[PATCH] drm/prime: fix drm_prime_add_buf_handle

[Simona Vetter]

On Fri, Jun 13, 2025 at 03:12:01PM +0200, Christian König wrote:
> It is possible through flink or IOCTLs like MODE_GETFB2 to create
> multiple handles for the same underlying GEM object.
> 
> But in prime we explicitely don't want to have multiple handles for the
> same DMA-buf. So just ignore it if a DMA-buf is exported with another
> handle.
> 
> This was made obvious by removing the extra check in
> drm_gem_prime_handle_to_dmabuf() to not add the handle if we could already
> find it in the housekeeping structures.
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>  drivers/gpu/drm/drm_prime.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 1d93b44c00c4..f5f30d947b61 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -113,6 +113,17 @@ static int drm_prime_add_buf_handle(struct drm_prime_file_private *prime_fpriv,
>  
>  		rb = *p;
>  		pos = rb_entry(rb, struct drm_prime_member, dmabuf_rb);
> +
> +		/*
> +		 * Just ignore the new handle if we already have an handle for
> +		 * this DMA-buf.
> +		 */
> +		if (dma_buf == pos->dma_buf) {
> +			dma_buf_put(dma_buf);
> +			kfree(member);
> +			return 0;

This feels a bit brittle, because this case should only be possible when
called from drm_gem_prime_handle_to_dmabuf and not from
drm_gem_prime_fd_to_handle() (where it would indicate a real race and
hence bug in our code).

I think  drm_gem_prime_fd_to_handle() should WARN_ON if it hits this case. 

Otherwise yes this is the functional change that I've missed :-/ Note that
there's no race in the original code, because it's all protected by the
file_priv->prime.lock. Which means I think you're claim that you've only
widened the race with your patch is wrong.

Cheers, Sima

> +
> +		}
>  		if (dma_buf > pos->dma_buf)
>  			p = &rb->rb_right;
>  		else
> -- 
> 2.34.1
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch