Re: [PATCH v2 1/4] dma-buf: Add dma_buf_try_get()

["Christian König" <christian.koenig@amd.com>]

Am 01.09.22 um 01:12 schrieb Jason Gunthorpe:
> Used to increment the refcount of the dma buf's struct file, only if the
> refcount is not zero. Useful to allow the struct file's lifetime to
> control the lifetime of the dmabuf while still letting the driver to keep
> track of created dmabufs.
>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   include/linux/dma-buf.h | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 71731796c8c3a8..a35f1554f2fb36 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -618,6 +618,19 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags);
>   struct dma_buf *dma_buf_get(int fd);
>   void dma_buf_put(struct dma_buf *dmabuf);
>   
> +/**
> + * dma_buf_try_get - try to get a reference on a dmabuf
> + * @dmabuf - the dmabuf to get
> + *
> + * Returns true if a reference was successfully obtained. The caller must
> + * interlock with the dmabuf's release function in some way, such as RCU, to
> + * ensure that this is not called on freed memory.

I still have a bad feeling about this, but I also see that we can only 
choose between evils here.

Could you just call get_file_rcu() from the exporter with a comment 
explaining why this works instead?

That would at least not give importers the opportunity to abuse this.

Thanks,
Christian.

> + */
> +static inline bool dma_buf_try_get(struct dma_buf *dmabuf)
> +{
> +	return get_file_rcu(dmabuf->file);
> +}
> +
>   struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
>   					enum dma_data_direction);
>   void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *,