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ThinLTOBitcodeWriter: Emit __cfi_check to full LTO part of bitcode file.

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The CrossDSOCFI pass runs on the full LTO module and fills in the
body of __cfi_check. This function must have the correct attributes in
order to be compatible with the rest of the program. For example, when
building with -mbranch-protection=standard, the function must have the
branch-target-enforcement attribute, which is normally added by Clang.
When __cfi_check is missing, CrossDSOCFI will give it the default set
of attributes, which are likely incorrect. Therefore, emit __cfi_check
to the full LTO part, where CrossDSOCFI will see it.

Reviewers: efriedma-quic, vitalybuka, fmayer

Reviewed By: efriedma-quic

Pull Request: https://github.com/llvm/llvm-project/pull/154833
This commit is contained in:
Peter Collingbourne 2025-08-21 16:31:32 -07:00 committed by GitHub
parent f961b61f88
commit ff85dbdf6b
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GPG Key ID: B5690EEEBB952194
2 changed files with 30 additions and 1 deletions
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12
llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
View File

@ -350,12 +350,20 @@ void splitAndWriteThinLTOBitcode(
});
}
auto MustEmitToMergedModule = [](const GlobalValue *GV) {
// The __cfi_check definition is filled in by the CrossDSOCFI pass which
// runs only in the merged module.
return GV->getName() == "__cfi_check";
};
ValueToValueMapTy VMap;
std::unique_ptr<Module> MergedM(
CloneModule(M, VMap, [&](const GlobalValue *GV) -> bool {
if (const auto *C = GV->getComdat())
if (MergedMComdats.count(C))
return true;
if (MustEmitToMergedModule(GV))
return true;
if (auto *F = dyn_cast<Function>(GV))
return EligibleVirtualFns.count(F);
if (auto *GVar =
@ -372,7 +380,7 @@ void splitAndWriteThinLTOBitcode(
cloneUsedGlobalVariables(M, *MergedM, /*CompilerUsed*/ true);
for (Function &F : *MergedM)
if (!F.isDeclaration()) {
if (!F.isDeclaration() && !MustEmitToMergedModule(&F)) {
// Reset the linkage of all functions eligible for virtual constant
// propagation. The canonical definitions live in the thin LTO module so
// that they can be imported.
@ -398,6 +406,8 @@ void splitAndWriteThinLTOBitcode(
if (const auto *C = GV->getComdat())
if (MergedMComdats.count(C))
return false;
if (MustEmitToMergedModule(GV))
return false;
return true;
});

19
llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-check.ll Normal file
View File

@ -0,0 +1,19 @@
; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
; Check that __cfi_check is emitted on the full LTO side with
; attributes preserved.
; M0: define void @f()
define void @f() !type !{!"f1", i32 0} {
ret void
}
; M1: define void @__cfi_check() #0
define void @__cfi_check() #0 {
ret void
}
; M1: attributes #0 = { "branch-target-enforcement" }
attributes #0 = { "branch-target-enforcement" }