->ppc64SmallCodeModelTocRelocs &&
!b->file->ppc64SmallCodeModelTocRelocs;
});
}
}
// If no layout was provided by linker script, we want to apply default
// sorting for special input sections. This also handles --symbol-ordering-file.
template <class ELFT> void Writer<ELFT>::sortInputSections() {
// Build the order once since it is expensive.
DenseMap<const InputSectionBase *, int> order = buildSectionOrder();
maybeShuffle(order);
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
sortSection(osd->osec, order);
}
template <class ELFT> void Writer<ELFT>::sortSections() {
llvm::TimeTraceScope timeScope("Sort sections");
// Don't sort if using -r. It is not necessary and we want to preserve the
// relative order for SHF_LINK_ORDER sections.
if (config->relocatable) {
script->adjustOutputSections();
return;
}
sortInputSections();
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
osd->osec.sortRank = getSectionRank(osd->osec);
if (!script->hasSectionsCommand) {
// OutputDescs are mostly contiguous, but may be interleaved with
// SymbolAssignments in the presence of INSERT commands.
auto mid = std::stable_partition(
script->sectionCommands.begin(), script->sectionCommands.end(),
[](SectionCommand *cmd) { return isa<OutputDesc>(cmd); });
std::stable_sort(script->sectionCommands.begin(), mid, compareSections);
}
// Process INSERT commands and update output section attributes. From this
// point onwards the order of script->sectionCommands is fixed.
script->processInsertCommands();
script->adjustOutputSections();
if (script->hasSectionsCommand)
sortOrphanSections();
script->adjustSectionsAfterSorting();
}
template <class ELFT> void Writer<ELFT>::sortOrphanSections() {
// Orphan sections are sections present in the input files which are
// not explicitly placed into the output file by the linker script.
//
// The sections in the linker script are already in the correct
// order. We have to figuere out where to insert the orphan
// sections.
//
// The order of the sections in the script is arbitrary and may not agree with
// compareSections. This means that we cannot easily define a strict weak
// ordering. To see why, consider a comparison of a section in the script and
// one not in the script. We have a two simple options:
// * Make them equivalent (a is not less than b, and b is not less than a).
// The problem is then that equivalence has to be transitive and we can
// have sections a, b and c with only b in a script and a less than c
// which breaks this property.
// * Use compareSectionsNonScript. Given that the script order doesn't have
// to match, we can end up with sections a, b, c, d where b and c are in the
// script and c is compareSectionsNonScript less than b. In which case d
// can be equivalent to c, a to b and d < a. As a concrete example:
// .a (rx) # not in script
// .b (rx) # in script
// .c (ro) # in script
// .d (ro) # not in script
//
// The way we define an order then is:
// * Sort only the orphan sections. They are in the end right now.
// * Move each orphan section to its preferred position. We try
// to put each section in the last position where it can share
// a PT_LOAD.
//
// There is some ambiguity as to where exactly a new entry should be
// inserted, because Commands contains not only output section
// commands but also other types of commands such as symbol assignment
// expressions. There's no correct answer here due to the lack of the
// formal specification of the linker script. We use heuristics to
// determine whether a new output command should be added before or
// after another commands. For the details, look at shouldSkip
// function.
auto i = script->sectionCommands.begin();
auto e = script->sectionCommands.end();
auto nonScriptI = std::find_if(i, e, [](SectionCommand *cmd) {
if (auto *osd = dyn_cast<OutputDesc>(cmd))
return osd->osec.sectionIndex == UINT32_MAX;
return false;
});
// Sort the orphan sections.
std::stable_sort(nonScriptI, e, compareSections);
// As a horrible special case, skip the first . assignment if it is before any
// section. We do this because it is common to set a load address by starting
// the script with ". = 0xabcd" and the expectation is that every section is
// after that.
auto firstSectionOrDotAssignment =
std::find_if(i, e, [](SectionCommand *cmd) { return !shouldSkip(cmd); });
if (firstSectionOrDotAssignment != e &&
isa<SymbolAssignment>(**firstSectionOrDotAssignment))
++firstSectionOrDotAssignment;
i = firstSectionOrDotAssignment;
while (nonScriptI != e) {
auto pos = findOrphanPos(i, nonScriptI);
OutputSection *orphan = &cast<OutputDesc>(*nonScriptI)->osec;
// As an optimization, find all sections with the same sort rank
// and insert them with one rotate.
unsigned rank = orphan->sortRank;
auto end = std::find_if(nonScriptI + 1, e, [=](SectionCommand *cmd) {
return cast<OutputDesc>(cmd)->osec.sortRank != rank;
});
std::rotate(pos, nonScriptI, end);
nonScriptI = end;
}
}
static bool compareByFilePosition(InputSection *a, InputSection *b) {
InputSection *la = a->flags & SHF_LINK_ORDER ? a->getLinkOrderDep() : nullptr;
InputSection *lb = b->flags & SHF_LINK_ORDER ? b->getLinkOrderDep() : nullptr;
// SHF_LINK_ORDER sections with non-zero sh_link are ordered before
// non-SHF_LINK_ORDER sections and SHF_LINK_ORDER sections with zero sh_link.
if (!la || !lb)
return la && !lb;
OutputSection *aOut = la->getParent();
OutputSection *bOut = lb->getParent();
if (aOut == bOut)
return la->outSecOff < lb->outSecOff;
if (aOut->addr == bOut->addr)
return aOut->sectionIndex < bOut->sectionIndex;
return aOut->addr < bOut->addr;
}
template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
llvm::TimeTraceScope timeScope("Resolve SHF_LINK_ORDER");
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_LINK_ORDER))
continue;
// The ARM.exidx section use SHF_LINK_ORDER, but we have consolidated
// this processing inside the ARMExidxsyntheticsection::finalizeContents().
if (!config->relocatable && config->emachine == EM_ARM &&
sec->type == SHT_ARM_EXIDX)
continue;
// Link order may be distributed across several InputSectionDescriptions.
// Sorting is performed separately.
SmallVector<InputSection **, 0> scriptSections;
SmallVector<InputSection *, 0> sections;
for (SectionCommand *cmd : sec->commands) {
auto *isd = dyn_cast<InputSectionDescription>(cmd);
if (!isd)
continue;
bool hasLinkOrder = false;
scriptSections.clear();
sections.clear();
for (InputSection *&isec : isd->sections) {
if (isec->flags & SHF_LINK_ORDER) {
InputSection *link = isec->getLinkOrderDep();
if (link && !link->getParent())
error(toString(isec) + ": sh_link points to discarded section " +
toString(link));
hasLinkOrder = true;
}
scriptSections.push_back(&isec);
sections.push_back(isec);
}
if (hasLinkOrder && errorCount() == 0) {
llvm::stable_sort(sections, compareByFilePosition);
for (int i = 0, n = sections.size(); i != n; ++i)
*scriptSections[i] = sections[i];
}
}
}
}
static void finalizeSynthetic(SyntheticSection *sec) {
if (sec && sec->isNeeded() && sec->getParent()) {
llvm::TimeTraceScope timeScope("Finalize synthetic sections", sec->name);
sec->finalizeContents();
}
}
// We need to generate and finalize the content that depends on the address of
// InputSections. As the generation of the content may also alter InputSection
// addresses we must converge to a fixed point. We do that here. See the comment
// in Writer<ELFT>::finalizeSections().
template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
llvm::TimeTraceScope timeScope("Finalize address dependent content");
ThunkCreator tc;
AArch64Err843419Patcher a64p;
ARMErr657417Patcher a32p;
script->assignAddresses();
// .ARM.exidx and SHF_LINK_ORDER do not require precise addresses, but they
// do require the relative addresses of OutputSections because linker scripts
// can assign Virtual Addresses to OutputSections that are not monotonically
// increasing. Anything here must be repeatable, since spilling may change
// section order.
