objtool: Allow no-op CFI ops in alternatives

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: 7117f16 ("objtool: Fix ORC vs alternatives")
Reported-by: Randy Dunlap <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Tested-by: Randy Dunlap <[email protected]>

Author: Peter Zijlstra

tools/objtool/check.c

@@ -2060,17 +2060,18 @@ static int handle_insn_ops(struct instruction *insn, struct insn_state *state)
 	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;
+		}
+
 		if (op->dest.type == OP_DEST_PUSHF) {
 			if (!state->uaccess_stack) {
 				state->uaccess_stack = 1;