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

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@ozlabs.org

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 *,