| Message ID | 1331913881-13105-1-git-send-email-rob.clark@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Fri, Mar 16, 2012 at 4:04 PM, Rob Clark <rob.clark@linaro.org> wrote: > From: Rob Clark <rob@ti.com> > > Works in a similar way to get_file(), and is needed in cases such as > when the exporter needs to also keep a reference to the dmabuf (that > is later released with a dma_buf_put()), and possibly other similar > cases. > > Signed-off-by: Rob Clark <rob@ti.com> Reviewed-by: Dave Airlie <airlied@redhat.com>
On 16 March 2012 23:23, Dave Airlie <airlied@gmail.com> wrote: > On Fri, Mar 16, 2012 at 4:04 PM, Rob Clark <rob.clark@linaro.org> wrote: >> From: Rob Clark <rob@ti.com> >> >> Works in a similar way to get_file(), and is needed in cases such as >> when the exporter needs to also keep a reference to the dmabuf (that >> is later released with a dma_buf_put()), and possibly other similar >> cases. >> >> Signed-off-by: Rob Clark <rob@ti.com> > > Reviewed-by: Dave Airlie <airlied@redhat.com> > Thanks; pulled into for-next. BR, ~me. > _______________________________________________ > Linaro-mm-sig mailing list > Linaro-mm-sig@lists.linaro.org > http://lists.linaro.org/mailman/listinfo/linaro-mm-sig
On Sun, Mar 18, 2012 at 01:12:22PM +0530, Sumit Semwal wrote: > On 16 March 2012 23:23, Dave Airlie <airlied@gmail.com> wrote: > > On Fri, Mar 16, 2012 at 4:04 PM, Rob Clark <rob.clark@linaro.org> wrote: > >> From: Rob Clark <rob@ti.com> > >> > >> Works in a similar way to get_file(), and is needed in cases such as > >> when the exporter needs to also keep a reference to the dmabuf (that > >> is later released with a dma_buf_put()), and possibly other similar > >> cases. > >> > >> Signed-off-by: Rob Clark <rob@ti.com> > > > > Reviewed-by: Dave Airlie <airlied@redhat.com> > > > Thanks; pulled into for-next. I'm back from vacation and already grumpily complaining about dma-buf patches ;-) For consistency with dma_buf_put we should call this dma_buf_get instead of get_dma_buf ... I'll write a bikeshed patch on top of your tree. -Daniel
On Sun, Mar 18, 2012 at 08:04:53PM +0100, Daniel Vetter wrote: > On Sun, Mar 18, 2012 at 01:12:22PM +0530, Sumit Semwal wrote: > > On 16 March 2012 23:23, Dave Airlie <airlied@gmail.com> wrote: > > > On Fri, Mar 16, 2012 at 4:04 PM, Rob Clark <rob.clark@linaro.org> wrote: > > >> From: Rob Clark <rob@ti.com> > > >> > > >> Works in a similar way to get_file(), and is needed in cases such as > > >> when the exporter needs to also keep a reference to the dmabuf (that > > >> is later released with a dma_buf_put()), and possibly other similar > > >> cases. > > >> > > >> Signed-off-by: Rob Clark <rob@ti.com> > > > > > > Reviewed-by: Dave Airlie <airlied@redhat.com> > > > > > Thanks; pulled into for-next. > > I'm back from vacation and already grumpily complaining about dma-buf > patches ;-) For consistency with dma_buf_put we should call this > dma_buf_get instead of get_dma_buf ... I'll write a bikeshed patch on top > of your tree. Oops, there's already a dma_buf_get around - Rob and Dave pointed that out on irc to dense me. And I can't come up with a saner naming scheme. I'll retract my bikeshed. -Daniel
On Tue, May 22, 2012 at 4:05 PM, Daniel Vetter <daniel@ffwll.ch> wrote: > On Tue, May 22, 2012 at 5:00 PM, Tomasz Stanislawski > <t.stanislaws@samsung.com> 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 ;-) Now I've thought about it and Tomasz has pointed it out I agree, Now we just have to work out when to drop that reference, which I don't see anyone addressing :-) I love lifetime rules. Dave.
On Tue, May 22, 2012 at 9:13 AM, Dave Airlie <airlied@gmail.com> wrote: > On Tue, May 22, 2012 at 4:05 PM, Daniel Vetter <daniel@ffwll.ch> wrote: >> On Tue, May 22, 2012 at 5:00 PM, Tomasz Stanislawski >> <t.stanislaws@samsung.com> 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 ;-) > > Now I've thought about it and Tomasz has pointed it out I agree, > > Now we just have to work out when to drop that reference, which I > don't see anyone addressing :-) > > I love lifetime rules. I think in the GEM case, where we keep a ref in obj->export_dma_buf, we can drop the extra ref to the dmabuf (if we decide dma_buf_fd() increases the refcnt), as long as we be sure to NULL out obj->export_dma_buf from dmabuf_ops->release (which is unfortunately in each driver at the moment).. This way obj->export_dma_buf behaves as a sort of weak-reference, not causing a memory leak. BR, -R > Dave.
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h index cbdff81..25eb287 100644 --- a/include/linux/dma-buf.h +++ b/include/linux/dma-buf.h @@ -132,6 +132,20 @@ struct dma_buf_attachment { void *priv; }; +/** + * get_dma_buf - convenience wrapper for get_file. + * @dmabuf: [in] pointer to dma_buf + * + * Increments the reference count on the dma-buf, needed in case of drivers + * that either need to create additional references to the dmabuf on the + * kernel side. For example, an exporter that needs to keep a dmabuf ptr + * so that subsequent exports don't create a new dmabuf. + */ +static inline void get_dma_buf(struct dma_buf *dmabuf) +{ + get_file(dmabuf->file); +} + #ifdef CONFIG_DMA_SHARED_BUFFER struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev);