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

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.

Cc: stable@vger.kernel.org # 3.0+
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b979426d16c..ec01803e0a5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1396,6 +1396,8 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
                 struct list_head *head_page_with_bit;
 
                 head_page = &rb_set_head_page(cpu_buffer)->list;
+                if (!head_page)
+                        break;
                 prev_page = head_page->prev;
 
                 first_page = pages->next;
@@ -2934,7 +2936,7 @@ 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;
@@ -2949,7 +2951,8 @@ 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);
-        ret = bpage->page->time_stamp;
+        if (bpage)
+                ret = bpage->page->time_stamp;
         raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
         return ret;
@@ -3260,6 +3263,8 @@ 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;