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

commit 54f7be5b831254199522523ccab4c3d954bbf576 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>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f5b7b5c..6fdc629 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/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;
@@ -2698,7 +2698,8 @@ 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;
@@ -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;