From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomasz Stanislawski Subject: Re: [PATCH] dma-buf: add get_dma_buf() Date: Tue, 22 May 2012 17:00:31 +0200 Message-ID: <4FBBAA0F.6090503@samsung.com> References: <1331913881-13105-1-git-send-email-rob.clark@linaro.org> <4FBB98E0.8040600@samsung.com> <20120522143234.GC4629@phenom.ffwll.local> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7BIT Return-path: In-reply-to: <20120522143234.GC4629@phenom.ffwll.local> Sender: linux-media-owner@vger.kernel.org To: Daniel Vetter Cc: Rob Clark , linaro-mm-sig@lists.linaro.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org, patches@linaro.org, sumit.semwal@linaro.org, airlied@redhat.com, Rob Clark , Marek Szyprowski , =?UTF-8?B?J+uwleqyveuvvCc=?= , InKi Dae List-Id: dri-devel@lists.freedesktop.org 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? Regards, Tomasz Stanislawski