Re: [PATCH] dma-buf: add get_dma_buf()

[Daniel Vetter <daniel@xxxxxxxx>]

On Tue, May 22, 2012 at 5:00 PM, Tomasz Stanislawski
<t.stanislaws@xxxxxxxxxxx> wrote:
> On 05/22/2012 04:32 PM, Daniel Vetter wrote:
>> On Tue, May 22, 2012 at 03:47:12PM +0200, Tomasz Stanislawski wrote:
>>> Hi,
>>> I think I discovered an interesting issue with dma_buf.
>>> I found out that dma_buf_fd does not increase reference
>>> count for dma_buf::file. This leads to potential kernel
>>> crash triggered by user space. Please, take a look on
>>> the scenario below:
>>>
>>> The applications spawns two thread. One of them is exporting DMABUF.
>>>
>>>       Thread I         |   Thread II       | Comments
>>> -----------------------+-------------------+-----------------------------------
>>> dbuf = dma_buf_export  |                   | dma_buf is creates, refcount is 1
>>> fd = dma_buf_fd(dbuf)  |                   | assume fd is set to 42, refcount is still 1
>>>                        |      close(42)    | The file descriptor is closed asynchronously, dbuf's refcount drops to 0
>>>                        |  dma_buf_release  | dbuf structure is freed, dbuf becomes a dangling pointer
>>> int size = dbuf->size; |                   | the dbuf is dereferenced, causing a kernel crash
>>> -----------------------+-------------------+-----------------------------------
>>>
>>> I think that the problem could be fixed in two ways.
>>> a) forcing driver developer to call get_dma_buf just before calling dma_buf_fd.
>>> b) increasing dma_buf->file's reference count at dma_buf_fd
>>>
>>> I prefer solution (b) because it prevents symmetry between dma_buf_fd and close.
>>> I mean that dma_buf_fd increases reference count, close decreases it.
>>>
>>> What is your opinion about the issue?
>>
>> I guess most exporters would like to hang onto the exported dma_buf a bit
>> and hence need a reference (e.g. to cache the dma_buf as long as the
>> underlying buffer object exists). So I guess we can change the semantics
>> of dma_buf_fd from transferring the reference you currently have (and
>> hence forbidding any further access by the caller) to grabbing a reference
>> of it's on for the fd that is created.
>> -Daniel
>
> Hi Daniel,
> Would it be simpler, safer and more intuitive if dma_buf_fd increased
> dmabuf->file's reference counter?

That's actually what I wanted to say. Message seems to have been lost
in transit ;-)
-Daniel
-- 
Daniel Vetter
daniel.vetter@xxxxxxxx - +41 (0) 79 364 57 48 - http://blog.ffwll.ch
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