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[ELF] Don't resolve relocations referencing SHN_ABS to tombstone in non-SHF_ALLOC sections #79238

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Merged
merged 3 commits into from
Jan 24, 2024
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[ELF] Don't resolve relocations referencing SHN_ABS to tombstone in non-SHF_ALLOC sections #79238

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97b500a
[𝘀𝗽𝗿] initial version
MaskRay Jan 24, 2024
5296604
edit description, include C example
MaskRay Jan 24, 2024
14fab61
fix typo. clarify .debug_addr
MaskRay Jan 24, 2024
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13 changes: 6 additions & 7 deletions lld/ELF/InputSection.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -961,20 +961,19 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) {
// vector. The computed value is st_value plus a non-negative offset.
// Negative values are invalid, so -1 can be used as the tombstone value.
//
// If the referenced symbol is discarded (made Undefined), or the
// section defining the referenced symbol is garbage collected,
// sym.getOutputSection() is nullptr. `ds->folded` catches the ICF folded
// case. However, resolving a relocation in .debug_line to -1 would stop
// debugger users from setting breakpoints on the folded-in function, so
// exclude .debug_line.
// If the referenced symbol is relative to a discarded section (due to
// --gc-sections, COMDAT, etc), it has been converted to a Undefined.
// `ds->folded` catches the ICF folded case. However, resolving a
// relocation in .debug_line to -1 would stop debugger users from setting
// breakpoints on the folded-in function, so exclude .debug_line.
//
// For pre-DWARF-v5 .debug_loc and .debug_ranges, -1 is a reserved value
// (base address selection entry), use 1 (which is used by GNU ld for
// .debug_ranges).
//
// TODO To reduce disruption, we use 0 instead of -1 as the tombstone
// value. Enable -1 in a future release.
if (!sym.getOutputSection() || (ds && ds->folded && !isDebugLine)) {
if (!ds || (ds->folded && !isDebugLine)) {
// If -z dead-reloc-in-nonalloc= is specified, respect it.
uint64_t value = SignExtend64<bits>(*tombstone);
// For a 32-bit local TU reference in .debug_names, X86_64::relocate
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2 changes: 1 addition & 1 deletion lld/test/ELF/dead-reloc-in-nonalloc.s
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Expand Up @@ -17,7 +17,7 @@
# AA: Contents of section .debug_info:
# AA-NEXT: 0000 [[ADDR]] 00000000 aaaaaaaa 00000000
# AA: Contents of section .not_debug:
# AA-NEXT: 0000 bbbbbbbb bbbbbbbb 00000000 .
# AA-NEXT: 0000 bbbbbbbb 2a000000 00000000 .

## Specifying zero can get a behavior similar to GNU ld.
# RUN: ld.lld --icf=all -z dead-reloc-in-nonalloc=.debug_info=0 %t.o %tabs.o -o %tzero
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