Re: [PATCH v6 3/9] dma-buf: Export dma_buf_set_name()

[Christian König]

On 9/11/26 23:41, Matt Evans wrote:
> dma_buf_set_name() originally took a __user string to duplicate for
> the buffer name.  Make this function a generic set-name helper, taking
> a kernel-allocated string.
> 
> Wrap this by a new dma_buf_set_name_user() to support the existing
> ioctl path for a user-provided string.

That just describes what the patch does and not why, in other words this needs a bit more justification for the change.

For example something like "Allow exporters/importers to set the name of buffers from pre-existing information".

It is also important to describe which role should set a name (exporter, importer or both). When an exporter gives a standard name for it's buffers that is most likely harmless, but when an importer bluntly overwrites a name previously set by an exporter or userspace then we really need a good reason for that.

> 
> Signed-off-by: Matt Evans <matt@xxxxxxxxxx>
> ---
>  drivers/dma-buf/dma-buf.c | 58 ++++++++++++++++++++++++++++++---------
>  include/linux/dma-buf.h   |  2 ++
>  2 files changed, 47 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d504c636dc29..8129ea11ff58 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -405,31 +405,29 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>  }
>  
>  /**
> - * 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.
> + * dma_buf_set_name_user - Set a dma_buf's name from a user string
> + *
> + * The string is up to DMA_BUF_NAME_LEN long, including the terminator.
>   *
>   * @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.
> - *
> + * Returns 0 on success, and any previously-set name is freed.
>   */

Kerneldoc for a static function is usually overkill.

And since this is used only once and very complicated I would completely merge the logic into dma_buf_ioctl().

Regards,
Christian.

> -static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> +static long dma_buf_set_name_user(struct dma_buf *dmabuf, const char __user *buf)
>  {
>  	char *name = strndup_user(buf, DMA_BUF_NAME_LEN);
> +	int ret;
>  
>  	if (IS_ERR(name))
>  		return PTR_ERR(name);
>  
> -	spin_lock(&dmabuf->name_lock);
> -	kfree(dmabuf->name);
> -	dmabuf->name = name;
> -	spin_unlock(&dmabuf->name_lock);
> +	ret = dma_buf_set_name(dmabuf, name);
> +	if (ret)
> +		kfree(name);
>  
> -	return 0;
> +	return ret;
>  }
>  
>  #if IS_ENABLED(CONFIG_SYNC_FILE)
> @@ -578,7 +576,7 @@ static long dma_buf_ioctl(struct file *file,
>  
>  	case DMA_BUF_SET_NAME_A:
>  	case DMA_BUF_SET_NAME_B:
> -		return dma_buf_set_name(dmabuf, (const char __user *)arg);
> +		return dma_buf_set_name_user(dmabuf, (const char __user *)arg);
>  
>  #if IS_ENABLED(CONFIG_SYNC_FILE)
>  	case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
> @@ -854,6 +852,40 @@ void dma_buf_put(struct dma_buf *dmabuf)
>  }
>  EXPORT_SYMBOL_NS_GPL(dma_buf_put, "DMA_BUF");
>  
> +/**
> + * dma_buf_set_name - Set a dma_buf's name
> + * 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.
> + * @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 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 d1203da56fc5..14d3950b63c8 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -570,6 +570,8 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags);
>  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 *,