Re: [PATCH v2] dma-buf: remove restriction of IOCTL:DMA_BUF_SET_NAME

[Sumit Semwal <sumit.semwal@linaro.org>]

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Hello Guangming, Christian,



On Tue, 12 Oct 2021, 14:09 , <guangming.cao@mediatek.com> wrote:

> From: Guangming Cao <Guangming.Cao@mediatek.com>
>
> > Am 09.10.21 um 07:55 schrieb guangming.cao@mediatek.com:
> > From: Guangming Cao <Guangming.Cao@mediatek.com>
> > >
> > > If dma-buf don't want userspace users to touch the dmabuf buffer,
> > > it seems we should add this restriction into dma_buf_ops.mmap,
> > > not in this IOCTL:DMA_BUF_SET_NAME.
> > >
> > > With this restriction, we can only know the kernel users of the dmabuf
> > > by attachments.
> > > However, for many userspace users, such as userpsace users of dma_heap,
> > > they also need to mark the usage of dma-buf, and they don't care about
> > > who attached to this dmabuf, and seems it's no meaning to be waiting
> for
> > > IOCTL:DMA_BUF_SET_NAME rather than mmap.
> >
> > Sounds valid to me, but I have no idea why this restriction was added in
> > the first place.
> >
> > Can you double check the git history and maybe identify when that was
> > added? Mentioning this change in the commit message then might make
> > things a bit easier to understand.
> >
> > Thanks,
> > Christian.
> It was add in this patch: https://patchwork.freedesktop.org/patch/310349/.
> However, there is no illustration about it.
> I guess it wants users to set_name when no attachments on the dmabuf,
> for case with attachments, we can find owner by device in attachments.
> But just I said in commit message, this is might not a good idea.
>
> Do you have any idea?
>

For the original series, the idea was that allowing name change mid-use
could confuse the users about the dma-buf. However, the rest of the series
also makes sure each dma-buf have a unique inode, and any accounting should
probably use that, without relying on the name as much.
So I don't have an objection to this change.

Best,
Sumit.

> >
> > >
> > > Signed-off-by: Guangming Cao <Guangming.Cao@mediatek.com>
> > > ---
> > >   drivers/dma-buf/dma-buf.c | 14 ++------------
> > >   1 file changed, 2 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > > index 511fe0d217a0..db2f4efdec32 100644
> > > --- a/drivers/dma-buf/dma-buf.c
> > > +++ b/drivers/dma-buf/dma-buf.c
> > > @@ -325,10 +325,8 @@ 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.
> > > - * The name of the dma-buf buffer can only be set when the dma-buf is
> not
> > > - * attached to any devices. It could theoritically support changing
> the
> > > - * name of the dma-buf if the same piece of memory is used for
> multiple
> > > - * purpose between different devices.
> > > + * It could theoretically 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
> > > @@ -346,19 +344,11 @@ static long dma_buf_set_name(struct dma_buf
> *dmabuf, const char __user *buf)
> > >     if (IS_ERR(name))
> > >             return PTR_ERR(name);
> > >
> > > -   dma_resv_lock(dmabuf->resv, NULL);
> > > -   if (!list_empty(&dmabuf->attachments)) {
> > > -           ret = -EBUSY;
> > > -           kfree(name);
> > > -           goto out_unlock;
> > > -   }
> > >     spin_lock(&dmabuf->name_lock);
> > >     kfree(dmabuf->name);
> > >     dmabuf->name = name;
> > >     spin_unlock(&dmabuf->name_lock);
> > >
> > > -out_unlock:
> > > -   dma_resv_unlock(dmabuf->resv);
> > >     return ret;
> > >   }
> > >
>

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