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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <153010024207.8693.14587899562244751472@mail.alporthouse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018-06-27 01:50 PM, Chris Wilson wrote: > Quoting Michel Dänzer (2018-06-26 15:31:47) >> From: Michel Dänzer >> >> Fixes the BUG_ON spuriously triggering under the following >> circumstances: >> >> * ttm_eu_reserve_buffers processes a list containing multiple BOs using >> the same reservation object, so it calls >> reservation_object_reserve_shared with that reservation object once >> for each such BO. >> * In reservation_object_reserve_shared, old->shared_count == >> old->shared_max - 1, so obj->staged is freed in preparation of an >> in-place update. >> * ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence >> once for each of the BOs above, always with the same fence. >> * The first call adds the fence in the remaining free slot, after which >> old->shared_count == old->shared_max. >> >> In the next call to reservation_object_add_shared_fence, the BUG_ON >> triggers. However, nothing bad would happen in >> reservation_object_add_shared_inplace, since the fence is already in the >> reservation object. >> >> Prevent this by moving the BUG_ON to where an overflow would actually >> happen (e.g. if a buggy caller didn't call >> reservation_object_reserve_shared before). >> >> Cc: stable@vger.kernel.org >> Signed-off-by: Michel Dänzer > > I've convinced myself (or rather have not found a valid argument > against) that being able to call reserve_shared + add_shared multiple > times for the same fence is an intended part of reservation_object API > > I'd double check with Christian though. Right, I'm interested in Christian's feedback. > Reviewed-by: Chris Wilson Thanks! >> drivers/dma-buf/reservation.c | 6 +++--- >> 1 file changed, 3 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c >> index 314eb1071cce..532545b9488e 100644 >> --- a/drivers/dma-buf/reservation.c >> +++ b/drivers/dma-buf/reservation.c >> @@ -141,6 +141,7 @@ reservation_object_add_shared_inplace(struct reservation_object *obj, >> if (signaled) { >> RCU_INIT_POINTER(fobj->shared[signaled_idx], fence); >> } else { >> + BUG_ON(fobj->shared_count >= fobj->shared_max); > > Personally I would just let kasan detect this and throw away the BUG_ON > or at least move it behind some DMABUF_BUG_ON(). Hmm. Normally, I'm not a fan of BUG(_ON) either. But in this case, it's clear that the caller is buggy, and proceeding to write beyond the end of the array could have far-reaching consequences. I'm leaving that to somebody else. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer