Messages in this thread | | | From | Nick Desaulniers <> | Date | Mon, 30 Oct 2023 08:24:16 -0700 | Subject | Re: [PATCH v2 05/11] x86/stackprotector/64: Convert stack protector to normal percpu variable |
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On Sun, Oct 29, 2023 at 10:01 AM Brian Gerst <brgerst@gmail.com> wrote: > > On Sun, Oct 29, 2023 at 2:56 AM Brian Gerst <brgerst@gmail.com> wrote: > > > > On Sat, Oct 28, 2023 at 9:26 PM 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/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@intel.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@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
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