objtool: UNWIND_HINT_RET_OFFSET should not check registers

UNWIND_HINT_RET_OFFSET will adjust a modified stack. However if a
callee-saved register was pushed on the stack then the stack frame
will still appear modified. So stop checking registers when
UNWIND_HINT_RET_OFFSET is used.
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200407073142.20659-3-alexandre.chartre@oracle.com

tools/objtool/check.c

@@ -1507,6 +1507,14 @@ static bool has_modified_stack_frame(struct instruction *insn, struct insn_state
 	if (cfi->stack_size != initial_func_cfi.cfa.offset + ret_offset)
 		return true;
 
+	/*
+	 * If there is a ret offset hint then don't check registers
+	 * because a callee-saved register might have been pushed on
+	 * the stack.
+	 */
+	if (ret_offset)
+		return false;
+
 	for (i = 0; i < CFI_NUM_REGS; i++) {
 		if (cfi->regs[i].base != initial_func_cfi.regs[i].base ||
 		    cfi->regs[i].offset != initial_func_cfi.regs[i].offset)