From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 8 May 2020 12:34:33 +0200
Subject: objtool: Allow no-op CFI ops in alternatives
Git-commit: ab3852ab5cb8fd2e2c5bfa176e5f953353836907
Patch-mainline: v5.8-rc1
References: bsc#1202396

Randy reported a false-positive:

  arch/x86/hyperv/hv_apic.o: warning: objtool: hv_apic_write()+0x25: alternative modifies stack

What happens is that:

	alternative_io("movl %0, %P1", "xchgl %0, %P1", X86_BUG_11AP,
 13d:   89 9d 00 d0 7f ff       mov    %ebx,-0x803000(%rbp)

decodes to an instruction with CFI-ops because it modifies RBP.
However, due to this being a !frame-pointer build, that should not in
fact change the CFI state.

So instead of dis-allowing any CFI-op, verify the op would've actually
changed the CFI state.

Fixes: 7117f16bf460 ("objtool: Fix ORC vs alternatives")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Miroslav Benes <mbenes@suse.cz>
---
 tools/objtool/check.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1842,16 +1842,17 @@ static int handle_insn_ops(struct instru
 	struct stack_op *op;
 
 	list_for_each_entry(op, &insn->stack_ops, list) {
+		struct cfi_state old_cfi = state->cfi;
 		int res;
 
-		if (insn->alt_group) {
-			WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
-			return -1;
-		}
-
 		res = update_cfi_state(insn, &state->cfi, op);
 		if (res)
 			return res;
+
+		if (insn->alt_group && memcmp(&state->cfi, &old_cfi, sizeof(struct cfi_state))) {
+			WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
+			return -1;
+		}
 	}
 
 	return 0;