Re: [PATCH] dma-buf: WARN on dmabuf release with pending attachments

[Christian König]

Am 23.07.21 um 14:31 schrieb Charan Teja Reddy:
> It is expected from the clients to follow the below steps on an imported
> dmabuf fd:
> a) dmabuf = dma_buf_get(fd) // Get the dmabuf from fd
> b) dma_buf_attach(dmabuf); // Clients attach to the dmabuf
>     o Here the kernel does some slab allocations, say for
> dma_buf_attachment and may be some other slab allocation in the
> dmabuf->ops->attach().
> c) Client may need to do dma_buf_map_attachment().
> d) Accordingly dma_buf_unmap_attachment() should be called.
> e) dma_buf_detach () // Clients detach to the dmabuf.
>     o Here the slab allocations made in b) are freed.
> f) dma_buf_put(dmabuf) // Can free the dmabuf if it is the last
> reference.
>
> Now say an erroneous client failed at step c) above thus it directly
> called dma_buf_put(), step f) above. Considering that it may be the last
> reference to the dmabuf, buffer will be freed with pending attachments
> left to the dmabuf which can show up as the 'memory leak'. This should
> at least be reported as the WARN().
>
> Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>

Good idea. I would expect a crash immediately, but from such a backtrace 
it is quite hard to tell what the problem is.

Patch is Reviewed-by: Christian König <christian.koenig@xxxxxxx> and I'm 
going to push this to drm-misc-next on Monday if nobody objects.

Thanks,
Christian.

> ---
>   drivers/dma-buf/dma-buf.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 511fe0d..733c8b1 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -79,6 +79,7 @@ static void dma_buf_release(struct dentry *dentry)
>   	if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
>   		dma_resv_fini(dmabuf->resv);
>   
> +	WARN_ON(!list_empty(&dmabuf->attachments));
>   	module_put(dmabuf->owner);
>   	kfree(dmabuf->name);
>   	kfree(dmabuf);