const auto finalizeOrderDependentContent = [this] {
for (Partition &part : partitions)
finalizeSynthetic(part.armExidx.get());
resolveShfLinkOrder();
};
finalizeOrderDependentContent();
// Converts call x@GDPLT to call __tls_get_addr
if (config->emachine == EM_HEXAGON)
hexagonTLSSymbolUpdate(outputSections);
uint32_t pass = 0, assignPasses = 0;
for (;;) {
bool changed = target->needsThunks ? tc.createThunks(pass, outputSections)
: target->relaxOnce(pass);
bool spilled = script->spillSections();
changed |= spilled;
++pass;
// With Thunk Size much smaller than branch range we expect to
// converge quickly; if we get to 30 something has gone wrong.
if (changed && pass >= 30) {
error(target->needsThunks ? "thunk creation not converged"
: "relaxation not converged");
break;
}
if (config->fixCortexA53Errata843419) {
if (changed)
script->assignAddresses();
changed |= a64p.createFixes();
}
if (config->fixCortexA8) {
if (changed)
script->assignAddresses();
changed |= a32p.createFixes();
}
finalizeSynthetic(in.got.get());
if (in.mipsGot)
in.mipsGot->updateAllocSize();
for (Partition &part : partitions) {
// The R_AARCH64_AUTH_RELATIVE has a smaller addend field as bits [63:32]
// encode the signing schema. We've put relocations in .relr.auth.dyn
// during RelocationScanner::processAux, but the target VA for some of
// them might be wider than 32 bits. We can only know the final VA at this
// point, so move relocations with large values from .relr.auth.dyn to
// .rela.dyn. See also AArch64::relocate.
if (part.relrAuthDyn) {
auto it = llvm::remove_if(
part.relrAuthDyn->relocs, [&part](const RelativeReloc &elem) {
const Relocation &reloc = elem.inputSec->relocs()[elem.relocIdx];
if (isInt<32>(reloc.sym->getVA(reloc.addend)))
return false;
part.relaDyn->addReloc({R_AARCH64_AUTH_RELATIVE, elem.inputSec,
reloc.offset,
DynamicReloc::AddendOnlyWithTargetVA,
*reloc.sym, reloc.addend, R_ABS});
return true;
});
changed |= (it != part.relrAuthDyn->relocs.end());
part.relrAuthDyn->relocs.erase(it, part.relrAuthDyn->relocs.end());
}
changed |= part.relaDyn->updateAllocSize();
if (part.relrDyn)
changed |= part.relrDyn->updateAllocSize();
if (part.relrAuthDyn)
changed |= part.relrAuthDyn->updateAllocSize();
if (part.memtagGlobalDescriptors)
changed |= part.memtagGlobalDescriptors->updateAllocSize();
}
std::pair<const OutputSection *, const Defined *> changes =
script->assignAddresses();
if (!changed) {
// Some symbols may be dependent on section addresses. When we break the
// loop, the symbol values are finalized because a previous
// assignAddresses() finalized section addresses.
if (!changes.first && !changes.second)
break;
if (++assignPasses == 5) {
if (changes.first)
errorOrWarn("address (0x" + Twine::utohexstr(changes.first->addr) +
") of section '" + changes.first->name +
"' does not converge");
if (changes.second)
errorOrWarn("assignment to symbol " + toString(*changes.second) +
" does not converge");
break;
}
} else if (spilled) {
// Spilling can change relative section order.
finalizeOrderDependentContent();
}
}
if (!config->relocatable)
target->finalizeRelax(pass);
if (config->relocatable)
for (OutputSection *sec : outputSections)
sec->addr = 0;
// If addrExpr is set, the address may not be a multiple of the alignment.
// Warn because this is error-prone.
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
OutputSection *osec = &osd->osec;
if (osec->addr % osec->addralign != 0)
warn("address (0x" + Twine::utohexstr(osec->addr) + ") of section " +
osec->name + " is not a multiple of alignment (" +
Twine(osec->addralign) + ")");
}
// Sizes are no longer allowed to grow, so all allowable spills have been
// taken. Remove any leftover potential spills.
script->erasePotentialSpillSections();
}
// If Input Sections have been shrunk (basic block sections) then
// update symbol values and sizes associated with these sections. With basic
// block sections, input sections can shrink when the jump instructions at
// the end of the section are relaxed.
static void fixSymbolsAfterShrinking() {
for (InputFile *File : ctx.objectFiles) {
parallelForEach(File->getSymbols(), [&](Symbol *Sym) {
auto *def = dyn_cast<Defined>(Sym);
if (!def)
return;
const SectionBase *sec = def->section;
if (!sec)
return;
const InputSectionBase *inputSec = dyn_cast<InputSectionBase>(sec);
if (!inputSec || !inputSec->bytesDropped)
return;
const size_t OldSize = inputSec->content().size();
const size_t NewSize = OldSize - inputSec->bytesDropped;
if (def->value > NewSize && def->value <= OldSize) {
LLVM_DEBUG(llvm::dbgs()
<< "Moving symbol " << Sym->getName() << " from "
<< def->value << " to "
<< def->value - inputSec->bytesDropped << " bytes\n");
def->value -= inputSec->bytesDropped;
return;
}
if (def->value + def->size > NewSize && def->value <= OldSize &&
def->value + def->size <= OldSize) {
LLVM_DEBUG(llvm::dbgs()
<< "Shrinking symbol " << Sym->getName() << " from "
<< def->size << " to " << def->size - inputSec->bytesDropped
<< " bytes\n");
def->size -= inputSec->bytesDropped;
}
});
}
}
// If basic block sections exist, there are opportunities to delete fall thru
// jumps and shrink jump instructions after basic block reordering. This
// relaxation pass does that. It is only enabled when --optimize-bb-jumps
// option is used.
template <class ELFT> void Writer<ELFT>::optimizeBasicBlockJumps() {
assert(config->optimizeBBJumps);
SmallVector<InputSection *, 0> storage;
script->assignAddresses();
// For every output section that has executable input sections, this
// does the following:
// 1. Deletes all direct jump instructions in input sections that
// jump to the following section as it is not required.
// 2. If there are two consecutive jump instructions, it checks
// if they can be flipped and one can be deleted.
for (OutputSection *osec : outputSections) {
if (!(osec->flags & SHF_EXECINSTR))
continue;
ArrayRef<InputSection *> sections = getInputSections(*osec, storage);
size_t numDeleted = 0;
// Delete all fall through jump instructions. Also, check if two
// consecutive jump instructions can be flipped so that a fall
// through jmp instruction can be deleted.
for (size_t i = 0, e = sections.size(); i != e; ++i) {
InputSection *next = i + 1 < sections.size() ? sections[i + 1] : nullptr;
InputSection &sec = *sections[i];
numDeleted += target->deleteFallThruJmpInsn(sec, sec.file, next);
}
if (numDeleted > 0) {
script->assignAddresses();
LLVM_DEBUG(llvm::dbgs()
<< "Removing " << numDeleted << " fall through jumps\n");
}
}
fixSymbolsAfterShrinking();
for (OutputSection *osec : outputSections)
for (InputSection *is : getInputSections(*osec, storage))
is->trim();
}
// In order to allow users to manipulate linker-synthesized sections,
// we had to add synthetic sections to the input section list early,
// even before we make decisions whether they are needed. This allows
// users to write scripts like this: ".mygot : { .got }".
//
// Doing it has an unintended side effects. If it turns out that we
// don't need a .got (for example) at all because there's no
// relocation that needs a .got, we don't want to emit .got.
