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    Subject[v3.2-v3.4 stable version][PATCH 1/2] ring-buffer: Fix NULL pointer if rb_set_head_page() fails
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    Date
    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>
    ---
    kernel/trace/ring_buffer.c | 9 +++++++--
    1 files changed, 7 insertions(+), 2 deletions(-)

    Index: linux-test.git/kernel/trace/ring_buffer.c
    ===================================================================
    --- linux-test.git.orig/kernel/trace/ring_buffer.c
    +++ linux-test.git/kernel/trace/ring_buffer.c
    @@ -2683,7 +2683,7 @@ unsigned long ring_buffer_oldest_event_t
    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_t
    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_pe
    * 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;




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