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[ELF] Eliminate symbols demoted due to /DISCARD/ discarded sections (#85167)

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#69295 demoted Defined symbols relative to discarded sections.
If such a symbol is unreferenced, the desired behavior is to
eliminate it from .symtab just like --gc-sections discarded
definitions.
Linux kernel's CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y configuration expects
that the unreferenced `unused` is not emitted to .symtab
(https://github.com/ClangBuiltLinux/linux/issues/2006).

For relocations referencing demoted symbols, the symbol index restores
to 0 like older lld (`R_X86_64_64 0` in `discard-section.s`).

Fix #85048

(cherry picked from commit 8fe3e70e810b409dce36f6d415e86f0f9b1cf22d)
This commit is contained in:
Fangrui Song 2024-03-14 09:51:27 -07:00 committed by llvmbot
parent 33c6b20276
commit 122ba9f100
2 changed files with 19 additions and 9 deletions
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3
lld/ELF/Writer.cpp
View File

@ -261,6 +261,9 @@ static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
/*discardedSecIdx=*/map.lookup(sym.section))
.overwrite(sym);
// Eliminate from the symbol table, otherwise we would leave an undefined
// symbol if the symbol is unreferenced in the absence of GC.
sym.isUsedInRegularObj = false;
}
// If all references to a DSO happen to be weak, the DSO is not added to

25
lld/test/ELF/linkerscript/discard-section.s
View File

@ -9,6 +9,9 @@
# RUN: ld.lld -r -T a.lds a.o b.o -o a.ro 2>&1 | FileCheck %s --check-prefix=WARNING --implicit-check-not=warning:
# RUN: llvm-readelf -r -s a.ro | FileCheck %s --check-prefix=RELOC
# RUN: ld.lld -r --gc-sections -T a.lds a.o b.o -o a.gc.ro --no-fatal-warnings
# RUN: llvm-readelf -r -s a.gc.ro | FileCheck %s --check-prefix=RELOC-GC
# LOCAL: error: relocation refers to a discarded section: .aaa
# LOCAL-NEXT: >>> defined in a.o
# LOCAL-NEXT: >>> referenced by a.o:(.bbb+0x0)
@ -32,16 +35,18 @@
# WARNING: warning: relocation refers to a discarded section: .aaa
# WARNING-NEXT: >>> referenced by a.o:(.rela.bbb+0x0)
## GNU ld reports "defined in discarded secion" errors even in -r mode.
## We set the symbol index to 0.
# RELOC: Relocation section '.rela.bbb' at offset {{.*}} contains 1 entries:
# RELOC-NEXT: Offset Info Type Symbol's Value Symbol's Name + Addend
# RELOC-NEXT: 0000000000000000 0000000000000000 R_X86_64_NONE 0
# RELOC-EMPTY:
# RELOC-NEXT: Relocation section '.rela.data' at offset {{.*}} contains 4 entries:
# RELOC-NEXT: Offset Info Type Symbol's Value Symbol's Name + Addend
# RELOC-NEXT: 0000000000000000 0000000500000001 R_X86_64_64 0000000000000000 global + 0
# RELOC-NEXT: 0000000000000008 0000000700000001 R_X86_64_64 0000000000000000 weak + 0
# RELOC-NEXT: 0000000000000010 0000000600000001 R_X86_64_64 0000000000000000 weakref1 + 0
# RELOC-NEXT: 0000000000000018 0000000800000001 R_X86_64_64 0000000000000000 weakref2 + 0
# RELOC-NEXT: 0000000000000000 0000000000000001 R_X86_64_64 0
# RELOC-NEXT: 0000000000000008 0000000000000001 R_X86_64_64 0
# RELOC-NEXT: 0000000000000010 0000000000000001 R_X86_64_64 0
# RELOC-NEXT: 0000000000000018 0000000000000001 R_X86_64_64 0
# RELOC: Num: Value Size Type Bind Vis Ndx Name
# RELOC-NEXT: 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
@ -49,23 +54,25 @@
# RELOC-NEXT: 2: 0000000000000000 0 SECTION LOCAL DEFAULT 2 .bbb
# RELOC-NEXT: 3: 0000000000000000 0 SECTION LOCAL DEFAULT 4 .data
# RELOC-NEXT: 4: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 1 _start
# RELOC-NEXT: 5: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND global
# RELOC-NEXT: 6: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weakref1
# RELOC-NEXT: 7: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weak
# RELOC-NEXT: 8: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weakref2
# RELOC-EMPTY:
# RELOC-GC: There are no relocations in this file.
#--- a.s
.globl _start
_start:
.section .aaa,"a"
.globl global, weakref1
.globl global, weakref1, unused
.weak weak, weakref2
global:
weak:
weakref1:
weakref2:
## Eliminate `unused` just like GC discarded definitions.
## Linux kernel's CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y configuration expects
## that the unreferenced `unused` is not emitted to .symtab.
unused:
.quad 0
.section .bbb,"aw"