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objtool: Fix skip_alt_group() for non-alternative STAC/CLAC

If an insn->alt points to a STAC/CLAC instruction, skip_alt_group()
assumes it's part of an alternative ("alt group") as opposed to some
other kind of "alt" such as an exception fixup.

While that assumption may hold true in the current code base, Linus has
an out-of-tree patch which breaks that assumption by replacing the
STAC/CLAC alternatives with raw STAC/CLAC instructions.

Make skip_alt_group() more robust by making sure it's actually an alt
group before continuing.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: 2d12c6fb7875 ("objtool: Remove ANNOTATE_IGNORE_ALTERNATIVE from CLAC/STAC")
Closes: https://lore.kernel.org/CAHk-=wi6goUT36sR8GE47_P-aVrd5g38=VTRHpktWARbyE-0ow@mail.gmail.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/3d22415f7b8e06a64e0873b21f48389290eeaa49.1761767616.git.jpoimboe@kernel.org

authored by

Josh Poimboeuf and committed by
Ingo Molnar c44b4b9e ba36dd5e

+4 -1
+4 -1
tools/objtool/check.c
··· 3516 3516 { 3517 3517 struct instruction *alt_insn = insn->alts ? insn->alts->insn : NULL; 3518 3518 3519 + if (!insn->alt_group) 3520 + return false; 3521 + 3519 3522 /* ANNOTATE_IGNORE_ALTERNATIVE */ 3520 - if (insn->alt_group && insn->alt_group->ignore) 3523 + if (insn->alt_group->ignore) 3521 3524 return true; 3522 3525 3523 3526 /*