//
// To deal with the above problem, this function is called after
// scanRelocations is called to remove synthetic sections that turn
// out to be empty.
static void removeUnusedSyntheticSections() {
// All input synthetic sections that can be empty are placed after
// all regular ones. Reverse iterate to find the first synthetic section
// after a non-synthetic one which will be our starting point.
auto start =
llvm::find_if(llvm::reverse(ctx.inputSections), [](InputSectionBase *s) {
return !isa<SyntheticSection>(s);
}).base();
// Remove unused synthetic sections from ctx.inputSections;
DenseSet<InputSectionBase *> unused;
auto end =
std::remove_if(start, ctx.inputSections.end(), [&](InputSectionBase *s) {
auto *sec = cast<SyntheticSection>(s);
if (sec->getParent() && sec->isNeeded())
return false;
// .relr.auth.dyn relocations may be moved to .rela.dyn in
// finalizeAddressDependentContent, making .rela.dyn no longer empty.
// Conservatively keep .rela.dyn. .relr.auth.dyn can be made empty, but
// we would fail to remove it here.
if (config->emachine == EM_AARCH64 && config->relrPackDynRelocs)
if (auto *relSec = dyn_cast<RelocationBaseSection>(sec))
if (relSec == mainPart->relaDyn.get())
return false;
unused.insert(sec);
return true;
});
ctx.inputSections.erase(end, ctx.inputSections.end());
// Remove unused synthetic sections from the corresponding input section
// description and orphanSections.
for (auto *sec : unused)
if (OutputSection *osec = cast<SyntheticSection>(sec)->getParent())
for (SectionCommand *cmd : osec->commands)
if (auto *isd = dyn_cast<InputSectionDescription>(cmd))
llvm::erase_if(isd->sections, [&](InputSection *isec) {
return unused.count(isec);
});
llvm::erase_if(script->orphanSections, [&](const InputSectionBase *sec) {
return unused.count(sec);
});
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
if (!config->relocatable) {
Out::preinitArray = findSection(".preinit_array");
Out::initArray = findSection(".init_array");
Out::finiArray = findSection(".fini_array");
// The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
// symbols for sections, so that the runtime can get the start and end
// addresses of each section by section name. Add such symbols.
addStartEndSymbols();
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
addStartStopSymbols(osd->osec);
// Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
// It should be okay as no one seems to care about the type.
// Even the author of gold doesn't remember why gold behaves that way.
// https://sourceware.org/ml/binutils/2002-03/msg00360.html
if (mainPart->dynamic->parent) {
Symbol *s = symtab.addSymbol(Defined{
ctx.internalFile, "_DYNAMIC", STB_WEAK, STV_HIDDEN, STT_NOTYPE,
/*value=*/0, /*size=*/0, mainPart->dynamic.get()});
s->isUsedInRegularObj = true;
}
// Define __rel[a]_iplt_{start,end} symbols if needed.
addRelIpltSymbols();
// RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800. This symbol
// should only be defined in an executable. If .sdata does not exist, its
// value/section does not matter but it has to be relative, so set its
// st_shndx arbitrarily to 1 (Out::elfHeader).
if (config->emachine == EM_RISCV) {
ElfSym::riscvGlobalPointer = nullptr;
if (!config->shared) {
OutputSection *sec = findSection(".sdata");
addOptionalRegular(
"__global_pointer$", sec ? sec : Out::elfHeader, 0x800, STV_DEFAULT);
// Set riscvGlobalPointer to be used by the optional global pointer
// relaxation.
if (config->relaxGP) {
Symbol *s = symtab.find("__global_pointer$");
if (s && s->isDefined())
ElfSym::riscvGlobalPointer = cast<Defined>(s);
}
}
}
if (config->emachine == EM_386 || config->emachine == EM_X86_64) {
// On targets that support TLSDESC, _TLS_MODULE_BASE_ is defined in such a
// way that:
//
// 1) Without relaxation: it produces a dynamic TLSDESC relocation that
// computes 0.
// 2) With LD->LE relaxation: _TLS_MODULE_BASE_@tpoff = 0 (lowest address
// in the TLS block).
//
// 2) is special cased in @tpoff computation. To satisfy 1), we define it
// as an absolute symbol of zero. This is different from GNU linkers which
// define _TLS_MODULE_BASE_ relative to the first TLS section.
Symbol *s = symtab.find("_TLS_MODULE_BASE_");
if (s && s->isUndefined()) {
s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL,
STV_HIDDEN, STT_TLS, /*value=*/0, 0,
/*section=*/nullptr});
ElfSym::tlsModuleBase = cast<Defined>(s);
}
}
// This responsible for splitting up .eh_frame section into
// pieces. The relocation scan uses those pieces, so this has to be
// earlier.
{
llvm::TimeTraceScope timeScope("Finalize .eh_frame");
for (Partition &part : partitions)
finalizeSynthetic(part.ehFrame.get());
}
}
demoteSymbolsAndComputeIsPreemptible();
if (config->copyRelocs && config->discard != DiscardPolicy::None)
markUsedLocalSymbols<ELFT>();
demoteAndCopyLocalSymbols();
if (config->copyRelocs)
addSectionSymbols();
// Change values of linker-script-defined symbols from placeholders (assigned
// by declareSymbols) to actual definitions.
script->processSymbolAssignments();
if (!config->relocatable) {
llvm::TimeTraceScope timeScope("Scan relocations");
// Scan relocations. This must be done after every symbol is declared so
// that we can correctly decide if a dynamic relocation is needed. This is
// called after processSymbolAssignments() because it needs to know whether
// a linker-script-defined symbol is absolute.
ppc64noTocRelax.clear();
scanRelocations<ELFT>();
reportUndefinedSymbols();
postScanRelocations();
if (in.plt && in.plt->isNeeded())
in.plt->addSymbols();
if (in.iplt && in.iplt->isNeeded())
in.iplt->addSymbols();
if (config->unresolvedSymbolsInShlib != UnresolvedPolicy::Ignore) {
auto diagnose =
config->unresolvedSymbolsInShlib == UnresolvedPolicy::ReportError
? errorOrWarn
: warn;
// Error on undefined symbols in a shared object, if all of its DT_NEEDED
// entries are seen. These cases would otherwise lead to runtime errors
// reported by the dynamic linker.
//
// ld.bfd traces all DT_NEEDED to emulate the logic of the dynamic linker
// to catch more cases. That is too much for us. Our approach resembles
// the one used in ld.gold, achieves a good balance to be useful but not
// too smart.
//
// If a DSO reference is resolved by a SharedSymbol, but the SharedSymbol
// is overridden by a hidden visibility Defined (which is later discarded
// due to GC), don't report the diagnostic. However, this may indicate an
// unintended SharedSymbol.
for (SharedFile *file : ctx.sharedFiles) {
bool allNeededIsKnown =
llvm::all_of(file->dtNeeded, [&](StringRef needed) {
return symtab.soNames.count(CachedHashStringRef(needed));
});
if (!allNeededIsKnown)
continue;
for (Symbol *sym : file->requiredSymbols) {
if (sym->dsoDefined)
continue;
if (sym->isUndefined() && !sym->isWeak()) {
diagnose("undefined reference: " + toString(*sym) +
"\n>>> referenced by " + toString(file) +
" (disallowed by --no-allow-shlib-undefined)");
} else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) {
diagnose("non-exported symbol '" + toString(*sym) + "' in '" +
toString(sym->file) + "' is referenced by DSO '" +
toString(file) + "'");
}
}
}
}
}
{
llvm::TimeTraceScope timeScope("Add symbols to symtabs");
// Now that we have defined all possible global symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
for (Symbol *sym : symtab.getSymbols()) {
if (!sym->isUsedInRegularObj || !includeInSymtab(*sym))
continue;
if (!config->relocatable)
sym->binding = sym->computeBinding();
if (in.symTab)
in.symTab->addSymbol(sym);
if (sym->includeInDynsym()) {
partitions[sym->partition - 1].dynSymTab->addSymbol(sym);
if (auto *file = dyn_cast_or_null<SharedFile>(sym->file))
if (file->isNeeded && !sym->isUndefined())
addVerneed(sym);
}
}
// We also need to scan the dynamic relocation tables of the other
// partitions and add any referenced symbols to the partition's dynsym.
