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

[Fangrui Song]

On Fri, Jan 6, 2023 at 1:27 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Fri, Jan 6, 2023 at 1:25 PM Fangrui Song <maskray@google.com> wrote:
> >
> >  On Fri, Jan 6, 2023 at 1:14 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > >
> > > On Fri, Jan 6, 2023 at 10:48 AM Fangrui Song <maskray@google.com> wrote:
> > > >
> > > > On Fri, Jan 6, 2023 at 9:04 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > >
> > > > > On Thu, Jan 5, 2023 at 1:06 PM Fangrui Song via Binutils
> > > > > <binutils@sourceware.org> wrote:
> > > > > >
> > > > > > _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.
> > > > > >
> > > > > >     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 914f82d0151..095fe2e0fe6 100644
> > > > > > --- a/bfd/elf64-x86-64.c
> > > > > > +++ b/bfd/elf64-x86-64.c
> > > > > > @@ -1241,7 +1241,7 @@ 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);
> > > > > >         }
> > > > > > --
> > > > > > 2.39.0.314.g84b9a713c41-goog
> > > > > >
> > > > >
> > > > > Since the new TLS sequence was added after R_X86_64_GOTPCRELX was
> > > > > required for call, R_X86_64_GOTPCREL should be invalid in this TLS sequence.
> > > > >
> > > > > --
> > > > > H.J.
> > > >
> > > > I have multiple arguments (albeit no single one is very strong) that
> > > > this 1-deletion-1-addition change provides benefits for users (IMHO
> > > > with no burden to binutils at all).
> > > >
> > > > Some projects may add -Wa,-mrelax-relocations=no to work around older
> > > > GNU ld. Then the project's toolchain requirement may increase and no
> > > > longer need to work around older GNU ld.
> > > > But a distribution may for some reason use a global -fno-plt (e.g.
> > > > Arch Linux) and then run into this TLS GD/LD->IE/LE optimization
> > > > issue.
> > > >
> > > > rust src/ci/docker/host-x86_64/*musl/Dockerfile
> > > > openjdk/jdk19u make/autoconf/flags-cflags.m4 (this file appears to be
> > > > copied into quite a few projects)
> > > > Linux kernel arch/x86/boot/compressed/Makefile (not a good example as
> > > > it doesn't use TLS AFAICT)
> > > >
> > > > R_X86_64_GOTPCREL isn't purely usefull. It may help linker design: for
> > > > R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX, the linker can make a
> > > > decision upfront whether a GOT entry is needed
> > > > (this affects the size of .got, which may affect section layout and
> > > > whether other relocations may overflow).
> > > > This may increase risk of 32-bit relocation overflow.
> > > > R_X86_64_GOTPCREL can mitigate the risk while being aware to the user.
> > > >
> > > > rustc somehow disables x86 relaxed relocations and defaults to `-Z
> > >
> > > Why is that?
> >
> > It's assuredly a rust's problem and I am trying to fix that in
> > https://github.com/rust-lang/rust/pull/106511
> >
> > The  -Wa,-mrelax-relocations=no problem may affect more packages.
>
> -mrelax-relocations=no should be a workaround for the older linker.   It
> shouldn't be used with the current linker.

A project may choose to work with many linker versions.
For simplicity, before it decides to drop compatibility with GNU
ld<2.26 (AIUI GOTPCRELX was supported in 2.26),
it may unconditionally add -Wa,-mrelax-relocations=no, instead of
doing configure work to check linker support.

Now a user may use -fno-plt (Arch Linux, rustc, maybe Alpine) and run
into the aforementioned TLS problem.

This 1-deletion-1-addition change can address this issue with no
maintenance burden on binutils side in my opinion,
so I made this patch.

The linker design I described is true as well. Whether GOTPCRELX leads
to a GOT entry can be decided at relocation scanning time, before the
section layout is decided.
Users may make a conscious decision to use GOTPCREL to avoid potential
relocation overflow risk.

GOTPCREL isn't really dead. It can be used with Intel LAM and tagged
global variables (with non-zero high address bits)
https://reviews.llvm.org/D111343
GOTPCREL instead of GOTPCRELX makes it clear an instruction
referencing the variable isn't supposed to be relaxed.

> > > > plt=no` and now relies on llvm-project to work around the GNU ld
> > > > compatibility issue.
> > > >
> > > >
> > > >
> > > >
> > > > --
> > > > 宋方睿
> > >
> > >
> > >
> > > --
> > > H.J.
> >
> >
> >
> > --
> > 宋方睿
>
>
>
> --
> H.J.



-- 
宋方睿