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Thu, 12 Mar 2026 16:13:00 +0000 (UTC) Received: from redhat.com (unknown [10.45.225.57]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3BADD19560B7; Thu, 12 Mar 2026 16:12:58 +0000 (UTC) Date: Thu, 12 Mar 2026 17:12:56 +0100 From: Kevin Wolf To: Peter Maydell Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org Subject: Re: [PULL 26/28] block/nfs: add support for libnfs v6 Message-ID: References: <20260310162622.333137-1-kwolf@redhat.com> <20260310162622.333137-27-kwolf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -3 X-Spam_score: -0.4 X-Spam_bar: / X-Spam_report: (-0.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.819, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.903, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 12.03.2026 um 10:41 hat Peter Maydell geschrieben: > On Tue, 10 Mar 2026 at 16:30, Kevin Wolf wrote: > > > > From: Peter Lieven > > > > libnfs v6 added a new api structure for read and write requests. > > > > This effectively also adds zero copy read support for cases where > > the qiov coming from the block layer has only one vector. > > > > The .brdv_refresh_limits implementation is needed because libnfs v6 > > silently dropped support for splitting large read/write request into > > chunks. > > > > Signed-off-by: Ronnie Sahlberg > > Signed-off-by: Peter Lieven > > Message-ID: <20260306142840.72923-1-pl@dlhnet.de> > > Reviewed-by: Kevin Wolf > > Signed-off-by: Kevin Wolf > > > Hi; Coverity reports a potential issue with this code > (CID 1645631): > > > @@ -266,13 +270,36 @@ static int coroutine_fn nfs_co_preadv(BlockDriverState *bs, int64_t offset, > > { > > NFSClient *client = bs->opaque; > > NFSRPC task; > > + char *buf = NULL; > > + bool my_buffer = false; > > > > nfs_co_init_task(bs, &task); > > - task.iov = iov; > > + > > +#ifdef LIBNFS_API_V2 > > + if (iov->niov != 1) { > > + buf = g_try_malloc(bytes); > > + if (bytes && buf == NULL) { > > + return -ENOMEM; > > + } > > + my_buffer = true; > > Here we have code that takes the "read zero bytes" case, and > still tries to malloc a 0-length buffer (which is of dubious > portability). Then it will continue... g_try_malloc() always returns NULL for allocating 0 bytes, so I don't think portability is a problem here. > > + } else { > > + buf = iov->iov[0].iov_base; > > + } > > +#endif > > > > WITH_QEMU_LOCK_GUARD(&client->mutex) { > > +#ifdef LIBNFS_API_V2 > > + if (nfs_pread_async(client->context, client->fh, > > + buf, bytes, offset, > > + nfs_co_generic_cb, &task) != 0) { > > +#else > > + task.iov = iov; > > if (nfs_pread_async(client->context, client->fh, > > offset, bytes, nfs_co_generic_cb, &task) != 0) { > > +#endif > > + if (my_buffer) { > > + g_free(buf); > > + } > > return -ENOMEM; > > } > > > > @@ -280,6 +307,13 @@ static int coroutine_fn nfs_co_preadv(BlockDriverState *bs, int64_t offset, > > } > > qemu_coroutine_yield(); > > > > + if (my_buffer) { > > + if (task.ret > 0) { > > + qemu_iovec_from_buf(iov, 0, buf, task.ret); > > ...and down here we use 'buf', but Coverity thinks it might be NULL > because we only returned -ENOMEM above for a NULL buffer if bytes == 0. > So we might be here with bytes == 0 and buf == NULL. Yes, buf might be NULL, but copying 0 bytes from NULL isn't a problem because you don't actually dereference the pointer then. I think the part that Coverity doesn't understand is probably that task.ret is limited to bytes (i.e. 0 in this case). > Maybe we can't get here, but maybe it would be simpler to handle > the "asked to read 0 bytes" case directly without calling into the > nfs library or allocating a 0 byte buffer? We could have an 'if (!bytes) { return 0; }' at the start of the function if we want to. Kevin