for (Partition &part : MutableArrayRef<Partition>(partitions).slice(1)) {
DenseSet<Symbol *> syms;
for (const SymbolTableEntry &e : part.dynSymTab->getSymbols())
syms.insert(e.sym);
for (DynamicReloc &reloc : part.relaDyn->relocs)
if (reloc.sym && reloc.needsDynSymIndex() &&
syms.insert(reloc.sym).second)
part.dynSymTab->addSymbol(reloc.sym);
}
}
if (in.mipsGot)
in.mipsGot->build();
removeUnusedSyntheticSections();
script->diagnoseOrphanHandling();
script->diagnoseMissingSGSectionAddress();
sortSections();
// Create a list of OutputSections, assign sectionIndex, and populate
// in.shStrTab.
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
OutputSection *osec = &osd->osec;
outputSections.push_back(osec);
osec->sectionIndex = outputSections.size();
osec->shName = in.shStrTab->addString(osec->name);
}
// Prefer command line supplied address over other constraints.
for (OutputSection *sec : outputSections) {
auto i = config->sectionStartMap.find(sec->name);
if (i != config->sectionStartMap.end())
sec->addrExpr = [=] { return i->second; };
}
// With the outputSections available check for GDPLT relocations
// and add __tls_get_addr symbol if needed.
if (config->emachine == EM_HEXAGON && hexagonNeedsTLSSymbol(outputSections)) {
Symbol *sym =
symtab.addSymbol(Undefined{ctx.internalFile, "__tls_get_addr",
STB_GLOBAL, STV_DEFAULT, STT_NOTYPE});
sym->isPreemptible = true;
partitions[0].dynSymTab->addSymbol(sym);
}
// This is a bit of a hack. A value of 0 means undef, so we set it
// to 1 to make __ehdr_start defined. The section number is not
// particularly relevant.
Out::elfHeader->sectionIndex = 1;
Out::elfHeader->size = sizeof(typename ELFT::Ehdr);
// Binary and relocatable output does not have PHDRS.
// The headers have to be created before finalize as that can influence the
// image base and the dynamic section on mips includes the image base.
if (!config->relocatable && !config->oFormatBinary) {
for (Partition &part : partitions) {
part.phdrs = script->hasPhdrsCommands() ? script->createPhdrs()
: createPhdrs(part);
if (config->emachine == EM_ARM) {
// PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
addPhdrForSection(part, SHT_ARM_EXIDX, PT_ARM_EXIDX, PF_R);
}
if (config->emachine == EM_MIPS) {
// Add separate segments for MIPS-specific sections.
addPhdrForSection(part, SHT_MIPS_REGINFO, PT_MIPS_REGINFO, PF_R);
addPhdrForSection(part, SHT_MIPS_OPTIONS, PT_MIPS_OPTIONS, PF_R);
addPhdrForSection(part, SHT_MIPS_ABIFLAGS, PT_MIPS_ABIFLAGS, PF_R);
}
if (config->emachine == EM_RISCV)
addPhdrForSection(part, SHT_RISCV_ATTRIBUTES, PT_RISCV_ATTRIBUTES,
PF_R);
}
Out::programHeaders->size = sizeof(Elf_Phdr) * mainPart->phdrs.size();
// Find the TLS segment. This happens before the section layout loop so that
// Android relocation packing can look up TLS symbol addresses. We only need
// to care about the main partition here because all TLS symbols were moved
// to the main partition (see MarkLive.cpp).
for (PhdrEntry *p : mainPart->phdrs)
if (p->p_type == PT_TLS)
Out::tlsPhdr = p;
}
// Some symbols are defined in term of program headers. Now that we
// have the headers, we can find out which sections they point to.
setReservedSymbolSections();
{
llvm::TimeTraceScope timeScope("Finalize synthetic sections");
finalizeSynthetic(in.bss.get());
finalizeSynthetic(in.bssRelRo.get());
finalizeSynthetic(in.symTabShndx.get());
finalizeSynthetic(in.shStrTab.get());
finalizeSynthetic(in.strTab.get());
finalizeSynthetic(in.got.get());
finalizeSynthetic(in.mipsGot.get());
finalizeSynthetic(in.igotPlt.get());
finalizeSynthetic(in.gotPlt.get());
finalizeSynthetic(in.relaPlt.get());
finalizeSynthetic(in.plt.get());
finalizeSynthetic(in.iplt.get());
finalizeSynthetic(in.ppc32Got2.get());
finalizeSynthetic(in.partIndex.get());
// Dynamic section must be the last one in this list and dynamic
// symbol table section (dynSymTab) must be the first one.
for (Partition &part : partitions) {
if (part.relaDyn) {
part.relaDyn->mergeRels();
// Compute DT_RELACOUNT to be used by part.dynamic.
part.relaDyn->partitionRels();
finalizeSynthetic(part.relaDyn.get());
}
if (part.relrDyn) {
part.relrDyn->mergeRels();
finalizeSynthetic(part.relrDyn.get());
}
if (part.relrAuthDyn) {
part.relrAuthDyn->mergeRels();
finalizeSynthetic(part.relrAuthDyn.get());
}
finalizeSynthetic(part.dynSymTab.get());
finalizeSynthetic(part.gnuHashTab.get());
finalizeSynthetic(part.hashTab.get());
finalizeSynthetic(part.verDef.get());
finalizeSynthetic(part.ehFrameHdr.get());
finalizeSynthetic(part.verSym.get());
finalizeSynthetic(part.verNeed.get());
finalizeSynthetic(part.dynamic.get());
}
}
if (!script->hasSectionsCommand && !config->relocatable)
fixSectionAlignments();
// This is used to:
// 1) Create "thunks":
// Jump instructions in many ISAs have small displacements, and therefore
// they cannot jump to arbitrary addresses in memory. For example, RISC-V
// JAL instruction can target only +-1 MiB from PC. It is a linker's
// responsibility to create and insert small pieces of code between
// sections to extend the ranges if jump targets are out of range. Such
// code pieces are called "thunks".
//
// We add thunks at this stage. We couldn't do this before this point
// because this is the earliest point where we know sizes of sections and
// their layouts (that are needed to determine if jump targets are in
// range).
//
// 2) Update the sections. We need to generate content that depends on the
// address of InputSections. For example, MIPS GOT section content or
// android packed relocations sections content.
