[libcamera-devel] [PATCH v2] libcamera: v4l2_videodevice: Handle unexpected buffers

[Kieran Bingham]

Sorry - this patch was already posted, so this is in fact v2.

On 09/09/2021 16:08, Kieran Bingham wrote:
> A kernel bug can lead to unexpected buffers being dequeued where we
> haven't entered the buffer in our queuedBuffers_ list.
> 
> This causes invalid accesses if not handled correctly within libcamera,
> and while it is a kernel issue, we can protect against unpatched
> kernels to provide a more suitable error message.
> 
> This is fixed in the kernel by c592b46907ad ("media: videobuf2-core:
> dequeue if start_streaming fails") [0]
> 
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c592b46907ad
> 
> Handle unexpected buffers by returning a nullptr, and move cache
> management after the validation of the buffer.
> 

Paul had already provided this:

Reviewed-by: Paul Elder <paul.elder at ideasonboard.com>

> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> ---

v2:
 - Update commit message to reference the now upstream kernel fix.
 - Fix spelling errors


>  src/libcamera/v4l2_videodevice.cpp | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index 837a59d9bae2..7bb28aea357a 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -1654,9 +1654,28 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
>  
>  	LOG(V4L2, Debug) << "Dequeuing buffer " << buf.index;
>  
> +	auto it = queuedBuffers_.find(buf.index);
> +	/*
> +	 * If the video node fails to stream-on successfully (which can occur
> +	 * when queuing a buffer), a vb2 kernel bug can lead to the buffer which
> +	 * returns a failure upon queuing, being mistakenly kept in the kernel.
> +	 * This leads to the kernel notifying us that a buffer is available to
> +	 * dequeue, which we have no awareness of being queued, and thus we will
> +	 * not find it in the queuedBuffers_ list.
> +	 *
> +	 * Whilst this is a kernel bug and should be fixed there, ensure that we
> +	 * safely ignore buffers which are unexpected to prevent crashes on
> +	 * unpatched kernels.
> +	 */
> +	if (it == queuedBuffers_.end()) {
> +		LOG(V4L2, Error)
> +			<< "Dequeued an unexpected buffer: " << buf.index;
> +
> +		return nullptr;
> +	}
> +
>  	cache_->put(buf.index);
>  
> -	auto it = queuedBuffers_.find(buf.index);
>  	FrameBuffer *buffer = it->second;
>  	queuedBuffers_.erase(it);
>  
>