[PATCH v2] drm/gem: Internally test import_attach for imported objects

[Boris Brezillon]

On Tue, 15 Apr 2025 16:02:20 +0200
Thomas Zimmermann <tzimmermann at suse.de> wrote:

> Test struct drm_gem_object.import_attach.dmabuf to detect imported
> objects. Warn if the stored state is inconsistent.
> 
> During object clenaup, the dma_buf field might be NULL. Testing it in
> an object's free callback then incorrectly does a cleanup as for native
> objects. Happens for calls to drm_mode_destroy_dumb_ioctl() that
> clears the dma_buf field in drm_gem_object_exported_dma_buf_free().
> 
> v2:
> - use import_attach.dmabuf instead of dma_buf (Christian)
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> Fixes: b57aa47d39e9 ("drm/gem: Test for imported GEM buffers with helper")
> Reported-by: Andy Yan <andyshrk at 163.com>
> Closes: https://lore.kernel.org/dri-devel/38d09d34.4354.196379aa560.Coremail.andyshrk@163.com/
> Tested-by: Andy Yan <andyshrk at 163.com>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: Anusha Srivatsa <asrivats at redhat.com>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: David Airlie <airlied at gmail.com>
> Cc: Simona Vetter <simona at ffwll.ch>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> Cc: "Christian König" <christian.koenig at amd.com>
> Cc: dri-devel at lists.freedesktop.org
> Cc: linux-media at vger.kernel.org
> Cc: linaro-mm-sig at lists.linaro.org
> ---
>  include/drm/drm_gem.h | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 9b71f7a9f3f8..464b9c7feec0 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -579,6 +579,21 @@ static inline bool drm_gem_object_is_shared_for_memory_stats(struct drm_gem_obje
>  	return (obj->handle_count > 1) || obj->dma_buf;
>  }
>  
> +/**
> + * drm_gem_owns_dma_buf() - Tests if GEM object backs a DMA-buffer object
> + * @obj: the GEM object
> + * @obj: the DMA buffer
> + *
> + * Returns:
> + * True if the DMA buffer refers to the GEM object's buffer.
> + */
> +static inline bool drm_gem_owns_dma_buf(const struct drm_gem_object *obj,
> +					const struct dma_buf *dma_buf)
> +{
> +	/* The dma-buf's priv field points to the original GEM object. */
> +	return dma_buf->priv == obj;
> +}
> +
>  /**
>   * drm_gem_is_imported() - Tests if GEM object's buffer has been imported
>   * @obj: the GEM object
> @@ -588,8 +603,15 @@ static inline bool drm_gem_object_is_shared_for_memory_stats(struct drm_gem_obje
>   */
>  static inline bool drm_gem_is_imported(const struct drm_gem_object *obj)
>  {
> -	/* The dma-buf's priv field points to the original GEM object. */
> -	return obj->dma_buf && (obj->dma_buf->priv != obj);
> +	const struct dma_buf *dma_buf = NULL;
> +
> +	if (!obj->import_attach)
> +		return false;
> +
> +	dma_buf = obj->import_attach->dmabuf;
> +
> +	/* Warn if we somehow reimported our own buffer. */
> +	return !drm_WARN_ON_ONCE(obj->dev, !dma_buf || drm_gem_owns_dma_buf(obj, dma_buf));

I'm honestly not sure I see the point of checking
obj->import_attach->dmabuf. If obj->import_attach != NULL, we're sure
it's a foreign buffer already, otherwise we would get the original GEM
object which has ->import_attach=NULL. So why not go for a simple

	return obj->import_attach != NULL;

check, and extend the check when you get to implement imports without
import attachments (not sure what those would look like BTW).