//
// 3) Assign the final values for the linker script symbols. Linker scripts
// sometimes using forward symbol declarations. We want to set the correct
// values. They also might change after adding the thunks.
finalizeAddressDependentContent();
// All information needed for OutputSection part of Map file is available.
if (errorCount())
return;
{
llvm::TimeTraceScope timeScope("Finalize synthetic sections");
// finalizeAddressDependentContent may have added local symbols to the
// static symbol table.
finalizeSynthetic(in.symTab.get());
finalizeSynthetic(in.debugNames.get());
finalizeSynthetic(in.ppc64LongBranchTarget.get());
finalizeSynthetic(in.armCmseSGSection.get());
}
// Relaxation to delete inter-basic block jumps created by basic block
// sections. Run after in.symTab is finalized as optimizeBasicBlockJumps
// can relax jump instructions based on symbol offset.
if (config->optimizeBBJumps)
optimizeBasicBlockJumps();
// Fill other section headers. The dynamic table is finalized
// at the end because some tags like RELSZ depend on result
// of finalizing other sections.
for (OutputSection *sec : outputSections)
sec->finalize();
script->checkFinalScriptConditions();
if (config->emachine == EM_ARM && !config->isLE && config->armBe8) {
addArmInputSectionMappingSymbols();
sortArmMappingSymbols();
}
}
// Ensure data sections are not mixed with executable sections when
// --execute-only is used. --execute-only make pages executable but not
// readable.
template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
if (!config->executeOnly)
return;
SmallVector<InputSection *, 0> storage;
for (OutputSection *osec : outputSections)
if (osec->flags & SHF_EXECINSTR)
for (InputSection *isec : getInputSections(*osec, storage))
if (!(isec->flags & SHF_EXECINSTR))
error("cannot place " + toString(isec) + " into " +
toString(osec->name) +
": --execute-only does not support intermingling data and code");
}
// The linker is expected to define SECNAME_start and SECNAME_end
// symbols for a few sections. This function defines them.
template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
// If a section does not exist, there's ambiguity as to how we
// define _start and _end symbols for an init/fini section. Since
// the loader assume that the symbols are always defined, we need to
// always define them. But what value? The loader iterates over all
// pointers between _start and _end to run global ctors/dtors, so if
// the section is empty, their symbol values don't actually matter
// as long as _start and _end point to the same location.
//
// That said, we don't want to set the symbols to 0 (which is
// probably the simplest value) because that could cause some
// program to fail to link due to relocation overflow, if their
// program text is above 2 GiB. We use the address of the .text
// section instead to prevent that failure.
//
// In rare situations, the .text section may not exist. If that's the
// case, use the image base address as a last resort.
OutputSection *Default = findSection(".text");
if (!Default)
Default = Out::elfHeader;
auto define = [=](StringRef start, StringRef end, OutputSection *os) {
if (os && !script->isDiscarded(os)) {
addOptionalRegular(start, os, 0);
addOptionalRegular(end, os, -1);
} else {
addOptionalRegular(start, Default, 0);
addOptionalRegular(end, Default, 0);
}
};
define("__preinit_array_start", "__preinit_array_end", Out::preinitArray);
define("__init_array_start", "__init_array_end", Out::initArray);
define("__fini_array_start", "__fini_array_end", Out::finiArray);
if (OutputSection *sec = findSection(".ARM.exidx"))
define("__exidx_start", "__exidx_end", sec);
}
// If a section name is valid as a C identifier (which is rare because of
// the leading '.'), linkers are expected to define __start_<secname> and
// __stop_<secname> symbols. They are at beginning and end of the section,
// respectively. This is not requested by the ELF standard, but GNU ld and
// gold provide the feature, and used by many programs.
template <class ELFT>
void Writer<ELFT>::addStartStopSymbols(OutputSection &osec) {
StringRef s = osec.name;
if (!isValidCIdentifier(s))
return;
addOptionalRegular(saver().save("__start_" + s), &osec, 0,
config->zStartStopVisibility);
addOptionalRegular(saver().save("__stop_" + s), &osec, -1,
config->zStartStopVisibility);
}
static bool needsPtLoad(OutputSection *sec) {
if (!(sec->flags & SHF_ALLOC))
return false;
// Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
// responsible for allocating space for them, not the PT_LOAD that
// contains the TLS initialization image.
if ((sec->flags & SHF_TLS) && sec->type == SHT_NOBITS)
return false;
return true;
}
// Adjust phdr flags according to certain options.
static uint64_t computeFlags(uint64_t flags) {
if (config->omagic)
return PF_R | PF_W | PF_X;
if (config->executeOnly && (flags & PF_X))
return flags & ~PF_R;
return flags;
}
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT>
SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
SmallVector<PhdrEntry *, 0> ret;
auto addHdr = [&](unsigned type, unsigned flags) -> PhdrEntry * {
ret.push_back(make<PhdrEntry>(type, flags));
return ret.back();
};
unsigned partNo = part.getNumber();
bool isMain = partNo == 1;
// Add the first PT_LOAD segment for regular output sections.
uint64_t flags = computeFlags(PF_R);
PhdrEntry *load = nullptr;
// nmagic or omagic output does not have PT_PHDR, PT_INTERP, or the readonly
// PT_LOAD.
if (!config->nmagic && !config->omagic) {
// The first phdr entry is PT_PHDR which describes the program header
// itself.
if (isMain)
addHdr(PT_PHDR, PF_R)->add(Out::programHeaders);
else
addHdr(PT_PHDR, PF_R)->add(part.programHeaders->getParent());
// PT_INTERP must be the second entry if exists.
if (OutputSection *cmd = findSection(".interp", partNo))
addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd);
// Add the headers. We will remove them if they don't fit.
// In the other partitions the headers are ordinary sections, so they don't
// need to be added here.
if (isMain) {
load = addHdr(PT_LOAD, flags);
load->add(Out::elfHeader);
load->add(Out::programHeaders);
}
}
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after processing relocations.
// Current dynamic loaders only support one PT_GNU_RELRO PHDR, give
// an error message if more than one PT_GNU_RELRO PHDR is required.
PhdrEntry *relRo = make<PhdrEntry>(PT_GNU_RELRO, PF_R);
bool inRelroPhdr = false;
OutputSection *relroEnd = nullptr;
for (OutputSection *sec : outputSections) {
if (sec->partition != partNo || !needsPtLoad(sec))
continue;
if (isRelroSection(sec)) {
inRelroPhdr = true;
if (!relroEnd)
relRo->add(sec);
else
error("section: " + sec->name + " is not contiguous with other relro" +
" sections");
} else if (inRelroPhdr) {
inRelroPhdr = false;
relroEnd = sec;
}
}
relRo->p_align = 1;
for (OutputSection *sec : outputSections) {
if (!needsPtLoad(sec))
continue;
// Normally, sections in partitions other than the current partition are
// ignored. But partition number 255 is a special case: it contains the
// partition end marker (.part.end). It needs to be added to the main
// partition so that a segment is created for it in the main partition,
// which will cause the dynamic loader to reserve space for the other
// partitions.
if (sec->partition != partNo) {
if (isMain && sec->partition == 255)
addHdr(PT_LOAD, computeFlags(sec->getPhdrFlags()))->add(sec);
continue;
}
// Segments are contiguous memory regions that has the same attributes
// (e.g. executable or writable). There is one phdr for each segment.
// Therefore, we need to create a new phdr when the next section has
// incompatible flags or is loaded at a discontiguous address or memory
// region using AT or AT> linker script command, respectively.
//
// As an exception, we don't create a separate load segment for the ELF
// headers, even if the first "real" output has an AT or AT> attribute.
//
// In addition, NOBITS sections should only be placed at the end of a LOAD
// segment (since it's represented as p_filesz < p_memsz). If we have a
// not-NOBITS section after a NOBITS, we create a new LOAD for the latter
// even if flags match, so as not to require actually writing the
// supposed-to-be-NOBITS section to the output file. (However, we cannot do
// so when hasSectionsCommand, since we cannot introduce the extra alignment
// needed to create a new LOAD)
uint64_t newFlags = computeFlags(sec->getPhdrFlags());
// When --no-rosegment is specified, RO and RX sections are compatible.
uint32_t incompatible = flags ^ newFlags;
if (config->singleRoRx && !(newFlags & PF_W))
incompatible &= ~PF_X;
if (incompatible)
load = nullptr;
bool sameLMARegion =
load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
if (load && sec != relroEnd &&
sec->memRegion == load->firstSec->memRegion &&
(sameLMARegion || load->lastSec == Out::programHeaders) &&
(script->hasSectionsCommand || sec->type == SHT_NOBITS ||
load->lastSec->type != SHT_NOBITS)) {
load->p_flags |= newFlags;
} else {
load = addHdr(PT_LOAD, newFlags);
flags = newFlags;
}
load->add(sec);
}
// Add a TLS segment if any.
