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SubjectRe: [PATCH 2/2] ring-buffer: Fix race between integrity check and readers
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On Fri, 2012-11-30 at 11:12 -0500, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.com>
>
> The function rb_check_pages() was added to make sure the ring buffer's
> pages were sane. This check is done when the ring buffer size is modified
> as well as when the iterator is released (closing the "trace" file),
> as that was considered a non fast path and a good place to do a sanity
> check.
>
> The problem is that the check does not have any locks around it.
> If one process were to read the trace file, and another were to read
> the raw binary file, the check could happen while the reader is reading
> the file.
>
> The issues with this is that the check requires to clear the HEAD page
> before doing the full check and it restores it afterward. But readers
> require the HEAD page to exist before it can read the buffer, otherwise
> it gives a nasty warning and disables the buffer.
>
> By adding the reader lock around the check, this keeps the race from
> happening.
>
> Cc: stable@vger.kernel.org # 3.6

Again, quilt failed to Cc stable :-(

-- Steve

> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
> kernel/trace/ring_buffer.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index ec01803..4cb5e51 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3783,12 +3783,17 @@ void
> ring_buffer_read_finish(struct ring_buffer_iter *iter)
> {
> struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
> + unsigned long flags;
>
> /*
> * Ring buffer is disabled from recording, here's a good place
> - * to check the integrity of the ring buffer.
> + * to check the integrity of the ring buffer.
> + * Must prevent readers from trying to read, as the check
> + * clears the HEAD page and readers require it.
> */
> + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> rb_check_pages(cpu_buffer);
> + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>
> atomic_dec(&cpu_buffer->record_disabled);
> atomic_dec(&cpu_buffer->buffer->resize_disabled);




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