[PATCH] D72197: [MC][ELF] Emit a relocation if target is defined in the same section and is non-local

[Xiang Zhang via Phabricator via llvm-commits]

xiangzhangllvm added a comment.

In D72197#1806225 <https://reviews.llvm.org/D72197#1806225>, @MaskRay wrote:

> In D72197#1805438 <https://reviews.llvm.org/D72197#1805438>, @bd1976llvm wrote:
>
> > I wonder if is this really a bug or intentional. Clang and LLVM seem to ignore the possibility of symbol interposition (see: -fno-semantic-interposition). What is the LLVM policy for this?
> >
> > Also, this code is missing a case for STB_GLOBAL and visibility != STV_DEFAULT.  Actually, I have noticed before that this case is missing in various places in MC. As it stands this patch may cause a performance regression on platforms that use -fvisibility=hidden.
>
>
> Visibility is irrelevant here. For a symbol defined in the same section as the instruction, the relocation record will reference a non-SECTION symbol instead of a STT_SECTION after this change. If the symbol is STV_HIDDEN, the linker will think the symbol is non-preemptible. So, no performance regression.
>
> There are many ways to make a symbol non-preemptible: binding (STB_LOCAL), visibility (STV_INTERNAL, STV_PROTECTED or STV_HIDDEN), --dynamic-list, VER_NDX_LOCAL, -Bsymbolic, -Bsymbolic-functions, etc. I have made enough refactorings to lld Symbol::isPreemptible and I am confident with these things :)
>
> GCC r212049 (first included in GCC 5) introduced -fsemantic-interposition. Yes, current Clang/LLVM behavior is like -fno-semantic-interposition. Our behavior is not ideal and @hfinkel had an RFC https://lists.llvm.org/pipermail/llvm-dev/2016-November/107625.html  . This change should help -fsemantic-interposition if we decide to move towards that goal.


Hi Fangrui, for "protected" visibility, that references within the defining module (mostly same section) will bind to the local symbol. So I think here should except "protected":

  -    if (SymA.getBinding() != ELF::STB_LOCAL
  +   if (SymA.getBinding() != ELF::STB_LOCAL  && SymA.getVisibility() != ELF::STV_PROTECTED

Now Clang also get a bug about the following test :

  $cat t.c
  __attribute__((visibility("protected"))) void * foo (void) { return (void *)foo; }
  $clang -c  -O0 -g  -fpic t.c
  $clang -o t.so  -O0 -g  -fpic t.o  -shared
  /usr/bin/ld: t.o: relocation R_X86_64_PC32 against protected symbol `foo' can not be used when making a shared object
  /usr/bin/ld: final link failed: Bad value
  clang-11: error: linker command failed with exit code 1 (use -v to see invocation)
  $/usr/bin/ld -v
  GNU ld version 2.30-49.el8


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72197/new/

https://reviews.llvm.org/D72197