PhdrEntry *tlsHdr = make<PhdrEntry>(PT_TLS, PF_R);
for (OutputSection *sec : outputSections)
if (sec->partition == partNo && sec->flags & SHF_TLS)
tlsHdr->add(sec);
if (tlsHdr->firstSec)
ret.push_back(tlsHdr);
// Add an entry for .dynamic.
if (OutputSection *sec = part.dynamic->getParent())
addHdr(PT_DYNAMIC, sec->getPhdrFlags())->add(sec);
if (relRo->firstSec)
ret.push_back(relRo);
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (part.ehFrame->isNeeded() && part.ehFrameHdr &&
part.ehFrame->getParent() && part.ehFrameHdr->getParent())
addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
->add(part.ehFrameHdr->getParent());
// PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
// the dynamic linker fill the segment with random data.
if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
if (config->zGnustack != GnuStackKind::None) {
// PT_GNU_STACK is a special section to tell the loader to make the
// pages for the stack non-executable. If you really want an executable
// stack, you can pass -z execstack, but that's not recommended for
// security reasons.
unsigned perm = PF_R | PF_W;
if (config->zGnustack == GnuStackKind::Exec)
perm |= PF_X;
addHdr(PT_GNU_STACK, perm)->p_memsz = config->zStackSize;
}
// PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
// is expected to perform W^X violations, such as calling mprotect(2) or
// mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
// OpenBSD.
if (config->zWxneeded)
addHdr(PT_OPENBSD_WXNEEDED, PF_X);
if (OutputSection *cmd = findSection(".note.gnu.property", partNo))
addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd);
// Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the
// same alignment.
PhdrEntry *note = nullptr;
for (OutputSection *sec : outputSections) {
if (sec->partition != partNo)
continue;
if (sec->type == SHT_NOTE && (sec->flags & SHF_ALLOC)) {
if (!note || sec->lmaExpr || note->lastSec->addralign != sec->addralign)
note = addHdr(PT_NOTE, PF_R);
note->add(sec);
} else {
note = nullptr;
}
}
return ret;
}
template <class ELFT>
void Writer<ELFT>::addPhdrForSection(Partition &part, unsigned shType,
unsigned pType, unsigned pFlags) {
unsigned partNo = part.getNumber();
auto i = llvm::find_if(outputSections, [=](OutputSection *cmd) {
return cmd->partition == partNo && cmd->type == shType;
});
if (i == outputSections.end())
return;
PhdrEntry *entry = make<PhdrEntry>(pType, pFlags);
entry->add(*i);
part.phdrs.push_back(entry);
}
// Place the first section of each PT_LOAD to a different page (of maxPageSize).
// This is achieved by assigning an alignment expression to addrExpr of each
// such section.
template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
const PhdrEntry *prev;
auto pageAlign = [&](const PhdrEntry *p) {
OutputSection *cmd = p->firstSec;
if (!cmd)
return;
cmd->alignExpr = [align = cmd->addralign]() { return align; };
if (!cmd->addrExpr) {
// Prefer advancing to align(dot, maxPageSize) + dot%maxPageSize to avoid
// padding in the file contents.
//
// When -z separate-code is used we must not have any overlap in pages
// between an executable segment and a non-executable segment. We align to
// the next maximum page size boundary on transitions between executable
// and non-executable segments.
//
// SHT_LLVM_PART_EHDR marks the start of a partition. The partition
// sections will be extracted to a separate file. Align to the next
// maximum page size boundary so that we can find the ELF header at the
// start. We cannot benefit from overlapping p_offset ranges with the
// previous segment anyway.
if (config->zSeparate == SeparateSegmentKind::Loadable ||
(config->zSeparate == SeparateSegmentKind::Code && prev &&
(prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
cmd->type == SHT_LLVM_PART_EHDR)
cmd->addrExpr = [] {
return alignToPowerOf2(script->getDot(), config->maxPageSize);
};
// PT_TLS is at the start of the first RW PT_LOAD. If `p` includes PT_TLS,
// it must be the RW. Align to p_align(PT_TLS) to make sure
// p_vaddr(PT_LOAD)%p_align(PT_LOAD) = 0. Otherwise, if
// sh_addralign(.tdata) < sh_addralign(.tbss), we will set p_align(PT_TLS)
// to sh_addralign(.tbss), while p_vaddr(PT_TLS)=p_vaddr(PT_LOAD) may not
// be congruent to 0 modulo p_align(PT_TLS).
//
// Technically this is not required, but as of 2019, some dynamic loaders
// don't handle p_vaddr%p_align != 0 correctly, e.g. glibc (i386 and
// x86-64) doesn't make runtime address congruent to p_vaddr modulo
// p_align for dynamic TLS blocks (PR/24606), FreeBSD rtld has the same
// bug, musl (TLS Variant 1 architectures) before 1.1.23 handled TLS
// blocks correctly. We need to keep the workaround for a while.
else if (Out::tlsPhdr && Out::tlsPhdr->firstSec == p->firstSec)
cmd->addrExpr = [] {
return alignToPowerOf2(script->getDot(), config->maxPageSize) +
alignToPowerOf2(script->getDot() % config->maxPageSize,
Out::tlsPhdr->p_align);
};
else
cmd->addrExpr = [] {
return alignToPowerOf2(script->getDot(), config->maxPageSize) +
script->getDot() % config->maxPageSize;
};
}
};
for (Partition &part : partitions) {
prev = nullptr;
for (const PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && p->firstSec) {
pageAlign(p);
prev = p;
}
}
}
// Compute an in-file position for a given section. The file offset must be the
// same with its virtual address modulo the page size, so that the loader can
// load executables without any address adjustment.
static uint64_t computeFileOffset(OutputSection *os, uint64_t off) {
// The first section in a PT_LOAD has to have congruent offset and address
// modulo the maximum page size.
if (os->ptLoad && os->ptLoad->firstSec == os)
return alignTo(off, os->ptLoad->p_align, os->addr);
// File offsets are not significant for .bss sections other than the first one
// in a PT_LOAD/PT_TLS. By convention, we keep section offsets monotonically
// increasing rather than setting to zero.
if (os->type == SHT_NOBITS &&
(!Out::tlsPhdr || Out::tlsPhdr->firstSec != os))
return off;
// If the section is not in a PT_LOAD, we just have to align it.
if (!os->ptLoad)
return alignToPowerOf2(off, os->addralign);
// If two sections share the same PT_LOAD the file offset is calculated
// using this formula: Off2 = Off1 + (VA2 - VA1).
