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ELF: Use index 0 for unversioned undefined symbols (#168189)

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The GNU documentation is ambiguous about the version index for
unversioned undefined symbols. The current specification at
https://sourceware.org/gnu-gabi/program-loading-and-dynamic-linking.txt
defines VER_NDX_LOCAL (0) as "The symbol is private, and is not
available outside this object."

However, this naming is misleading for undefined symbols. As suggested
in
discussions, VER_NDX_LOCAL should conceptually be VER_NDX_NONE and apply
to unversioned undefined symbols as well.

GNU ld has used index 0 for unversioned undefined symbols both before
version 2.35 (see https://sourceware.org/PR26002) and in the upcoming
2.46 release (see https://sourceware.org/PR33577). This change aligns
with GNU ld's behavior by switching from index 1 to index 0.

While here, add a test to dso-undef-extract-lazy.s that undefined
symbols of index 0 in DSO are treated as unversioned symbols.
This commit is contained in:
Fangrui Song
2025-11-22 12:54:50 -08:00
committed by GitHub
parent 080ca902c6
commit d96a93ff00
6 changed files with 12 additions and 8 deletions
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5
lld/ELF/InputFiles.cpp
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@@ -1676,8 +1676,9 @@ template <class ELFT> void SharedFile::parse() {
const uint16_t ver = versyms[i], idx = ver & ~VERSYM_HIDDEN;
if (sym.isUndefined()) {
// For unversioned undefined symbols, VER_NDX_GLOBAL makes more sense but
// as of binutils 2.34, GNU ld produces VER_NDX_LOCAL.
// Index 0 (VER_NDX_LOCAL) is used for unversioned undefined symbols.
// GNU ld versions between 2.35 and 2.45 also generate VER_NDX_GLOBAL
// for this case (https://sourceware.org/PR33577).
if (ver != VER_NDX_LOCAL && ver != VER_NDX_GLOBAL) {
if (idx >= verneeds.size()) {
ErrAlways(ctx) << "corrupt input file: version need index " << idx

2
lld/ELF/Symbols.h
View File

@@ -313,6 +313,8 @@ public:
// represents the Verdef index within the input DSO, which will be converted
// to a Verneed index in the output. Otherwise, this represents the Verdef
// index (VER_NDX_LOCAL, VER_NDX_GLOBAL, or a named version).
// VER_NDX_LOCAL indicates a defined symbol that has been localized by a
// version script's local: directive or --exclude-libs.
uint16_t versionId;
LLVM_PREFERRED_TYPE(bool)
uint8_t versionScriptAssigned : 1;

5
lld/ELF/SyntheticSections.cpp
View File

@@ -3784,9 +3784,10 @@ void VersionTableSection::writeTo(uint8_t *buf) {
buf += 2;
for (const SymbolTableEntry &s : getPartition(ctx).dynSymTab->getSymbols()) {
// For an unextracted lazy symbol (undefined weak), it must have been
// converted to Undefined and have VER_NDX_GLOBAL version here.
// converted to Undefined.
assert(!s.sym->isLazy());
write16(ctx, buf, s.sym->versionId);
// Undefined symbols should use index 0 when unversioned.
write16(ctx, buf, s.sym->isUndefined() ? 0 : s.sym->versionId);
buf += 2;
}
}

2
lld/test/ELF/linkerscript/version-script.s
View File

@@ -17,7 +17,7 @@
# CHECK-NEXT: Name:
# CHECK-NEXT: }
# CHECK-NEXT: Symbol {
# CHECK-NEXT: Version: 1
# CHECK-NEXT: Version: 0
# CHECK-NEXT: Name: und
# CHECK-NEXT: }
# CHECK-NEXT: Symbol {

2
lld/test/ELF/version-script-extern-undefined.s
View File

@@ -11,7 +11,7 @@
# CHECK-NEXT: Name:
# CHECK-NEXT: }
# CHECK-NEXT: Symbol {
# CHECK-NEXT: Version: 1
# CHECK-NEXT: Version: 0
# CHECK-NEXT: Name: _Z3abbi
# CHECK-NEXT: }
# CHECK-NEXT: ]

4
llvm/include/llvm/BinaryFormat/ELF.h
View File

@@ -1710,8 +1710,8 @@ enum { VER_FLG_BASE = 0x1, VER_FLG_WEAK = 0x2, VER_FLG_INFO = 0x4 };
// Special constants for the version table. (SHT_GNU_versym/.gnu.version)
enum {
VER_NDX_LOCAL = 0, // Unversioned local symbol
VER_NDX_GLOBAL = 1, // Unversioned global symbol
VER_NDX_LOCAL = 0, // Unversioned undefined or localized defined symbol
VER_NDX_GLOBAL = 1, // Unversioned non-local defined symbol
VERSYM_VERSION = 0x7fff, // Version Index mask
VERSYM_HIDDEN = 0x8000 // Hidden bit (non-default version)
};