Re: [PATCH v7 3/9] dma-buf: Provide dma_buf_set_name()

[Christian König]

On 9/24/26 17:21, Matt Evans wrote:
> Allow exporters to set the name of buffers, during export or for
> subsequent updates.  This is useful to express buffer-specific debug
> information, indicate state particular to the exporter's usage, etc.
> Only exporters and userspace are expected to set names; it isn't
> sensible for importers to do it.
> 
> Originally, the static dma_buf_set_name() was used for
> DMA_BUF_SET_NAME ioctls, taking a __user string parameter.  Export
> this function as a generic set-name helper, taking a kernel-allocated
> string.  The SET_NAME ioctls now locally duplicate the __user string,
> and use the new function to set the name.
> 
> Signed-off-by: Matt Evans <matt@xxxxxxxxxx>
> ---
>  drivers/dma-buf/dma-buf.c | 78 ++++++++++++++++++++++++---------------
>  include/linux/dma-buf.h   |  2 +
>  2 files changed, 50 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 4c9add51f9ef..e4ec504e9288 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -404,34 +404,6 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>  	return events;
>  }
>  
> -/**
> - * dma_buf_set_name - Set a name to a specific dma_buf to track the usage.
> - * It could support changing the name of the dma-buf if the same
> - * piece of memory is used for multiple purpose between different devices.
> - *
> - * @dmabuf: [in]     dmabuf buffer that will be renamed.
> - * @buf:    [in]     A piece of userspace memory that contains the name of
> - *                   the dma-buf.
> - *
> - * Returns 0 on success. If the dma-buf buffer is already attached to
> - * devices, return -EBUSY.
> - *
> - */
> -static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> -{
> -	char *name = strndup_user(buf, DMA_BUF_NAME_LEN);
> -
> -	if (IS_ERR(name))
> -		return PTR_ERR(name);
> -
> -	spin_lock(&dmabuf->name_lock);
> -	kfree(dmabuf->name);
> -	dmabuf->name = name;
> -	spin_unlock(&dmabuf->name_lock);
> -
> -	return 0;
> -}
> -
>  #if IS_ENABLED(CONFIG_SYNC_FILE)
>  static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
>  				     void __user *user_data)
> @@ -577,8 +549,19 @@ static long dma_buf_ioctl(struct file *file,
>  		return ret;
>  
>  	case DMA_BUF_SET_NAME_A:
> -	case DMA_BUF_SET_NAME_B:
> -		return dma_buf_set_name(dmabuf, (const char __user *)arg);
> +	case DMA_BUF_SET_NAME_B: {
> +		char *name = strndup_user((const char __user *)arg,
> +					  DMA_BUF_NAME_LEN);
> +
> +		if (IS_ERR(name))
> +			return PTR_ERR(name);
> +
> +		ret = dma_buf_set_name(dmabuf, name);
> +		if (ret)
> +			kfree(name);
> +
> +		return ret;
> +	}
>  
>  #if IS_ENABLED(CONFIG_SYNC_FILE)
>  	case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
> @@ -874,6 +857,41 @@ void dma_buf_put(struct dma_buf *dmabuf)
>  }
>  EXPORT_SYMBOL_NS_GPL(dma_buf_put, "DMA_BUF");
>  
> +/**
> + * dma_buf_set_name - Set a dmabuf's name
> + * Intended to be used by the exporter to set a name for debug
> + * purposes.  This can also change an existing name if the same piece
> + * of memory is used for multiple purposes over time.

Add something like "Importers should *NOT* use this function to overwrite the name." here and add some note under "DOC: locking convention" how this function is supposed to be called.

Apart from that looks good to me,
Christian.

> + *
> + * @dmabuf: [in]     dmabuf buffer that will be renamed.
> + * @name:   [in]     The name of the dma-buf, allocated with kmalloc() or
> + *                   similar.  This takes ownership of the allocation
> + *                   on success, which will be kfree()d when the
> + *                   dmabuf is released or a new name assigned.
> + *
> + * Returns 0 on success, -EINVAL if the name is NULL, or -E2BIG if the
> + * name (including terminator) exceeds DMA_BUF_NAME_LEN.
> + */
> +int dma_buf_set_name(struct dma_buf *dmabuf, char *name)
> +{
> +	if (!name)
> +		return -EINVAL;
> +
> +	/* dmabuffs_dname() won't use the string if the length
> +	 * (including terminator) exceeds DMA_BUF_NAME_LEN:
> +	 */
> +	if (strlen(name) >= DMA_BUF_NAME_LEN)
> +		return -E2BIG;
> +
> +	spin_lock(&dmabuf->name_lock);
> +	kfree(dmabuf->name);
> +	dmabuf->name = name;
> +	spin_unlock(&dmabuf->name_lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_set_name, "DMA_BUF");
> +
>  static int dma_buf_wrap_sg_table(struct sg_table **sg_table)
>  {
>  	struct scatterlist *to_sg, *from_sg;
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index d15b2b31d3c9..952a2c196ad4 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -571,6 +571,8 @@ void dma_buf_fd_install(struct dma_buf *dmabuf, int fd);
>  struct dma_buf *dma_buf_get(int fd);
>  void dma_buf_put(struct dma_buf *dmabuf);
>  
> +int dma_buf_set_name(struct dma_buf *dmabuf, char *name);
> +
>  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 *,