OutputSection *first = os->ptLoad->firstSec;
return first->offset + os->addr - first->addr;
}
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
// Compute the minimum LMA of all non-empty non-NOBITS sections as minAddr.
auto needsOffset = [](OutputSection &sec) {
return sec.type != SHT_NOBITS && (sec.flags & SHF_ALLOC) && sec.size > 0;
};
uint64_t minAddr = UINT64_MAX;
for (OutputSection *sec : outputSections)
if (needsOffset(*sec)) {
sec->offset = sec->getLMA();
minAddr = std::min(minAddr, sec->offset);
}
// Sections are laid out at LMA minus minAddr.
fileSize = 0;
for (OutputSection *sec : outputSections)
if (needsOffset(*sec)) {
sec->offset -= minAddr;
fileSize = std::max(fileSize, sec->offset + sec->size);
}
}
static std::string rangeToString(uint64_t addr, uint64_t len) {
return "[0x" + utohexstr(addr) + ", 0x" + utohexstr(addr + len - 1) + "]";
}
// Assign file offsets to output sections.
template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
Out::programHeaders->offset = Out::elfHeader->size;
uint64_t off = Out::elfHeader->size + Out::programHeaders->size;
PhdrEntry *lastRX = nullptr;
for (Partition &part : partitions)
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
lastRX = p;
// Layout SHF_ALLOC sections before non-SHF_ALLOC sections. A non-SHF_ALLOC
// will not occupy file offsets contained by a PT_LOAD.
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_ALLOC))
continue;
off = computeFileOffset(sec, off);
sec->offset = off;
if (sec->type != SHT_NOBITS)
off += sec->size;
// If this is a last section of the last executable segment and that
// segment is the last loadable segment, align the offset of the
// following section to avoid loading non-segments parts of the file.
if (config->zSeparate != SeparateSegmentKind::None && lastRX &&
lastRX->lastSec == sec)
off = alignToPowerOf2(off, config->maxPageSize);
}
for (OutputSection *osec : outputSections)
if (!(osec->flags & SHF_ALLOC)) {
osec->offset = alignToPowerOf2(off, osec->addralign);
off = osec->offset + osec->size;
}
sectionHeaderOff = alignToPowerOf2(off, config->wordsize);
fileSize = sectionHeaderOff + (outputSections.size() + 1) * sizeof(Elf_Shdr);
// Our logic assumes that sections have rising VA within the same segment.
// With use of linker scripts it is possible to violate this rule and get file
// offset overlaps or overflows. That should never happen with a valid script
// which does not move the location counter backwards and usually scripts do
// not do that. Unfortunately, there are apps in the wild, for example, Linux
// kernel, which control segment distribution explicitly and move the counter
// backwards, so we have to allow doing that to support linking them. We
// perform non-critical checks for overlaps in checkSectionOverlap(), but here
// we want to prevent file size overflows because it would crash the linker.
for (OutputSection *sec : outputSections) {
if (sec->type == SHT_NOBITS)
continue;
if ((sec->offset > fileSize) || (sec->offset + sec->size > fileSize))
error("unable to place section " + sec->name + " at file offset " +
rangeToString(sec->offset, sec->size) +
"; check your linker script for overflows");
}
}
// Finalize the program headers. We call this function after we assign
// file offsets and VAs to all sections.
template <class ELFT> void Writer<ELFT>::setPhdrs(Partition &part) {
for (PhdrEntry *p : part.phdrs) {
OutputSection *first = p->firstSec;
OutputSection *last = p->lastSec;
// .ARM.exidx sections may not be within a single .ARM.exidx
// output section. We always want to describe just the
// SyntheticSection.
if (part.armExidx && p->p_type == PT_ARM_EXIDX) {
p->p_filesz = part.armExidx->getSize();
p->p_memsz = part.armExidx->getSize();
p->p_offset = first->offset + part.armExidx->outSecOff;
p->p_vaddr = first->addr + part.armExidx->outSecOff;
p->p_align = part.armExidx->addralign;
if (part.elfHeader)
p->p_offset -= part.elfHeader->getParent()->offset;
if (!p->hasLMA)
p->p_paddr = first->getLMA() + part.armExidx->outSecOff;
return;
}
if (first) {
p->p_filesz = last->offset - first->offset;
if (last->type != SHT_NOBITS)
p->p_filesz += last->size;
p->p_memsz = last->addr + last->size - first->addr;
p->p_offset = first->offset;
p->p_vaddr = first->addr;
// File offsets in partitions other than the main partition are relative
// to the offset of the ELF headers. Perform that adjustment now.
if (part.elfHeader)
p->p_offset -= part.elfHeader->getParent()->offset;
if (!p->hasLMA)
p->p_paddr = first->getLMA();
}
}
}
// A helper struct for checkSectionOverlap.
namespace {
struct SectionOffset {
OutputSection *sec;
uint64_t offset;
};
} // namespace
// Check whether sections overlap for a specific address range (file offsets,
// load and virtual addresses).
static void checkOverlap(StringRef name, std::vector<SectionOffset> &sections,
bool isVirtualAddr) {
llvm::sort(sections, [=](const SectionOffset &a, const SectionOffset &b) {
return a.offset < b.offset;
});
// Finding overlap is easy given a vector is sorted by start position.
// If an element starts before the end of the previous element, they overlap.
for (size_t i = 1, end = sections.size(); i < end; ++i) {
SectionOffset a = sections[i - 1];
SectionOffset b = sections[i];
if (b.offset >= a.offset + a.sec->size)
continue;
// If both sections are in OVERLAY we allow the overlapping of virtual
// addresses, because it is what OVERLAY was designed for.
if (isVirtualAddr && a.sec->inOverlay && b.sec->inOverlay)
continue;
errorOrWarn("section " + a.sec->name + " " + name +
" range overlaps with " + b.sec->name + "\n>>> " + a.sec->name +
" range is " + rangeToString(a.offset, a.sec->size) + "\n>>> " +
b.sec->name + " range is " +
rangeToString(b.offset, b.sec->size));
}
}
// Check for overlapping sections and address overflows.
//
// In this function we check that none of the output sections have overlapping
// file offsets. For SHF_ALLOC sections we also check that the load address
// ranges and the virtual address ranges don't overlap
template <class ELFT> void Writer<ELFT>::checkSections() {
// First, check that section's VAs fit in available address space for target.
for (OutputSection *os : outputSections)
if ((os->addr + os->size < os->addr) ||
(!ELFT::Is64Bits && os->addr + os->size > uint64_t(UINT32_MAX) + 1))
errorOrWarn("section " + os->name + " at 0x" + utohexstr(os->addr) +
" of size 0x" + utohexstr(os->size) +
" exceeds available address space");
// Check for overlapping file offsets. In this case we need to skip any
// section marked as SHT_NOBITS. These sections don't actually occupy space in
// the file so Sec->Offset + Sec->Size can overlap with others. If --oformat
// binary is specified only add SHF_ALLOC sections are added to the output
// file so we skip any non-allocated sections in that case.
std::vector<SectionOffset> fileOffs;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && sec->type != SHT_NOBITS &&
(!config->oFormatBinary || (sec->flags & SHF_ALLOC)))
fileOffs.push_back({sec, sec->offset});
checkOverlap("file", fileOffs, false);
// When linking with -r there is no need to check for overlapping virtual/load
// addresses since those addresses will only be assigned when the final
// executable/shared object is created.
if (config->relocatable)
return;
// Checking for overlapping virtual and load addresses only needs to take
// into account SHF_ALLOC sections since others will not be loaded.
// Furthermore, we also need to skip SHF_TLS sections since these will be
// mapped to other addresses at runtime and can therefore have overlapping
// ranges in the file.
std::vector<SectionOffset> vmas;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
vmas.push_back({sec, sec->addr});
checkOverlap("virtual address", vmas, true);
// Finally, check that the load addresses don't overlap. This will usually be
// the same as the virtual addresses but can be different when using a linker
// script with AT().
std::vector<SectionOffset> lmas;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
lmas.push_back({sec, sec->getLMA()});
checkOverlap("load address", lmas, false);
}
// The entry point address is chosen in the following ways.
