On Mon, Nov 16, 2015 at 02:10:36PM +0800, Fam Zheng wrote: > When a request R is absorbed by request M, it is appended to the > "mr_next" queue led by M, and is completed together with the completion > of M, in virtio_blk_rw_complete. > > With error policy equals stop, if M has an I/O error, now R also gets > prepended to the per device DMA restart queue, which will be retried > when VM resumes. It leads to a double completion (in symptoms of memory > corruption or use after free). > > Adding R to the queue is superfluous, only M needs to be in the queue. > > Fix this by marking request R as "merged" and skipping it in > virtio_blk_handle_rw_error. > > Cc: qemu-sta...@nongnu.org > Signed-off-by: Fam Zheng <f...@redhat.com> > > --- > > v2: Don't lose the request in migration. [Paolo] > --- > hw/block/virtio-blk.c | 7 +++++++ > include/hw/virtio/virtio-blk.h | 1 + > 2 files changed, 8 insertions(+) > > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c > index e70fccf..5cdb06f 100644 > --- a/hw/block/virtio-blk.c > +++ b/hw/block/virtio-blk.c > @@ -36,6 +36,7 @@ VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s) > req->in_len = 0; > req->next = NULL; > req->mr_next = NULL; > + req->merged = false; > return req; > } > > @@ -344,6 +345,7 @@ static inline void submit_requests(BlockBackend *blk, > MultiReqBuffer *mrb, > for (i = start + 1; i < start + num_reqs; i++) { > qemu_iovec_concat(qiov, &mrb->reqs[i]->qiov, 0, > mrb->reqs[i]->qiov.size); > + mrb->reqs[i]->merged = true; > mrb->reqs[i - 1]->mr_next = mrb->reqs[i]; > nb_sectors += mrb->reqs[i]->qiov.size / BDRV_SECTOR_SIZE; > } > @@ -511,6 +513,11 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, > MultiReqBuffer *mrb) > - sizeof(struct virtio_blk_inhdr); > iov_discard_back(in_iov, &in_num, sizeof(struct virtio_blk_inhdr)); > > + if (req->merged) { > + /* Enough for restarting a (migrated) merged request, no need to > + * actually submit I/O. */ > + return; > + } > type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type); > > /* VIRTIO_BLK_T_OUT defines the command direction. VIRTIO_BLK_T_BARRIER > diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h > index 6bf5905..db4adf4 100644 > --- a/include/hw/virtio/virtio-blk.h > +++ b/include/hw/virtio/virtio-blk.h > @@ -70,6 +70,7 @@ typedef struct VirtIOBlockReq { > size_t in_len; > struct VirtIOBlockReq *next; > struct VirtIOBlockReq *mr_next; > + bool merged; > BlockAcctCookie acct; > } VirtIOBlockReq;
I'm not sure if this patch truly fixes the bug: virtio_blk_rw_complete() doesn't do req->mr_next = NULL. There is a potential double-free if resubmitting failed requests doesn't overwrite themr_next field. This can be fixed by adding req->mr_next = NULL to the loop in virtio_blk_rw_complete(). Is that enough to solve the bug? I don't think adding a new field is necessary. If not, please explain the double-free.
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