[libcamera-devel] [PATCH] libcamera: v4l2_videodevice: Handle unexpected buffers
Kieran Bingham
kieran.bingham at ideasonboard.com
Thu Sep 9 17:08:03 CEST 2021
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.
Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
---
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);
--
2.30.2
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