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

[Josh Poimboeuf <jpoimboe@kernel.org>]

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.

Fixes: 2d12c6fb7875 ("objtool: Remove ANNOTATE_IGNORE_ALTERNATIVE from CLAC/STAC")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/CAHk-=wi6goUT36sR8GE47_P-aVrd5g38=VTRHpktWARbyE-0ow@mail.gmail.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/check.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 620854fdaaf63..9004fbc067693 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3516,8 +3516,11 @@ static bool skip_alt_group(struct instruction *insn)
 {
 	struct instruction *alt_insn = insn->alts ? insn->alts->insn : NULL;
 
+	if (!insn->alt_group)
+		return false;
+
 	/* ANNOTATE_IGNORE_ALTERNATIVE */
-	if (insn->alt_group && insn->alt_group->ignore)
+	if (insn->alt_group->ignore)
 		return true;
 
 	/*
-- 
2.51.0