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libcamera: v4l2_videodevice: Handle unexpected buffers

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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>
This commit is contained in:
Kieran Bingham 2021-09-09 16:08:03 +01:00
parent 3bcb7a90c1
commit 4cf7a8fc0b
1 changed files with 20 additions and 1 deletions
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21
src/libcamera/v4l2_videodevice.cpp
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@ -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);