libcamera: v4l2_videodevice: Handle unexpected buffers

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 commit 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.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/v4l2_videodevice.cpp

@@ -1654,9 +1654,28 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
 
 	LOG(V4L2, Debug) << "Dequeuing buffer " << 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 kernel bug has been fixed in mainline, ensure that we
+	 * safely ingore buffers which are unexpected to prevetn crashes on
+	 * older kernels.
+	 */
+	auto it = queuedBuffers_.find(buf.index);
+	if (it == queuedBuffers_.end()) {
+		LOG(V4L2, Error)
+			<< "Dequeued unexpected buffer index " << buf.index;
+
+		return nullptr;
+	}
+
 	cache_->put(buf.index);
 
-	auto it = queuedBuffers_.find(buf.index);
 	FrameBuffer *buffer = it->second;
 	queuedBuffers_.erase(it);