tree 35344b9a4158bf6262adbdad02be3875057e9de5
parent 7f3294a9b784ad011624cace3a1e81e32f273030
author Fangrui Song <maskray@google.com> 1678478012 -0800
committer Fangrui Song <i@maskray.me> 1678478012 -0800

ld: Allow R_X86_64_GOTPCREL for call *__tls_get_addr@GOTPCREL(%rip)

_Thread_local int a;
int main() { return a; }

% gcc -fno-plt -fpic a.c -fuse-ld=bfd -Wa,-mrelax-relocations=no
/usr/bin/ld.bfd: /tmp/ccSSBgrg.o: TLS transition from R_X86_64_TLSGD to R_X86_64_GOTTPOFF against `a' at 0xd in section `.text' failed
/usr/bin/ld.bfd: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

This commit fixes the issue.

There is an argument that the -fno-plt TLS sequence was added after
R_X86_64_GOTPCRELX was required for call, so R_X86_64_GOTPCREL was
intended to be unsupported.

Unfortunately this standpoint has caused interop difficulty: some
projects specify -mrelax-relocations=no to build relocatable object
files compatible with older linkers (e.g.
https://github.com/IHaskell/IHaskell/issues/636) or do so by accident
(e.g. https://github.com/rust-lang/rust/pull/106511 not addressed as of
today).  Many uses have not been cleaned up in practice, and compiling
with -fno-plt will lead to the `TLS transition from R_X86_64_TLSGD ...`
error which is hard to reason about.

There is another argument which may be weaker but relevant to the
necessity of -mrelax-relocations=no: HWAddressSanitizer x86-64 will
likely need some assembler support to disable relaxation.  Without the
support and if the compiler needs to support many gas version, the
simplest solution would be to use -Wa,-mrelax-relocations=no.

    PR ld/24784
    * bfd/elf64-x86-64.c (elf_x86_64_check_tls_transition): Allow
      R_X86_64_GOTPCREL.