Messages in this thread Patch in this message | | | Date | Mon, 14 Nov 2022 16:23:45 +0100 | From | Peter Zijlstra <> | Subject | [PATCH] lkdtm: Avoid objtool/ibt warning |
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On Mon, Nov 14, 2022 at 08:26:17PM +0800, kernel test robot wrote: > tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master > head: 094226ad94f471a9f19e8f8e7140a09c2625abaa > commit: 656d054e0a15ec327bd82801ccd58201e59f6896 jump_label,noinstr: Avoid instrumentation for JUMP_LABEL=n builds > date: 6 months ago > config: x86_64-buildonly-randconfig-r001-20221114 > compiler: gcc-11 (Debian 11.3.0-8) 11.3.0 > reproduce (this is a W=1 build): > # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=656d054e0a15ec327bd82801ccd58201e59f6896 > git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git > git fetch --no-tags linus master > git checkout 656d054e0a15ec327bd82801ccd58201e59f6896 > # save the config file > mkdir build_dir && cp config build_dir/.config > make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash
> >> vmlinux.o: warning: objtool: lkdtm_UNSET_SMEP+0x1c3: relocation to !ENDBR: native_write_cr4+0x41
--- Subject: lkdtm: Avoid objtool/ibt warning From: Peter Zijlstra <peterz@infradead.org> Date: Mon Nov 14 16:17:50 CET 2022
For certain configs objtool will complain like:
vmlinux.o: warning: objtool: lkdtm_UNSET_SMEP+0x1c3: relocation to !ENDBR: native_write_cr4+0x41
What happens is that GCC optimizes the loop:
insn = (unsigned char *)native_write_cr4; for (i = 0; i < MOV_CR4_DEPTH; i++)
to read something like:
for (insn = (unsigned char *)native_write_cr4; insn < (unsigned char *)native_write_cr4 + MOV_CR4_DEPTH; insn++)
Which then obviously generates the text reference native_write_cr4+041. Since none of this is a fast path, simply confuse GCC enough to inhibit this optimization.
Reported-by: kernel test robot <lkp@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- drivers/misc/lkdtm/bugs.c | 1 + 1 file changed, 1 insertion(+)
--- a/drivers/misc/lkdtm/bugs.c +++ b/drivers/misc/lkdtm/bugs.c @@ -487,6 +487,7 @@ static void lkdtm_UNSET_SMEP(void) * the cr4 writing instruction. */ insn = (unsigned char *)native_write_cr4; + OPTIMIZER_HIDE_VAR(insn); for (i = 0; i < MOV_CR4_DEPTH; i++) { /* mov %rdi, %cr4 */ if (insn[i] == 0x0f && insn[i+1] == 0x22 && insn[i+2] == 0xe7)
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