[llvm-objdump] Create name for fake sections

It doesn't have a section header string table so add a vector to have
the strings and create name based on the program header type and the
index.

Differential Revision: https://reviews.llvm.org/D131290

llvm/include/llvm/Object/ELF.h

@@ -14,6 +14,7 @@
 #define LLVM_OBJECT_ELF_H
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/BinaryFormat/ELF.h"
@@ -182,6 +183,7 @@ public:
 private:
   StringRef Buf;
   std::vector<Elf_Shdr> FakeSections;
+  SmallString<0> FakeSectionStrings;
 
   ELFFile(StringRef Object);
 
@@ -648,8 +650,11 @@ ELFFile<ELFT>::getSectionStringTable(Elf_Shdr_Range Sections,
     Index = Sections[0].sh_link;
   }
 
-  if (!Index) // no section string table.
-    return "";
+  // There is no section name string table. Return FakeSectionStrings which
+  // is non-empty if we have created fake sections.
+  if (!Index)
+    return FakeSectionStrings;
+
   if (Index >= Sections.size())
     return createError("section header string table index " + Twine(Index) +
                        " does not exist");
@@ -770,8 +775,9 @@ template <class ELFT> void ELFFile<ELFT>::createFakeSections() {
   if (!PhdrsOrErr)
     return;
 
-  for (auto Phdr : *PhdrsOrErr) {
-    if (!(Phdr.p_type & ELF::PT_LOAD) || !(Phdr.p_flags & ELF::PF_X))
+  FakeSectionStrings += '\0';
+  for (auto [Idx, Phdr] : llvm::enumerate(*PhdrsOrErr)) {
+    if (Phdr.p_type != ELF::PT_LOAD || !(Phdr.p_flags & ELF::PF_X))
       continue;
     Elf_Shdr FakeShdr = {};
     FakeShdr.sh_type = ELF::SHT_PROGBITS;
@@ -779,6 +785,10 @@ template <class ELFT> void ELFFile<ELFT>::createFakeSections() {
     FakeShdr.sh_addr = Phdr.p_vaddr;
     FakeShdr.sh_size = Phdr.p_memsz;
     FakeShdr.sh_offset = Phdr.p_offset;
+    // Create a section name based on the p_type and index.
+    FakeShdr.sh_name = FakeSectionStrings.size();
+    FakeSectionStrings += ("PT_LOAD#" + Twine(Idx)).str();
+    FakeSectionStrings += '\0';
     FakeSections.push_back(FakeShdr);
   }
 }

llvm/test/Object/objdump-no-sectionheaders.test

@@ -1,8 +1,59 @@
-; RUN: llvm-objdump -h %p/Inputs/no-sections.elf-x86-64 \
-; RUN:              | FileCheck %s
+## Check llvm-objdump -h can handle ELF files without section info.
+## Only PT_LOAD segments with the PF_X flag will be displayed as fake sections.
 
-; CHECK:      Sections:
-; CHECK-NEXT: Idx Name          Size     VMA              Type
-; CHECK-NEXT:   0               000006ec 0000000000400000 TEXT
-; CHECK-NEXT:   1               00000000 0000000000000000 TEXT
-; CHECK-NOT:  {{.}}
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-objdump -h %t | FileCheck %s
+
+# CHECK:      Sections:
+# CHECK-NEXT: Idx Name          Size     VMA              Type
+# CHECK-NEXT:   0 PT_LOAD#1     00000100 0000000000400000 TEXT
+# CHECK-NEXT:   1 PT_LOAD#3     00000200 0000000000600400 TEXT
+# CHECK-NOT:  {{.}}
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_CORE
+  Machine:         EM_X86_64
+Sections:
+  - Type:            SectionHeaderTable
+    NoHeaders:       true
+  - Type:            Fill
+    Name:            code1
+    Pattern:         "cc"
+    Size:            0x100
+    Offset:          0x200
+  - Type:            Fill
+    Name:            data1
+    Pattern:         "aa55"
+    Size:            0x100
+    Offset:          0x300
+  - Type:            Fill
+    Name:            code2
+    Pattern:         "ff"
+    Size:            0x200
+    Offset:          0x400
+ProgramHeaders:
+  - Type:            PT_PHDR
+    Flags:           [ PF_X ]
+    VAddr:           0x400000
+    MemSize:         0x100
+  - Type:            PT_LOAD
+    Flags:           [ PF_X ]
+    VAddr:           0x400000
+    MemSize:         0x100
+    FirstSec:        code1
+    LastSec:         code1
+  - Type:            PT_LOAD
+    Flags:           [ PF_R ]
+    VAddr:           0x500300
+    MemSize:         0x100
+    FirstSec:        data1
+    LastSec:         data1
+  - Type:            PT_LOAD
+    Flags:           [ PF_R, PF_X ]
+    VAddr:           0x600400
+    MemSize:         0x200
+    FirstSec:        code2
+    LastSec:         code2

llvm/test/tools/llvm-objdump/X86/disassemble-no-section.test

@@ -6,9 +6,9 @@
 # RUN: yaml2obj %s -o %t
 # RUN: llvm-objdump -d %t | FileCheck %s
 
-# CHECK:       Disassembly of section :
+# CHECK:       Disassembly of section PT_LOAD#0:
 # CHECK-EMPTY:
-# CHECK-NEXT:  <>:
+# CHECK-NEXT:  <PT_LOAD#0>:
 # CHECK-NEXT:  55                    pushq   %rbp
 # CHECK-NEXT:  48 89 e5              movq    %rsp, %rbp
 # CHECK-NEXT:  0f 1f 40 00           nopl    (%rax)
@@ -22,9 +22,9 @@
 
 # RANGE:       no section overlaps the range
 # RANGE-EMPTY:
-# RANGE-NEXT:  Disassembly of section :
+# RANGE-NEXT:  Disassembly of section PT_LOAD#0:
 # RANGE-EMPTY:
-# RANGE-NEXT:  <>:
+# RANGE-NEXT:  <PT_LOAD#0>:
 # RANGE-NEXT:  55                    pushq   %rbp
 # RANGE-NEXT:  48 89 e5              movq    %rsp, %rbp
 # RANGE-EMPTY:

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -2166,6 +2166,9 @@ static size_t getMaxSectionNameWidth(const ObjectFile &Obj) {
 }
 
 void objdump::printSectionHeaders(ObjectFile &Obj) {
+  if (Obj.isELF() && Obj.sections().empty())
+    createFakeELFSections(Obj);
+
   size_t NameWidth = getMaxSectionNameWidth(Obj);
   size_t AddressWidth = 2 * Obj.getBytesInAddress();
   bool HasLMAColumn = shouldDisplayLMA(Obj);
@@ -2178,9 +2181,6 @@ void objdump::printSectionHeaders(ObjectFile &Obj) {
     outs() << "Idx " << left_justify("Name", NameWidth) << " Size     "
            << left_justify("VMA", AddressWidth) << " Type\n";
 
-  if (Obj.isELF() && Obj.sections().empty())
-    createFakeELFSections(Obj);
-
   uint64_t Idx;
   for (const SectionRef &Section : ToolSectionFilter(Obj, &Idx)) {
     StringRef Name = unwrapOrError(Section.getName(), Obj.getFileName());