linux-kernel - Re: [PATCH v2 05/11] x86/stackprotector/64: Convert stack protector to normal percpu variable

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Message-ID: <CAMzpN2iF9-AdEcpkGOVooAn_6XTYaOb6gM+GYqy2SOc0S+dRqQ@mail.gmail.com>
Date:   Mon, 30 Oct 2023 13:19:21 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     kernel test robot <lkp@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, llvm@...ts.linux.dev,
        oe-kbuild-all@...ts.linux.dev, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Uros Bizjak <ubizjak@...il.com>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v2 05/11] x86/stackprotector/64: Convert stack protector
 to normal percpu variable

On Mon, Oct 30, 2023 at 11:24 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Sun, Oct 29, 2023 at 10:01 AM Brian Gerst <brgerst@...il.com> wrote:
> >
> > On Sun, Oct 29, 2023 at 2:56 AM Brian Gerst <brgerst@...il.com> wrote:
> > >
> > > On Sat, Oct 28, 2023 at 9:26 PM kernel test robot <lkp@...el.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/auto-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-brgerst%40gmail.com
> > > > patch subject: [PATCH v2 05/11] x86/stackprotector/64: Convert stack protector to normal percpu variable
> > > > config: x86_64-rhel-8.3-rust (https://download.01.org/0day-ci/archive/20231029/202310290927.2MuJJdu9-lkp@intel.com/config)
> > > > compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.git ae42196bc493ffe877a7e3dff8be32035dea4d07)
> > > > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231029/202310290927.2MuJJdu9-lkp@intel.com/reproduce)
> > > >
> > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > > > the same patch/commit), kindly add following tags
> > > > | Reported-by: kernel test robot <lkp@...el.com>
> > > > | Closes: https://lore.kernel.org/oe-kbuild-all/202310290927.2MuJJdu9-lkp@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@...PCREL(%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=clang LD=ld:
> > 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=clang LD=ld.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.

Correction:  It should be R_X86_64_PC32 for the LEA instruction.

Brian Gerst