ring-buffer: Fix NULL pointer if rb_set_head_page() fails

commit 54f7be5b upstream.

The function rb_set_head_page() searches the list of ring buffer
pages for a the page that has the HEAD page flag set. If it does
not find it, it will do a WARN_ON(), disable the ring buffer and
return NULL, as this should never happen.

But if this bug happens to happen, not all callers of this function
can handle a NULL pointer being returned from it. That needs to be
fixed.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/trace/ring_buffer.c

@@ -2683,7 +2683,7 @@ unsigned long ring_buffer_oldest_event_ts(struct ring_buffer *buffer, int cpu)
 	unsigned long flags;
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct buffer_page *bpage;
-	unsigned long ret;
+	unsigned long ret = 0;
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return 0;
@@ -2698,6 +2698,7 @@ unsigned long ring_buffer_oldest_event_ts(struct ring_buffer *buffer, int cpu)
 		bpage = cpu_buffer->reader_page;
 	else
 		bpage = rb_set_head_page(cpu_buffer);
+	if (bpage)
 		ret = bpage->page->time_stamp;
 	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
@@ -3005,6 +3006,8 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 	 * Splice the empty reader page into the list around the head.
 	 */
 	reader = rb_set_head_page(cpu_buffer);
+	if (!reader)
+		goto out;
 	cpu_buffer->reader_page->list.next = rb_list_head(reader->list.next);
 	cpu_buffer->reader_page->list.prev = reader->list.prev;