[libcamera-devel] [PATCH] libcamera: v4l2_videodevice: close dmabuf fd

[Laurent Pinchart]

Hi Karthik,

Thank you for the patch.

On Thu, Oct 14, 2021 at 11:38:33PM +0000, Karthik Poduval via libcamera-devel wrote:
> 
> The dmabuf fd that comes from V4L2_EXPBUF is passed to FileDescriptor
> which makes a dup of the fd and stores it. However the original fd
> isn't closed and this causes the vb2's buf reference count to remain at
> 3 (one for kernel, one for V4L2_EXPBUF and one for the dup fd by File
> Descriptor). When requestbufs is called with zero count, it unable to
> free the buffer as the refcount is still 2. After FileDecriptor dup the
> fd the original fd must be closed so that the buf refcount is maintained
> properly by vb2.
> 
> Signed-off-by: Karthik Poduval <kpoduval at lab126.com>
> ---
>  src/libcamera/v4l2_videodevice.cpp | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index ba5f88cd..8c85da22 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -1384,6 +1384,7 @@ FileDescriptor V4L2VideoDevice::exportDmabufFd(unsigned int index,
>  {
>  	struct v4l2_exportbuffer expbuf = {};
>  	int ret;
> +	FileDescriptor fd;
>  
>  	expbuf.type = bufferType_;
>  	expbuf.index = index;
> @@ -1397,7 +1398,14 @@ FileDescriptor V4L2VideoDevice::exportDmabufFd(unsigned int index,
>  		return FileDescriptor();
>  	}
>  
> -	return FileDescriptor(std::move(expbuf.fd));
> +	fd = FileDescriptor(std::move(expbuf.fd));

With the std::move(), the following FileDescriptor constructor is
called:

FileDescriptor::FileDescriptor(int &&fd)
{
	if (fd < 0)
		return;

	fd_ = std::make_shared<Descriptor>(fd, false);
	/*
	 * The Descriptor constructor can't have failed here, as it took over
	 * the fd without duplicating it. Just set the original fd to -1 to
	 * implement move semantics.
	 */
	fd = -1;
}

The inner Descriptor constructor thus doesn't call dup(). After this
line, expbuf.fd will be equal to -1, so the close() call below will be a
no-op. Unless I'm missing something :-)

> +	/*
> +	 * since FileDesciptor makes a dup of the fd, original fd must be
> +	 * closed or else driver considers it as an orphaned buffer (due to
> +	 * additional buffer refcount) thus causing a frame buffer leak
> +	 */
> +	::close(expbuf.fd);
> +	return fd;
>  }
>  
>  /**

-- 
Regards,

Laurent Pinchart