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[BOLT] Emit .eh_frame_hdr before .eh_frame to prevent table scans from libdw (#201917)

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resolves https://github.com/llvm/llvm-project/issues/201701 by moving
the `.eh_frame_hdr` section-header entry before `.eh_frame` before
assigning final section indices.

This lets BOLT binaries work with existing [libdw library logic
expecting the index before the
table](67199e1c97/libdw/dwarf_getcfi_elf.c (L301)),
preventing linear `.eh_frame` scanning.
This commit is contained in:
Daniel Hill 2026-06-22 17:15:51 -04:00 committed by GitHub
parent 33e8502336
commit ef752a0780
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GPG key ID: B5690EEEBB952194
2 changed files with 26 additions and 1 deletions
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16
bolt/lib/Rewrite/RewriteInstance.cpp
View file

@ -5104,6 +5104,22 @@ RewriteInstance::getOutputSections(ELFObjectFile<ELFT> *File,
addSection(NewSection, Section);
}
// Some consumers, including elfutils/libdw, stop scanning section headers
// once they find .eh_frame and only use .eh_frame_hdr if it appeared first.
// Keep layout-driven ordering for size calculations above, but preserve the
// conventional section-header order before assigning final indices.
auto HasOutputName = [](StringRef Name) {
return [Name](const auto &SectionKV) {
return SectionKV.first && SectionKV.first->getOutputName() == Name;
};
};
auto EHFrameHdrIt =
llvm::find_if(OutputSections, HasOutputName(getEHFrameHdrSectionName()));
auto EHFrameIt = llvm::find_if(OutputSections, HasOutputName(".eh_frame"));
if (EHFrameHdrIt != OutputSections.end() &&
EHFrameIt != OutputSections.end() && EHFrameIt < EHFrameHdrIt)
std::rotate(EHFrameIt, EHFrameHdrIt, std::next(EHFrameHdrIt));
// Assign indices to sections.
for (uint32_t Index = 1; Index < OutputSections.size(); ++Index)
OutputSections[Index].first->setIndex(Index);

11
bolt/test/eh-frame-hdr.test
View file

@ -1,4 +1,6 @@
# Check that llvm-bolt overwrites .eh_frame_hdr in-place.
# Check that llvm-bolt overwrites .eh_frame_hdr in-place and keeps the
# .eh_frame_hdr section header before .eh_frame when it has to create a new
# header.
REQUIRES: system-linux
@ -6,7 +8,14 @@ RUN: %clang %cflags %p/Inputs/hello.c -o %t -Wl,-q
RUN: llvm-bolt %t -o %t.bolt --use-old-text \
RUN: | FileCheck %s --check-prefix=CHECK-BOLT
RUN: llvm-readelf -WS %t.bolt | FileCheck %s
RUN: touch %t.empty
RUN: llvm-objcopy --update-section .eh_frame_hdr=%t.empty %t %t.small-hdr
RUN: llvm-bolt %t.small-hdr -o %t.small-hdr.bolt
RUN: llvm-readelf -WS %t.small-hdr.bolt | FileCheck %s --check-prefix=ORDER
CHECK-BOLT: rewriting .eh_frame_hdr in-place
CHECK-NOT: .bolt.org.eh_frame_hdr
ORDER: ] .eh_frame_hdr
ORDER-NEXT: ] .eh_frame