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lkdtm: Avoid objtool/ibt warning

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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>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/Y3JdgbXRV0MNZ+9h@hirez.programming.kicks-ass.net
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Peter Zijlstra 2022-11-14 16:23:45 +01:00 committed by Josh Poimboeuf
parent d49d1666aa
commit 4a03aa3443
1 changed files with 1 additions and 0 deletions
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1
drivers/misc/lkdtm/bugs.c
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@ -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)