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.

diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
index 0aa9af5d8fc0a22a71cb1da698323c396c626a0f..dd987ee011bb8afac8b05775059de03a35fe1834 100644 (file)
--- a/bfd/elf64-x86-64.c
+++ b/bfd/elf64-x86-64.c
@@ -1253,7 +1253,7 @@ elf_x86_64_check_tls_transition (bfd *abfd,
          if (largepic)
            return r_type == R_X86_64_PLTOFF64;
          else if (indirect_call)
-           return r_type == R_X86_64_GOTPCRELX;
+           return (r_type == R_X86_64_GOTPCRELX || r_type == R_X86_64_GOTPCREL);
          else
            return (r_type == R_X86_64_PC32 || r_type == R_X86_64_PLT32);
        }