[PATCH] D54145: [ELF] - Fix R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX when target is IFUNC.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added a reviewer: ruiu.
Herald added subscribers: arichardson, emaste.
Herald added a reviewer: espindola.

This is https://bugs.llvm.org/show_bug.cgi?id=38074. This simple reproducible is below.

Imagine the following code:

**user.cpp:**

  extern "C" int myfunc();
  
  int main() {
   int (*p)()  = &myfunc;
  
   return p();
  }

**library.cpp:**

  int myfunc2() {
     return 2;
  }
  
  extern "C" int myfunc();
  __asm__ (".type myfunc, @gnu_indirect_function");
  
  typeof(myfunc) * myfunc_dispatch (void) __asm__ ("myfunc");
  typeof(myfunc) * myfunc_dispatch (void)  {
    return myfunc2;
  }

If you run the application linked with gold, the output will be correct:

  > clang library.o user.o -fPIC -static -o test
  > ./test 
  > echo $?
  2

The application linked with LLD (-fuse-ld=lld) shows `32` for me instead.

The issue is that when calling a function, for example,
LLD generates a .got entry that points to the IFUNC resolver function when
instead, it should use the PLT entries properly for handling the IFUNC.

So we should create a got entry that points to PLT entry, which itself loads the value from 
`.got.iplt`, relocated with R_X86_64_IRELATIVE to make things work. Patch do that.


https://reviews.llvm.org/D54145

Files:
  ELF/InputSection.cpp
  ELF/Relocations.cpp
  ELF/Relocations.h
  test/ELF/gotpc-relax.s

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