[PATCH v1] vhost: fix build

[David Marchand]

On Mon, Aug 22, 2022 at 9:42 AM Min Zhou <zhoumin at loongson.cn> wrote:
>
> This patch fixes the following build failure seen on CentOS 8
> with gcc 12.1 because of uninitialized struct variable:
>
>   [..]
> ../lib/vhost/virtio_net.c:1159:18: warning: 'buf_vec[0].buf_addr' may be used uninitialized [-Wmaybe-uninitialized]
> 1159 |         buf_addr = buf_vec[vec_idx].buf_addr;
>      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
>   [..]

I don't like setting the whole variable to 0 just to silence a
warning, like pushing something under the rug.
This is all the more suspicious as there is other code in this file
that does almost the same.


I had seen a similar warning during 22.07 when cross compiling but did
not investigate much.
The patch that I had written at the time was:

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index 35fa4670fd..9446e33aa7 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;


Could you see if this fixes your issue?

If it is the case, it may be worth better understanding what bothers
the compiler in the current code.


-- 
David Marchand