objtool: Allow UNWIND_HINT to suppress dodgy stack modifications

rewind_stack_do_exit()
	UNWIND_HINT_FUNC
	/* Prevent any naive code from trying to unwind to our caller. */

	xorl	%ebp, %ebp
	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rax
	leaq	-PTREGS_SIZE(%rax), %rsp
	UNWIND_HINT_REGS

	call	do_exit

Does unspeakable things to the stack, which objtool currently fails to
detect due to a limitation in instruction decoding. This will be
rectified after which the above will result in:

arch/x86/entry/entry_64.o: warning: objtool: .text+0xab: unsupported stack register modification

Allow the UNWIND_HINT on the next instruction to suppress this, it
will overwrite the state anyway.
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20210211173626.918498579@infradead.org

tools/objtool/check.c

@@ -1959,8 +1959,9 @@ static void restore_reg(struct cfi_state *cfi, unsigned char reg)
  *   41 5d			pop    %r13
  *   c3				retq
  */
-static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,
-			     struct stack_op *op)
+static int update_cfi_state(struct instruction *insn,
+			    struct instruction *next_insn,
+			    struct cfi_state *cfi, struct stack_op *op)
 {
 	struct cfi_reg *cfa = &cfi->cfa;
 	struct cfi_reg *regs = cfi->regs;
@@ -2161,7 +2162,7 @@ static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,
 				break;
 			}
 
-			if (op->dest.reg == cfi->cfa.base) {
+			if (op->dest.reg == cfi->cfa.base && !(next_insn && next_insn->hint)) {
 				WARN_FUNC("unsupported stack register modification",
 					  insn->sec, insn->offset);
 				return -1;
@@ -2433,13 +2434,15 @@ static int propagate_alt_cfi(struct objtool_file *file, struct instruction *insn
 	return 0;
 }
 
-static int handle_insn_ops(struct instruction *insn, struct insn_state *state)
+static int handle_insn_ops(struct instruction *insn,
+			   struct instruction *next_insn,
+			   struct insn_state *state)
 {
 	struct stack_op *op;
 
 	list_for_each_entry(op, &insn->stack_ops, list) {
 
-		if (update_cfi_state(insn, &state->cfi, op))
+		if (update_cfi_state(insn, next_insn, &state->cfi, op))
 			return 1;
 
 		if (op->dest.type == OP_DEST_PUSHF) {
@@ -2719,7 +2722,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				return 0;
 		}
 
-		if (handle_insn_ops(insn, &state))
+		if (handle_insn_ops(insn, next_insn, &state))
 			return 1;
 
 		switch (insn->type) {