Messages in this thread | | | From | Brian Gerst <> | Date | Sun, 29 Oct 2023 13:00:49 -0400 | 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 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.
Brian Gerst
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