[PATCH v3] vhost: compilation fix for GCC-12

[Amit Prakash Shukla]

Ping. 

> -----Original Message-----
> From: Amit Prakash Shukla <amitprakashs at marvell.com>
> Sent: Friday, September 2, 2022 8:36 PM
> To: Maxime Coquelin <maxime.coquelin at redhat.com>; Chenbo Xia
> <chenbo.xia at intel.com>
> Cc: dev at dpdk.org; Jerin Jacob Kollanukkaran <jerinj at marvell.com>;
> stable at dpdk.org; Ruifeng.Wang at arm.com; Amit Prakash Shukla
> <amitprakashs at marvell.com>
> Subject: [PATCH v3] vhost: compilation fix for GCC-12
> 
> GCC-12 complains about the possible use of un-initialized array. At compile
> time it seems like it is not able to evaluate the size as it involves run-time
> variable and at compile time it seems like gcc assumes value of "size" variable
> to be zero which makes gcc-12 to jump the while loop.
> "size = pkt->pkt_len + sizeof(struct virtio_net_hdr_mrg_rxbuf);"
> 
> As part of the fix, "while (){}" is replaced by "do {} while()" which make the
> compiler to generate a code in which buf_vec will never be used un-
> initialized.
> 
> ../lib/vhost/virtio_net.c:941:35: error:
> 		'buf_vec[0].buf_len' may be used uninitialized
> 		[-Werror=maybe-uninitialized]
>   941 |         buf_len = buf_vec[vec_idx].buf_len;
>       |                   ~~~~~~~~~~~~~~~~^~~~~~~~
> ../lib/vhost/virtio_net.c: In function 'virtio_dev_rx_packed':
> ../lib/vhost/virtio_net.c:1285:27: note: 'buf_vec' declared here
>  1285 |         struct buf_vector buf_vec[BUF_VECTOR_MAX];
>       |                           ^~~~~~~
> cc1: all warnings being treated as errors
> 
> Fixes: 93520085efda ("vhost: add packed ring single enqueue")
> Cc: stable at dpdk.org
> 
> Signed-off-by: Amit Prakash Shukla <amitprakashs at marvell.com>
> ---
> v2:
> - Changes for code review suggestion
> 
> v3:
> - Added a description
> 
>  lib/vhost/virtio_net.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c index
> b3d954aab4..9b77d3d10f 100644
> --- a/lib/vhost/virtio_net.c
> +++ b/lib/vhost/virtio_net.c
> @@ -1069,7 +1069,7 @@ vhost_enqueue_single_packed(struct virtio_net
> *dev,
>  	else
>  		max_tries = 1;
> 
> -	while (size > 0) {
> +	do {
>  		/*
>  		 * if we tried all available ring items, and still
>  		 * can't get enough buf, it means something abnormal @@ -
> 1097,7 +1097,7 @@ vhost_enqueue_single_packed(struct virtio_net *dev,
>  		avail_idx += desc_count;
>  		if (avail_idx >= vq->size)
>  			avail_idx -= vq->size;
> -	}
> +	} while (size > 0);
> 
>  	if (mbuf_to_desc(dev, vq, pkt, buf_vec, nr_vec, num_buffers, false)
> < 0)
>  		return -1;
> @@ -1574,7 +1574,7 @@ vhost_enqueue_async_packed(struct virtio_net
> *dev,
>  	else
>  		max_tries = 1;
> 
> -	while (size > 0) {
> +	do {
>  		/*
>  		 * if we tried all available ring items, and still
>  		 * can't get enough buf, it means something abnormal @@ -
> 1601,7 +1601,7 @@ vhost_enqueue_async_packed(struct virtio_net *dev,
>  		avail_idx += desc_count;
>  		if (avail_idx >= vq->size)
>  			avail_idx -= vq->size;
> -	}
> +	} while (size > 0);
> 
>  	if (unlikely(mbuf_to_desc(dev, vq, pkt, buf_vec, nr_vec,
> *nr_buffers, true) < 0))
>  		return -1;
> --
> 2.25.1