Re: [PATCH v2] ld: Allow R_X86_64_GOTPCREL for call *__tls_get_addr@GOTPCREL(%rip)

[Jan Beulich <jbeulich@suse.com>]

On 02.03.2023 21:10, Fangrui Song wrote:
>  From a8df373aec097dcdabe46717dd95cdd9b16ef7d7 Mon Sep 17 00:00:00 2001
> From: Fangrui Song <maskray@google.com>
> Date: Thu, 5 Jan 2023 12:45:27 -0800
> Subject: [PATCH v2] 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 versions, the
> simplest solutation 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.
> ---
>   bfd/elf64-x86-64.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
> index 0aa9af5d8fc..dd987ee011b 100644
> --- 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);
>   	}

No further comments from H.J., so please feel free to commit.

Jan