Patch "ftrace: Check for null ret_stack on profile function graph entry function" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ftrace: Check for null ret_stack on profile function graph entry function

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ftrace-check-for-null-ret_stack-on-profile-function-graph-entry-function.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Steven Rostedt (VMware)" <[email protected]>
Date: Thu, 17 Aug 2017 16:37:25 -0400
Subject: ftrace: Check for null ret_stack on profile function graph entry function

From: Steven Rostedt (VMware) <[email protected]>

commit a8f0f9e49956a74718874b800251455680085600 upstream.

There's a small race when function graph shutsdown and the calling of the registered function graph entry callback. The callback must not reference the task's ret_stack without first checking that it is not NULL. Note, when a ret_stack is allocated for a task, it stays allocated until the task exits. The problem here, is that function_graph is shutdown, and a new task was created, which doesn't have its ret_stack allocated. But since some of the functions are still being traced, the callbacks can still be called.

The normal function_graph code handles this, but starting with commit 8861dd303c ("ftrace: Access ret_stack->subtime only in the function profiler") the profiler code references the ret_stack on function entry, but doesn't check if it is NULL first.

---
 kernel/trace/ftrace.c |    4 ++++
 1 file changed, 4 insertions(+)

@@ -876,6 +876,10 @@ static int profile_graph_entry(struct ft
 
 	function_profile_call(trace->func, 0, NULL, NULL);
 
+	/* If function graph is shutting down, ret_stack can be NULL */
+	if (!current->ret_stack)
+		return 0;
+
 	if (index >= 0 && index < FTRACE_RETFUNC_DEPTH)
 		current->ret_stack[index].subtime = 0;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ftrace-check-for-null-ret_stack-on-profile-function-graph-entry-function.patch queue-4.9/tracing-fix-freeing-of-filter-in-create_filter-when-set_str-is-false.patch queue-4.9/tracing-call-clear_boot_tracer-at-lateinit_sync.patch queue-4.9/tracing-fix-kmemleak-in-tracing_map_array_free.patch