[PATCH 1/1] vhost: fix GCC 13 build error

[Luca Vizzarro <luca.vizzarro@arm.com>]

This patch resolves a build error with GCC 13 and arm/aarch32 as
targets:

In function ‘mbuf_to_desc’,
    inlined from ‘vhost_enqueue_async_packed’ at
      ../lib/vhost/virtio_net.c:1828:6,
    inlined from ‘virtio_dev_rx_async_packed’ at
      ../lib/vhost/virtio_net.c:1842:6,
    inlined from ‘virtio_dev_rx_async_submit_packed’ at
      ../lib/vhost/virtio_net.c:1900:7:
../lib/vhost/virtio_net.c:1159:18: error: ‘buf_vec[0].buf_addr’ may
    be used uninitialized [-Werror=maybe-uninitialized]
 1159 |         buf_addr = buf_vec[vec_idx].buf_addr;
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
<snip>
../lib/vhost/virtio_net.c:1160:18: error: ‘buf_vec[0].buf_iova’ may
    be used uninitialized [-Werror=maybe-uninitialized]
 1160 |         buf_iova = buf_vec[vec_idx].buf_iova;
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
<snip>
../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;
      |                   ~~~~~~~~~~~~~~~~^~~~~~~~

GCC complains about the possible runtime path where the while loop
which fills buf_vec (in vhost_enqueue_async_packed) is not run. As a
consequence it correctly thinks that buf_vec is not initialized while
being accessed anyways.

This scenario is actually very unlikely as the only way this can occur
is if size has overflowed to 0. Meaning that the total packet length
would be close to UINT64_MAX (or actually UINT32_MAX). At first glance,
the code suggests that this may never happen as the type of size has
been changed to 64-bit. For a 32-bit architecture such as arm
(e.g. armv7-a) and aarch32, this still happens because the operand types
(pkt->pkt_len and sizeof) are 32-bit wide, performing 32-bit arithmetic
first (where the overflow can happen) and widening to 64-bit later.

The proposed fix simply guarantees to the compiler that the scope which
fills buf_vec is accessed at least once, while not disrupting the actual
logic. This is based on the assumption that size will always be greater
than 0, as suggested by the sizeof, and the packet length will never be
as big as UINT32_MAX, and causing an overflow.

Fixes: 873e8dad6f49 ("vhost: support packed ring in async datapath")
Cc: stable@dpdk.org

Signed-off-by: Luca Vizzarro <luca.vizzarro@arm.com>
Reviewed-by: Paul Szczepanek <paul.szczepanek@arm.com>
Reviewed-by: Nick Connolly <nick.connolly@arm.com>
---
 lib/vhost/virtio_net.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index 1359c5fb1f..6a2ca295f5 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -1935,7 +1935,7 @@ 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
@@ -1962,7 +1962,7 @@ 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;
-- 
2.34.1