function_graph: Use new curr_ret_depth to manage depth instead of curr_ret_stack

Currently, the depth of the ret_stack is determined by curr_ret_stack index.
The issue is that there's a race between setting of the curr_ret_stack and
calling of the callback attached to the return of the function.

Commit 03274a3f ("tracing/fgraph: Adjust fgraph depth before calling
trace return callback") moved the calling of the callback to after the
setting of the curr_ret_stack, even stating that it was safe to do so, when
in fact, it was the reason there was a barrier() there (yes, I should have
commented that barrier()).

Not only does the curr_ret_stack keep track of the current call graph depth,
it also keeps the ret_stack content from being overwritten by new data.

The function profiler, uses the "subtime" variable of ret_stack structure
and by moving the curr_ret_stack, it allows for interrupts to use the same
structure it was using, corrupting the data, and breaking the profiler.

To fix this, there needs to be two variables to handle the call stack depth
and the pointer to where the ret_stack is being used, as they need to change
at two different locations.

Cc: stable@kernel.org
Fixes: 03274a3f ("tracing/fgraph: Adjust fgraph depth before calling trace return callback")
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/sched.h

@@ -1116,6 +1116,7 @@ struct task_struct {
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	/* Index of current stored address in ret_stack: */
 	int				curr_ret_stack;
+	int				curr_ret_depth;
 
 	/* Stack of return addresses for return function tracing: */
 	struct ftrace_ret_stack		*ret_stack;

kernel/trace/ftrace.c

@@ -6814,6 +6814,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
 			atomic_set(&t->tracing_graph_pause, 0);
 			atomic_set(&t->trace_overrun, 0);
 			t->curr_ret_stack = -1;
+			t->curr_ret_depth = -1;
 			/* Make sure the tasks see the -1 first: */
 			smp_wmb();
 			t->ret_stack = ret_stack_list[start++];
@@ -7038,6 +7039,7 @@ graph_init_task(struct task_struct *t, struct ftrace_ret_stack *ret_stack)
 void ftrace_graph_init_idle_task(struct task_struct *t, int cpu)
 {
 	t->curr_ret_stack = -1;
+	t->curr_ret_depth = -1;
 	/*
 	 * The idle task has no parent, it either has its own
 	 * stack or no stack at all.
@@ -7068,6 +7070,7 @@ void ftrace_graph_init_task(struct task_struct *t)
 	/* Make sure we do not use the parent ret_stack */
 	t->ret_stack = NULL;
 	t->curr_ret_stack = -1;
+	t->curr_ret_depth = -1;
 
 	if (ftrace_graph_active) {
 		struct ftrace_ret_stack *ret_stack;

kernel/trace/trace_functions_graph.c

@@ -119,7 +119,7 @@ print_graph_duration(struct trace_array *tr, unsigned long long duration,
 
 /* Add a function return address to the trace stack on thread info.*/
 static int
-ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth,
+ftrace_push_return_trace(unsigned long ret, unsigned long func,
 			 unsigned long frame_pointer, unsigned long *retp)
 {
 	unsigned long long calltime;
@@ -177,8 +177,6 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth,
 #ifdef HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 	current->ret_stack[index].retp = retp;
 #endif
-	*depth = current->curr_ret_stack;
-
 	return 0;
 }
 
@@ -188,14 +186,20 @@ int function_graph_enter(unsigned long ret, unsigned long func,
 	struct ftrace_graph_ent trace;
 
 	trace.func = func;
-	trace.depth = current->curr_ret_stack + 1;
+	trace.depth = ++current->curr_ret_depth;
 
 	/* Only trace if the calling function expects to */
 	if (!ftrace_graph_entry(&trace))
-		return -EBUSY;
+		goto out;
 
-	return ftrace_push_return_trace(ret, func, &trace.depth,
-					frame_pointer, retp);
+	if (ftrace_push_return_trace(ret, func,
+				     frame_pointer, retp))
+		goto out;
+
+	return 0;
+ out:
+	current->curr_ret_depth--;
+	return -EBUSY;
 }
 
 /* Retrieve a function return address to the trace stack on thread info.*/
@@ -257,7 +261,7 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
 	trace->func = current->ret_stack[index].func;
 	trace->calltime = current->ret_stack[index].calltime;
 	trace->overrun = atomic_read(&current->trace_overrun);
-	trace->depth = index;
+	trace->depth = current->curr_ret_depth;
 }
 
 /*
@@ -273,6 +277,7 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
 	trace.rettime = trace_clock_local();
 	barrier();
 	current->curr_ret_stack--;
+	current->curr_ret_depth--;
 	/*
 	 * The curr_ret_stack can be less than -1 only if it was
 	 * filtered out and it's about to return from the function.