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[LLD] Make lld define __eh_frame_* symbols #162638

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[LLD] Make lld define __eh_frame_* symbols #162638

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6 changes: 6 additions & 0 deletions lld/ELF/Writer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2269,6 +2269,12 @@ template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
define("__preinit_array_start", "__preinit_array_end", ctx.out.preinitArray);
define("__init_array_start", "__init_array_end", ctx.out.initArray);
define("__fini_array_start", "__fini_array_end", ctx.out.finiArray);
// Define __eh_frame_* symbols, libunwind needs these symbols.
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@smithp35 smithp35 Oct 9, 2025 •
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The __eh_frame_* symbols are equivalent to __exidx_start and __exidx_end so this seems reasonable to me.

llvm libunwind reference with suggested linker script fragment (https://github.com/llvm/llvm-project/blob/main/libunwind/src/AddressSpace.hpp#L75)

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yes, with this patch we won't have to use that fragment just to provide these symbols

IIUC __exidx_start/end are ARM's version of __eh_frame_start/end, right?
does lld define them?

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Yes, just below on line 2281. I think I added that before we had good linker script support, could also be that some unwinders even on Linux might use those symbols.

// Define them only if the corresponding output section exists.
if (OutputSection *sec = findSection(ctx, ".eh_frame"))
define("__eh_frame_start", "__eh_frame_end", sec);
if (OutputSection *sec = findSection(ctx, ".eh_frame_hdr"))
define("__eh_frame_hdr_start", "__eh_frame_hdr_end", sec);

// As a special case, don't unnecessarily retain .ARM.exidx, which would
// create an empty PT_ARM_EXIDX.
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25 changes: 25 additions & 0 deletions lld/test/ELF/eh-frame-syms.s
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@@ -0,0 +1,25 @@
## Verify that lld defines __eh_frame_start/end and __eh_frame_hdr_start/end symbols

# REQUIRES: x86

# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
# RUN: ld.lld %t.o -eh-frame-hdr -o %t
# RUN: llvm-readelf -s %t | FileCheck %s --check-prefix=SYM

# SYM: __eh_frame_start
# SYM: __eh_frame_end
# SYM: __eh_frame_hdr_start
# SYM: __eh_frame_hdr_end

.text
.globl _start
.type _start, @function
_start:
nop

# Emit references so the link will fail if these are not defined
check_symbol:
.quad __eh_frame_start
.quad __eh_frame_end
.quad __eh_frame_hdr_start
.quad __eh_frame_hdr_end