//
// 1. the '-e' entry command-line option;
// 2. the ENTRY(symbol) command in a linker control script;
// 3. the value of the symbol _start, if present;
// 4. the number represented by the entry symbol, if it is a number;
// 5. the address 0.
static uint64_t getEntryAddr() {
// Case 1, 2 or 3
if (Symbol *b = symtab.find(config->entry))
return b->getVA();
// Case 4
uint64_t addr;
if (to_integer(config->entry, addr))
return addr;
// Case 5
if (config->warnMissingEntry)
warn("cannot find entry symbol " + config->entry +
"; not setting start address");
return 0;
}
static uint16_t getELFType() {
if (config->isPic)
return ET_DYN;
if (config->relocatable)
return ET_REL;
return ET_EXEC;
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
writeEhdr<ELFT>(Out::bufferStart, *mainPart);
writePhdrs<ELFT>(Out::bufferStart + sizeof(Elf_Ehdr), *mainPart);
auto *eHdr = reinterpret_cast<Elf_Ehdr *>(Out::bufferStart);
eHdr->e_type = getELFType();
eHdr->e_entry = getEntryAddr();
eHdr->e_shoff = sectionHeaderOff;
// Write the section header table.
//
// The ELF header can only store numbers up to SHN_LORESERVE in the e_shnum
// and e_shstrndx fields. When the value of one of these fields exceeds
// SHN_LORESERVE ELF requires us to put sentinel values in the ELF header and
// use fields in the section header at index 0 to store
// the value. The sentinel values and fields are:
// e_shnum = 0, SHdrs[0].sh_size = number of sections.
// e_shstrndx = SHN_XINDEX, SHdrs[0].sh_link = .shstrtab section index.
auto *sHdrs = reinterpret_cast<Elf_Shdr *>(Out::bufferStart + eHdr->e_shoff);
size_t num = outputSections.size() + 1;
if (num >= SHN_LORESERVE)
sHdrs->sh_size = num;
else
eHdr->e_shnum = num;
uint32_t strTabIndex = in.shStrTab->getParent()->sectionIndex;
if (strTabIndex >= SHN_LORESERVE) {
sHdrs->sh_link = strTabIndex;
eHdr->e_shstrndx = SHN_XINDEX;
} else {
eHdr->e_shstrndx = strTabIndex;
}
for (OutputSection *sec : outputSections)
sec->writeHeaderTo<ELFT>(++sHdrs);
}
// Open a result file.
template <class ELFT> void Writer<ELFT>::openFile() {
uint64_t maxSize = config->is64 ? INT64_MAX : UINT32_MAX;
if (fileSize != size_t(fileSize) || maxSize < fileSize) {
std::string msg;
raw_string_ostream s(msg);
s << "output file too large: " << Twine(fileSize) << " bytes\n"
<< "section sizes:\n";
for (OutputSection *os : outputSections)
s << os->name << ' ' << os->size << "\n";
error(s.str());
return;
}
unlinkAsync(config->outputFile);
unsigned flags = 0;
if (!config->relocatable)
flags |= FileOutputBuffer::F_executable;
if (!config->mmapOutputFile)
flags |= FileOutputBuffer::F_no_mmap;
Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
FileOutputBuffer::create(config->outputFile, fileSize, flags);
if (!bufferOrErr) {
error("failed to open " + config->outputFile + ": " +
llvm::toString(bufferOrErr.takeError()));
return;
}
buffer = std::move(*bufferOrErr);
Out::bufferStart = buffer->getBufferStart();
}
template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (sec->flags & SHF_ALLOC)
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
static void fillTrap(uint8_t *i, uint8_t *end) {
for (; i + 4 <= end; i += 4)
memcpy(i, &target->trapInstr, 4);
}
// Fill the last page of executable segments with trap instructions
// instead of leaving them as zero. Even though it is not required by any
// standard, it is in general a good thing to do for security reasons.
//
// We'll leave other pages in segments as-is because the rest will be
// overwritten by output sections.
template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
for (Partition &part : partitions) {
// Fill the last page.
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
fillTrap(Out::bufferStart +
alignDown(p->firstSec->offset + p->p_filesz, 4),
Out::bufferStart +
alignToPowerOf2(p->firstSec->offset + p->p_filesz,
config->maxPageSize));
// Round up the file size of the last segment to the page boundary iff it is
// an executable segment to ensure that other tools don't accidentally
// trim the instruction padding (e.g. when stripping the file).
PhdrEntry *last = nullptr;
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD)
last = p;
if (last && (last->p_flags & PF_X))
last->p_memsz = last->p_filesz =
alignToPowerOf2(last->p_filesz, config->maxPageSize);
}
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
llvm::TimeTraceScope timeScope("Write sections");
{
// In -r or --emit-relocs mode, write the relocation sections first as in
// ELf_Rel targets we might find out that we need to modify the relocated
// section while doing it.
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (isStaticRelSecType(sec->type))
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
{
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (!isStaticRelSecType(sec->type))
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
// Finally, check that all dynamic relocation addends were written correctly.
if (config->checkDynamicRelocs && config->writeAddends) {
for (OutputSection *sec : outputSections)
if (isStaticRelSecType(sec->type))
sec->checkDynRelAddends(Out::bufferStart);
}
}
// Computes a hash value of Data using a given hash function.
// In order to utilize multiple cores, we first split data into 1MB
// chunks, compute a hash for each chunk, and then compute a hash value
// of the hash values.
static void
computeHash(llvm::MutableArrayRef<uint8_t> hashBuf,
llvm::ArrayRef<uint8_t> data,
std::function<void(uint8_t *dest, ArrayRef<uint8_t> arr)> hashFn) {
std::vector<ArrayRef<uint8_t>> chunks = split(data, 1024 * 1024);
const size_t hashesSize = chunks.size() * hashBuf.size();
std::unique_ptr<uint8_t[]> hashes(new uint8_t[hashesSize]);
// Compute hash values.
parallelFor(0, chunks.size(), [&](size_t i) {
hashFn(hashes.get() + i * hashBuf.size(), chunks[i]);
});
// Write to the final output buffer.
hashFn(hashBuf.data(), ArrayRef(hashes.get(), hashesSize));
}
template <class ELFT> void Writer<ELFT>::writeBuildId() {
if (!mainPart->buildId || !mainPart->buildId->getParent())
return;
if (config->buildId == BuildIdKind::Hexstring) {
for (Partition &part : partitions)
part.buildId->writeBuildId(config->buildIdVector);
return;
}
// Compute a hash of all sections of the output file.
size_t hashSize = mainPart->buildId->hashSize;
std::unique_ptr<uint8_t[]> buildId(new uint8_t[hashSize]);
MutableArrayRef<uint8_t> output(buildId.get(), hashSize);
llvm::ArrayRef<uint8_t> input{Out::bufferStart, size_t(fileSize)};
// Fedora introduced build ID as "approximation of true uniqueness across all
// binaries that might be used by overlapping sets of people". It does not
// need some security goals that some hash algorithms strive to provide, e.g.
// (second-)preimage and collision resistance. In practice people use 'md5'
// and 'sha1' just for different lengths. Implement them with the more
// efficient BLAKE3.
switch (config->buildId) {
case BuildIdKind::Fast:
computeHash(output, input, [](uint8_t *dest, ArrayRef<uint8_t> arr) {
write64le(dest, xxh3_64bits(arr));
});
break;
case BuildIdKind::Md5:
computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, BLAKE3::hash<16>(arr).data(), hashSize);
});
break;
case BuildIdKind::Sha1:
computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, BLAKE3::hash<20>(arr).data(), hashSize);
});
break;
case BuildIdKind::Uuid:
if (auto ec = llvm::getRandomBytes(buildId.get(), hashSize))
error("entropy source failure: " + ec.message());
break;
default:
llvm_unreachable("unknown BuildIdKind");
}
for (Partition &part : partitions)
part.buildId->writeBuildId(output);
}
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();
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