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

[Matt Evans]

Hi Christian,

On 14/09/2026 12:06, Christian König wrote:
> 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".

Fair enough, am adding missing rationale.

> 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.

Good point; my expectation was that exporters would use this to either
set or update the name, and importers would never do this.  I will make
this clear in the commit message/comments, thanks.

>>
>> 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.

The original static function had some so I kept it, but no worries. Removed.

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

Will do (no function, no kerneldoc).


Many thanks,


Matt


> 
> 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 *,
>