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

[Nick Desaulniers]

On Sun, Oct 29, 2023 at 10:01 AM Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> On Sun, Oct 29, 2023 at 2:56 AM Brian Gerst <brgerst@xxxxxxxxx> wrote:
> >
> > On Sat, Oct 28, 2023 at 9:26 PM kernel test robot <lkp@xxxxxxxxx> 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@xxxxxxxxx/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@xxxxxxxxx/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@xxxxxxxxx>
> > > | Closes: https://lore.kernel.org/oe-kbuild-all/202310290927.2MuJJdu9-lkp@xxxxxxxxx/
> > >
> > > 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=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.
>
> 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.

Thanks for the report.  + Fangrui for thoughts on relocations against
__stack_chk_guard; clang has similar issues for 32b x86 as well.

>
> Brian Gerst
>


-- 
Thanks,
~Nick Desaulniers