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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #11 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot 
gnu.org> ---
The master branch has been updated by Fangrui Song <maskray@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d58854b6dd88e05dbf2a5d1c32c5acb7bd6ea274

commit d58854b6dd88e05dbf2a5d1c32c5acb7bd6ea274
Author: Fangrui Song <maskray@google.com>
Date:   Fri Mar 10 11:53:32 2023 -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.

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