[dpdk-dev] [PATCH v2] virtio: fix segfault when transmit pkts

[Jianfeng Tan]

Issue: when using virtio nic to transmit pkts, it causes segment fault.

How to reproduce:
Basically, we need to construct a case with vm send packets to vhost-user,
and this issue does not happen when transmitting packets using indirect
desc. Besides, make sure all descriptors are exhausted before vhost
dequeues any packets.

a. start testpmd with vhost.
  $ testpmd -c 0x3 -n 4 --socket-mem 1024,0 --no-pci \
    --vdev 'eth_vhost0,iface=/tmp/sock0,queues=1' -- -i --nb-cores=1

b. start a qemu with a virtio nic connected with the vhost-user port, just
make sure mrg_rxbuf is enabled.

c. enable testpmd on the host.
  testpmd> set fwd io
  testpmd> start (better without start vhost-user)

d. start testpmd in VM.
  $testpmd -c 0x3 -n 4 -m 1024 -- -i --disable-hw-vlan-filter --txqflags=0xf01
  testpmd> set fwd txonly
  testpmd> start

How to fix: this bug is because inside virtqueue_enqueue_xmit(), the flag of
desc has been updated inside the do {} while (), not necessary to update after
the loop. (And if we do that after the loop, if all descs could have run out,
idx is VQ_RING_DESC_CHAIN_END (32768), use this idx to reference the start_dp
array will lead to segment fault.)

Fixes: dd856dfcb9e ("virtio: use any layout on Tx")

Signed-off-by: Jianfeng Tan <jianfeng.tan at intel.com>
---
 v2: refine the commit message.

 drivers/net/virtio/virtio_rxtx.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index ef21d8e..432aeab 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -271,8 +271,6 @@ virtqueue_enqueue_xmit(struct virtqueue *txvq, struct rte_mbuf *cookie,
 		idx = start_dp[idx].next;
 	} while ((cookie = cookie->next) != NULL);
 
-	start_dp[idx].flags &= ~VRING_DESC_F_NEXT;
-
 	if (use_indirect)
 		idx = txvq->vq_ring.desc[head_idx].next;
 
-- 
2.1.4