s390/unwind: start unwinding from reliable state

A comment in arch/s390/include/asm/unwind.h says:
> If 'first_frame' is not zero unwind_start skips unwind frames until it
> reaches the specified stack pointer.
> The end of the unwinding is indicated with unwind_done, this can be true
> right after unwind_start, e.g. with first_frame!=0 that can not be found.
> unwind_next_frame skips to the next frame.
> Once the unwind is completed unwind_error() can be used to check if there
> has been a situation where the unwinder could not correctly understand
> the tasks call chain.

With this change backchain unwinder now comply with behaviour
described. As well as matches orc unwinder implementation.  Now unwinder
starts from reliable state, i.e. __unwind_start own stack frame is
taken or stack frame generated by __switch_to (ksp) - both known to be
valid. In case of pt_regs %r15 is better match for pt_regs psw, than
sometimes random "sp" caller passed.

Reviewed-by: Heiko Carstens <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>

Author: Vasily Gorbik

arch/s390/include/asm/unwind.h

@@ -58,11 +58,11 @@ static inline bool unwind_error(struct unwind_state *state)
 static inline void unwind_start(struct unwind_state *state,
 				struct task_struct *task,
 				struct pt_regs *regs,
-				unsigned long sp)
+				unsigned long first_frame)
 {
 	task = task ?: current;
-	sp = sp ?: get_stack_pointer(task, regs);
-	__unwind_start(state, task, regs, sp);
+	first_frame = first_frame ?: get_stack_pointer(task, regs);
+	__unwind_start(state, task, regs, first_frame);
 }
 
 static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)

arch/s390/kernel/unwind_bc.c

@@ -105,13 +105,11 @@ bool unwind_next_frame(struct unwind_state *state)
 EXPORT_SYMBOL_GPL(unwind_next_frame);
 
 void __unwind_start(struct unwind_state *state, struct task_struct *task,
-		    struct pt_regs *regs, unsigned long sp)
+		    struct pt_regs *regs, unsigned long first_frame)
 {
 	struct stack_info *info = &state->stack_info;
-	unsigned long *mask = &state->stack_mask;
-	bool reliable;
 	struct stack_frame *sf;
-	unsigned long ip;
+	unsigned long ip, sp;
 
 	memset(state, 0, sizeof(*state));
 	state->task = task;
@@ -123,30 +121,46 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 		return;
 	}
 
+	/* Get the instruction pointer from pt_regs or the stack frame */
+	if (regs) {
+		ip = regs->psw.addr;
+		sp = regs->gprs[15];
+	} else if (task == current) {
+		sp = current_frame_address();
+	} else {
+		sp = task->thread.ksp;
+	}
+
 	/* Get current stack pointer and initialize stack info */
-	if (get_stack_info(sp, task, info, mask) != 0 ||
-	    !on_stack(info, sp, sizeof(struct stack_frame))) {
+	if (!update_stack_info(state, sp)) {
 		/* Something is wrong with the stack pointer */
 		info->type = STACK_TYPE_UNKNOWN;
 		state->error = true;
 		return;
 	}
 
-	/* Get the instruction pointer from pt_regs or the stack frame */
-	if (regs) {
-		ip = READ_ONCE_NOCHECK(regs->psw.addr);
-		reliable = true;
-	} else {
-		sf = (struct stack_frame *) sp;
+	if (!regs) {
+		/* Stack frame is within valid stack */
+		sf = (struct stack_frame *)sp;
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
-		reliable = false;
 	}
 
 	ip = ftrace_graph_ret_addr(state->task, &state->graph_idx, ip, NULL);
 
 	/* Update unwind state */
 	state->sp = sp;
 	state->ip = ip;
-	state->reliable = reliable;
+	state->reliable = true;
+
+	if (!first_frame)
+		return;
+	/* Skip through the call chain to the specified starting frame */
+	while (!unwind_done(state)) {
+		if (on_stack(&state->stack_info, first_frame, sizeof(struct stack_frame))) {
+			if (state->sp >= first_frame)
+				break;
+		}
+		unwind_next_frame(state);
+	}
 }
 EXPORT_SYMBOL_GPL(__unwind_start);