[PATCH v2] vhost: fix build

[Min Zhou <zhoumin@loongson.cn>]

On CentOS 8 or Debian 10.4 systems using gcc 12.1 to cross
compile DPDK, gcc shows a following warning which will cause
build to fail when build is run with -werror:

In function 'mbuf_to_desc',
    inlined from 'vhost_enqueue_async_packed' at ../lib/vhost/virtio_net.c:1826:6,
    inlined from 'virtio_dev_rx_async_packed' at ../lib/vhost/virtio_net.c:1840:6,
    inlined from 'virtio_dev_rx_async_submit_packed.constprop' at ../lib/vhost/virtio_net.c:1900:7:
../lib/vhost/virtio_net.c:1161:35: error: 'buf_vec[0].buf_len' may be used uninitialized [-Werror=maybe-uninitialized]
 1161 |         buf_len = buf_vec[vec_idx].buf_len;
      |                   ~~~~~~~~~~~~~~~~^~~~~~~~
../lib/vhost/virtio_net.c: In function 'virtio_dev_rx_async_submit_packed.constprop':
../lib/vhost/virtio_net.c:1838:27: note: 'buf_vec' declared here
 1838 |         struct buf_vector buf_vec[BUF_VECTOR_MAX];
      |                           ^~~~~~~
cc1: all warnings being treated as errors

Actually, there are eight places to see the same codes in the file
lib/vhost/virtio_net.c, and all these `buf_vec` arraies are
initialized by sub-function calls under various conditions.

Although It's hard to understand why gcc just emits warning at one
of the eight places, adding validity checks for array length is
reasonable and can also fix the warning.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Min Zhou <zhoumin@loongson.cn>
---
 lib/vhost/virtio_net.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index 35fa4670fd..99233f1759 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -1153,7 +1153,7 @@ mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	struct virtio_net_hdr_mrg_rxbuf tmp_hdr, *hdr = NULL;
 	struct vhost_async *async = vq->async;
 
-	if (unlikely(m == NULL))
+	if (unlikely(m == NULL || nr_vec == 0))
 		return -1;
 
 	buf_addr = buf_vec[vec_idx].buf_addr;
@@ -2673,6 +2673,9 @@ desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	struct vhost_async *async = vq->async;
 	struct async_inflight_info *pkts_info;
 
+	if (unlikely(nr_vec == 0))
+		return -1;
+
 	buf_addr = buf_vec[vec_idx].buf_addr;
 	buf_iova = buf_vec[vec_idx].buf_iova;
 	buf_len = buf_vec[vec_idx].buf_len;
-- 
2.31.1