[ELF] Add -Bsymbolic-non-weak-functions
This option is a subset of -Bsymbolic-functions. It applies to STB_GLOBAL
STT_FUNC definitions.
The address of a vague linkage function (STB_WEAK STT_FUNC, e.g. an inline
function, a template instantiation) seen by a -Bsymbolic-functions linked
shared object may be different from the address seen from outside the shared
object. Such cases are uncommon. (ELF/Mach-O programs may use
`-fvisibility-inlines-hidden` to break such pointer equality. On Windows,
correct dllexport and dllimport are needed to make pointer equality work.
Windows link.exe enables /OPT:ICF by default so different inline functions may
have the same address.)
```
// a.cc -> a.o -> a.so (-Bsymbolic-functions)
inline void f() {}
void *g() { return (void *)&f; }
// b.cc -> b.o -> exe
// The address is different!
inline void f() {}
```
-Bsymbolic-non-weak-functions is a safer (C++ conforming) subset of
-Bsymbolic-functions, which can make such programs work.
Implementations usually emit a vague linkage definition in a COMDAT group. We
could detect the group (with more code) but I feel that we should just check
STB_WEAK for simplicity. A weak definition will thus serve as an escape hatch
for rare cases when users want interposition on definitions.
GNU ld feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=27871
Longer write-up: https://maskray.me/blog/2021-05-16-elf-interposition-and-bsymbolic
If Linux distributions migrate to protected non-vague-linkage external linkage
functions by default, the linker option can still be handy because it allows
rapid experiment without recompilation. Protected function addresses currently
have deep issues in GNU ld.
Reviewed By: peter.smith
Differential Revision: https://reviews.llvm.org/D102570
diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index 1039be3..496be33 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -368,8 +368,12 @@
// If -Bsymbolic or --dynamic-list is specified, or -Bsymbolic-functions is
// specified and the symbol is STT_FUNC, the symbol is preemptible iff it is
- // in the dynamic list.
- if (config->symbolic || (config->bsymbolicFunctions && sym.isFunc()))
+ // in the dynamic list. -Bsymbolic-non-weak-functions is a non-weak subset of
+ // -Bsymbolic-functions.
+ if (config->symbolic ||
+ (config->bsymbolic == BsymbolicKind::Functions && sym.isFunc()) ||
+ (config->bsymbolic == BsymbolicKind::NonWeakFunctions && sym.isFunc() &&
+ sym.binding != STB_WEAK))
return sym.inDynamicList;
return true;
}
