ELF: Use index 0 for unversioned undefined symbols

[MaskRay]

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.

[llvmbot]

@llvm/pr-subscribers-lld
@llvm/pr-subscribers-llvm-binary-utilities

@llvm/pr-subscribers-lld-elf

Author: Fangrui Song (MaskRay)

Changes

The ELF specification 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.

---

Full diff: https://github.com/llvm/llvm-project/pull/168189.diff

5 Files Affected:

• (modified) lld/ELF/Symbols.h (+2)
• (modified) lld/ELF/SyntheticSections.cpp (+3-2)
• (modified) lld/test/ELF/linkerscript/version-script.s (+1-1)
• (modified) lld/test/ELF/version-script-extern-undefined.s (+1-1)
• (modified) llvm/include/llvm/BinaryFormat/ELF.h (+2-2)

diff --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h index c117e3b716c1b..a7d61f48ed3d5 100644 --- a/lld/ELF/Symbols.h +++ b/lld/ELF/Symbols.h @@ -313,6 +313,8 @@ class Symbol { // 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; diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp index 9a70c0d19c41d..0a4888fd0b196 100644 --- a/lld/ELF/SyntheticSections.cpp +++ b/lld/ELF/SyntheticSections.cpp @@ -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; } } diff --git a/lld/test/ELF/linkerscript/version-script.s b/lld/test/ELF/linkerscript/version-script.s index 52382eeb1245c..6b97fede00c37 100644 --- a/lld/test/ELF/linkerscript/version-script.s +++ b/lld/test/ELF/linkerscript/version-script.s @@ -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 { diff --git a/lld/test/ELF/version-script-extern-undefined.s b/lld/test/ELF/version-script-extern-undefined.s index 38114229e0ce3..010b4d5d6b63d 100644 --- a/lld/test/ELF/version-script-extern-undefined.s +++ b/lld/test/ELF/version-script-extern-undefined.s @@ -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: ] diff --git a/llvm/include/llvm/BinaryFormat/ELF.h b/llvm/include/llvm/BinaryFormat/ELF.h index 39e9611c7190e..bfcf5dab47722 100644 --- a/llvm/include/llvm/BinaryFormat/ELF.h +++ b/llvm/include/llvm/BinaryFormat/ELF.h @@ -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) };