[Bug ld/24784] elf_x86_64_check_tls_transition should allow R_X86_64_GOTPCREL (-fno-plt)

[maskray at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=24784

--- Comment #5 from Fangrui Song <maskray at google dot com> ---
I don't see how this bug is "invalid". I've already worked around the bug in
clang, but I hope this hack can be deleted in a few years.

> g++ -fno-plt -fpic a.cc -Wa,-mrelax-relocations=no -fuse-ld=bfd

I mentioned the combination of -fno-plt and -Wa,-mrelax-relocations=no doesn't
work. So we have 3 resolutions:

1. Ignore it.
  gcc -fno-plt -fpic -Wa,-mrelax-relocations=no  has the linker error
2. Document in Chapter 11 Alternate Code Sequences For Security of x86-64
psABI,
  call *__tls_get_addr@GOTPCREL(%rip)
  must use R_X86_64_GOTPCRELX.

  This kinda justifies 1., though it is not obvious why R_X86_64_GOTPCREL can't
be used.
3. One line change to bfd/elf-x86-64.c:elf_x86_64_check_tls_transition
- return r_type == R_X86_64_GOTPCRELX; // It should allow R_X86_64_GOTPCREL
+ return r_type == R_X86_64_GOTPCREL || r_type == R_X86_64_GOTPCRELX;
 
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=9b3777ae068e67e08b27914abbab11297f1f8d1b

With 3, the `gcc -fno-plt -fpic -Wa,-mrelax-relocations=no` issue can be fixed.

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