Re: [RFC PATCH 4/7] dma-buf: uapi: Mechanism to revoke DMABUFs via ioctl()

[Christian König]

On 2/26/26 21:22, Matt Evans wrote:
> Add a new dma-buf ioctl() op, DMA_BUF_IOCTL_REVOKE, connected to a new
> (optional) dma_buf_ops callback, revoke().  An exporter receiving this
> will _permanently_ revoke the DMABUF, meaning it can no longer be
> mapped/attached/mmap()ed.  It also guarantees that existing
> importers have been detached (e.g. via move_notify) and all mappings
> made inaccessible.
> 
> This is useful for lifecycle management in scenarios where a process
> has created a DMABUF representing a resource, then delegated it to
> a client process; access to the resource is revoked when the client is
> deemed "done", and the resource can be safely re-used elsewhere.

Well that means revoking from the importer side. That absolutely doesn't make sense to me.

Why would you do that?

Regards,
Christian.

> 
> Signed-off-by: Matt Evans <mattev@xxxxxxxx>
> ---
>  drivers/dma-buf/dma-buf.c    |  5 +++++
>  include/linux/dma-buf.h      | 22 ++++++++++++++++++++++
>  include/uapi/linux/dma-buf.h |  1 +
>  3 files changed, 28 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index edaa9e4ee4ae..b9b315317f2d 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -561,6 +561,11 @@ static long dma_buf_ioctl(struct file *file,
>         case DMA_BUF_IOCTL_IMPORT_SYNC_FILE:
>                 return dma_buf_import_sync_file(dmabuf, (const void __user *)arg);
>  #endif
> +       case DMA_BUF_IOCTL_REVOKE:
> +               if (dmabuf->ops->revoke)
> +                       return dmabuf->ops->revoke(dmabuf);
> +               else
> +                       return -EINVAL;
> 
>         default:
>                 return -ENOTTY;
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 0bc492090237..a68c9ad7aebd 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -277,6 +277,28 @@ struct dma_buf_ops {
> 
>         int (*vmap)(struct dma_buf *dmabuf, struct iosys_map *map);
>         void (*vunmap)(struct dma_buf *dmabuf, struct iosys_map *map);
> +
> +       /**
> +        * @revoke:
> +        *
> +        * This callback is invoked from a userspace
> +        * DMA_BUF_IOCTL_REVOKE operation, and requests that access to
> +        * the buffer is immediately and permanently revoked.  On
> +        * successful return, the buffer is not accessible through any
> +        * mmap() or dma-buf import.  The request fails if the buffer
> +        * is pinned; otherwise, the exporter marks the buffer as
> +        * inaccessible and uses the move_notify callback to inform
> +        * importers of the change.  The buffer is permanently
> +        * disabled, and the exporter must refuse all map, mmap,
> +        * attach, etc. requests.
> +        *
> +        * Returns:
> +        *
> +        * 0 on success, or a negative error code on failure:
> +        * -ENODEV if the associated device no longer exists/is closed.
> +        * -EBADFD if the buffer has already been revoked.
> +        */
> +       int (*revoke)(struct dma_buf *dmabuf);
>  };
> 
>  /**
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index 5a6fda66d9ad..84bf2dd2d0f3 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -178,5 +178,6 @@ struct dma_buf_import_sync_file {
>  #define DMA_BUF_SET_NAME_B     _IOW(DMA_BUF_BASE, 1, __u64)
>  #define DMA_BUF_IOCTL_EXPORT_SYNC_FILE _IOWR(DMA_BUF_BASE, 2, struct dma_buf_export_sync_file)
>  #define DMA_BUF_IOCTL_IMPORT_SYNC_FILE _IOW(DMA_BUF_BASE, 3, struct dma_buf_import_sync_file)
> +#define DMA_BUF_IOCTL_REVOKE   _IO(DMA_BUF_BASE, 4)
> 
>  #endif
> --
> 2.47.3
>