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From:   Brian Gerst <brgerst@gmail.com>
Date:   Sun, 29 Oct 2023 13:00:49 -0400
Message-ID: <CAMzpN2jbA=ZCbbFq=gyVAtKBr4kyYcNPLpWaEcAhEAN2J0wwTw@mail.gmail.com>
Subject: Re: [PATCH v2 05/11] x86/stackprotector/64: Convert stack protector
 to normal percpu variable
To:     kernel test robot <lkp@intel.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, llvm@lists.linux.dev,
        oe-kbuild-all@lists.linux.dev, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Uros Bizjak <ubizjak@gmail.com>
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On Sun, Oct 29, 2023 at 2:56=E2=80=AFAM Brian Gerst <brgerst@gmail.com> wro=
te:
>
> On Sat, Oct 28, 2023 at 9:26=E2=80=AFPM kernel test robot <lkp@intel.com>=
 wrote:
> >
> > Hi Brian,
> >
> > kernel test robot noticed the following build errors:
> >
> > [auto build test ERROR on tip/master]
> > [also build test ERROR on next-20231027]
> > [cannot apply to tip/x86/core dennis-percpu/for-next linus/master tip/a=
uto-latest v6.6-rc7]
> > [If your patch is applied to the wrong git tree, kindly drop us a note.
> > And when submitting patch, we suggest to use '--base' as documented in
> > https://git-scm.com/docs/git-format-patch#_base_tree_information]
> >
> > url:    https://github.com/intel-lab-lkp/linux/commits/Brian-Gerst/x86-=
stackprotector-32-Remove-stack-protector-test-script/20231027-000533
> > base:   tip/master
> > patch link:    https://lore.kernel.org/r/20231026160100.195099-6-brgers=
t%40gmail.com
> > patch subject: [PATCH v2 05/11] x86/stackprotector/64: Convert stack pr=
otector to normal percpu variable
> > config: x86_64-rhel-8.3-rust (https://download.01.org/0day-ci/archive/2=
0231029/202310290927.2MuJJdu9-lkp@intel.com/config)
> > compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.gi=
t ae42196bc493ffe877a7e3dff8be32035dea4d07)
> > reproduce (this is a W=3D1 build): (https://download.01.org/0day-ci/arc=
hive/20231029/202310290927.2MuJJdu9-lkp@intel.com/reproduce)
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new ve=
rsion of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202310290927.2MuJJdu9-l=
kp@intel.com/
> >
> > All errors (new ones prefixed by >>):
> >
> > >> Unsupported relocation type: unknown type rel type name (42)
>
> Clang is generating a new relocation type (R_X86_64_REX_GOTPCRELX)
> that the relocs tool doesn't know about.  This is supposed to allow
>         movq    __stack_chk_guard@GOTPCREL(%rip), %rax
>         movq    %gs:(%rax), %rax
> to be relaxed to
>         leaq    __stack_chk_guard(%rip), %rax
>         movq    %gs:(%rax), %rax
>
> But why is clang doing this instead of what GCC does?
>         movq    %gs:__stack_chk_guard(%rip), %rax

Digging a bit deeper, there also appears to be differences in how the
linkers behave with this new relocation:

make CC=3Dclang LD=3Dld:
ffffffff81002838:       48 c7 c0 c0 5c 42 83    mov    $0xffffffff83425cc0,=
%rax
                        ffffffff8100283b: R_X86_64_32S  __stack_chk_guard
ffffffff8100283f:       65 48 8b 00             mov    %gs:(%rax),%rax

make CC=3Dclang LD=3Dld.lld:
ffffffff81002838:       48 8d 05 81 34 42 02    lea
0x2423481(%rip),%rax        # ffffffff83425cc0 <__stack_chk_guard>
                        ffffffff8100283b: R_X86_64_REX_GOTPCRELX
 __stack_chk_guard-0x4
ffffffff8100283f:       65 48 8b 00             mov    %gs:(%rax),%rax

The LLVM linker keeps the R_X86_64_REX_GOTPCRELX even after performing
the relaxation.  It should be R_X86_64_32S based on it changing to an
LEA instruction.  The GNU linker changes it to R_X86_64_32S and a MOV
immediate.

So I think there are two issues here.  1) clang is producing code
referencing the GOT for stack protector accesses, despite -fno-PIE on
the command line and no other GOT references, and 2) ld.lld is using
the wrong relocation type after the relaxation step is performed.

I think the quick fix here is to teach the relocs tool about this new
relocation.  It should be able to be safely ignored since it's
PC-relative.  The code clang produces is functionally correct,
although not optimal.

Brian Gerst