[PATCH] net/net.c: Fix qemu crash when hot-pluging a vhost-net failed.

[Ming Yang]

Hot-pluging a vhost-net may cause virtual machine crash in following steps:
1. Starting a vm without net devices.
2. Hot-pluging 70 memory devices.
3. Hot-pluging a vhost-net device.

The reason is : if hotplug a vhost-net failed, the nc cannot be found via 
function qemu_find_netdev, as
it has been cleaned up through function qemu_cleanup_net_client. Which leads to 
the result
that assert(nc) failed, then qemu crashed.

While, the root reason is that, in commit 46d4d36d0bf2 if not both 
has_vhostforce and vhostforce flags
are true, the errp would not be set. Then net_init_tap would not return a 
negative value, fallowed by founding nc
and assert nc.

In this patch, asserting nc is replaced with setting an error message.

Fixes: 46d4d36d0bf2("tap: setting error appropriately when calling 
net_init_tap_one()")
Signed-off-by: Ming Yang <yangming73@huawei.com>
Signed-off-by: Liang Zhang <zhangliang5@huawei.com>
---
 net/net.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/net.c b/net/net.c
index 840ad9dca5..1d1d7e54c4 100644
--- a/net/net.c
+++ b/net/net.c
@@ -1103,7 +1103,16 @@ static int net_client_init1(const Netdev *netdev, bool 
is_netdev, Error **errp)
 
     if (is_netdev) {
         nc = qemu_find_netdev(netdev->id);
-        assert(nc);
+        /*
+         * If the tap of hotpluged net device do not has both has_vhostforce 
flag and vhostforce flags,
+         * when error occurs, the error messags will be report but not set to 
errp. Thus net_client_init_fun
+         * will not return a negatave value. Therefore the value of nc might 
be NULL. To make qemu robust,
+         * it is better to judge if nc is NULL.
+         */
+        if (!nc) {
+            error_setg(errp, "Device '%s' could not be initialized", 
netdev->id);
+            return -1;
+        }
         nc->is_netdev = true;
     }
 
-- 
2.33.0