s390/unwind: fix mixing regs and sp

unwind_for_each_frame stops after the first frame if regs->gprs[15] <=
sp.

The reason is that in case regs are specified, the first frame should be
regs->psw.addr and the second frame should be sp->gprs[8]. However,
currently the second frame is regs->gprs[15], which confuses
outside_of_stack().

Fix by introducing a flag to distinguish this special case from
unwinding the interrupt handler, for which the current behavior is
appropriate.

Fixes: 78c98f90 ("s390/unwind: introduce stack unwind API")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: stable@vger.kernel.org # v5.2+
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/unwind.h

@@ -35,6 +35,7 @@ struct unwind_state {
 	struct task_struct *task;
 	struct pt_regs *regs;
 	unsigned long sp, ip;
+	bool reuse_sp;
 	int graph_idx;
 	bool reliable;
 	bool error;

arch/s390/kernel/unwind_bc.c

@@ -46,11 +46,16 @@ bool unwind_next_frame(struct unwind_state *state)
 
 	regs = state->regs;
 	if (unlikely(regs)) {
+		if (state->reuse_sp) {
+			sp = state->sp;
+			state->reuse_sp = false;
+		} else {
 			sp = READ_ONCE_NOCHECK(regs->gprs[15]);
 			if (unlikely(outside_of_stack(state, sp))) {
 				if (!update_stack_info(state, sp))
 					goto out_err;
 			}
+		}
 		sf = (struct stack_frame *) sp;
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
 		reliable = false;
@@ -107,9 +112,9 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 {
 	struct stack_info *info = &state->stack_info;
 	unsigned long *mask = &state->stack_mask;
+	bool reliable, reuse_sp;
 	struct stack_frame *sf;
 	unsigned long ip;
-	bool reliable;
 
 	memset(state, 0, sizeof(*state));
 	state->task = task;
@@ -134,10 +139,12 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	if (regs) {
 		ip = READ_ONCE_NOCHECK(regs->psw.addr);
 		reliable = true;
+		reuse_sp = true;
 	} else {
 		sf = (struct stack_frame *) sp;
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
 		reliable = false;
+		reuse_sp = false;
 	}
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -151,5 +158,6 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	state->sp = sp;
 	state->ip = ip;
 	state->reliable = reliable;
+	state->reuse_sp = reuse_sp;
 }
 EXPORT_SYMBOL_GPL(__unwind_start);