Don't expose 'vnet%d' to the user

https://bugzilla.redhat.com/517371

Matt Booth points out that if you use a non-existent bridge name when
start a guest you get a weird error message:

  Failed to add tap interface 'vnet%d' to bridge 'virbr0'

and dev='vnet%d' appears in the dumpxml output.

Fix that by not including 'vnet%d' in the error message and freeing the
'vnet%d' string if adding the tap device to the bridge fails.

* src/qemu_conf.c, src/uml_conf.c: fix qemudNetworkIfaceConnect()
  and umlConnectTapDevice() to not expose 'vnet%d' to the user

src/qemu_conf.c

@@ -1038,6 +1038,7 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
     int err;
     int tapfd = -1;
     int vnet_hdr = 0;
+    int template_ifname = 0;
 
     if (net->type == VIR_DOMAIN_NET_TYPE_NETWORK) {
         virNetworkPtr network = virNetworkLookupByName(conn,
@@ -1059,6 +1060,14 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
         return -1;
     }
 
+    char ebuf[1024];
+    if (!driver->brctl && (err = brInit(&driver->brctl))) {
+        qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                         _("cannot initialize bridge support: %s"),
+                         virStrerror(err, ebuf, sizeof ebuf));
+        return -1;
+    }
+
     if (!net->ifname ||
         STRPREFIX(net->ifname, "vnet") ||
         strchr(net->ifname, '%')) {
@@ -1067,14 +1076,8 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
             virReportOOMError(conn);
             return -1;
         }
-    }
-
-    char ebuf[1024];
-    if (!driver->brctl && (err = brInit(&driver->brctl))) {
-        qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
-                         _("cannot initialize bridge support: %s"),
-                         virStrerror(err, ebuf, sizeof ebuf));
-        return -1;
+        /* avoid exposing vnet%d in dumpxml or error outputs */
+        template_ifname = 1;
     }
 
     if (qemuCmdFlags & QEMUD_CMD_FLAG_VNET_HDR &&
@@ -1088,12 +1091,18 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
             qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
                              _("Failed to add tap interface to bridge. "
                                "%s is not a bridge device"), brname);
+        } else if (template_ifname) {
+            qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                             _("Failed to add tap interface to bridge '%s' : %s"),
+                             brname, virStrerror(err, ebuf, sizeof ebuf));
         } else {
             qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
                              _("Failed to add tap interface '%s' "
                                "to bridge '%s' : %s"),
                              net->ifname, brname, virStrerror(err, ebuf, sizeof ebuf));
         }
+        if (template_ifname)
+            VIR_FREE(net->ifname);
         return -1;
     }
 

src/uml_conf.c

@@ -104,17 +104,10 @@ umlConnectTapDevice(virConnectPtr conn,
                     virDomainNetDefPtr net,
                     const char *bridge)
 {
+    brControl *brctl = NULL;
     int tapfd = -1;
+    int template_ifname = 0;
     int err;
-    brControl *brctl = NULL;
-
-    if (!net->ifname ||
-        STRPREFIX(net->ifname, "vnet") ||
-        strchr(net->ifname, '%')) {
-        VIR_FREE(net->ifname);
-        if (!(net->ifname = strdup("vnet%d")))
-            goto no_memory;
-    }
 
     if ((err = brInit(&brctl))) {
         char ebuf[1024];
@@ -124,6 +117,16 @@ umlConnectTapDevice(virConnectPtr conn,
         goto error;
     }
 
+    if (!net->ifname ||
+        STRPREFIX(net->ifname, "vnet") ||
+        strchr(net->ifname, '%')) {
+        VIR_FREE(net->ifname);
+        if (!(net->ifname = strdup("vnet%d")))
+            goto no_memory;
+        /* avoid exposing vnet%d in dumpxml or error outputs */
+        template_ifname = 1;
+    }
+
     if ((err = brAddTap(brctl, bridge,
                         &net->ifname, BR_TAP_PERSIST, &tapfd))) {
         if (errno == ENOTSUP) {
@@ -131,6 +134,11 @@ umlConnectTapDevice(virConnectPtr conn,
             umlReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
                            _("Failed to add tap interface to bridge. "
                              "%s is not a bridge device"), bridge);
+        } else if (template_ifname) {
+            char ebuf[1024];
+            umlReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                           _("Failed to add tap interface to bridge '%s' : %s"),
+                           bridge, virStrerror(err, ebuf, sizeof ebuf));
         } else {
             char ebuf[1024];
             umlReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
@@ -138,6 +146,8 @@ umlConnectTapDevice(virConnectPtr conn,
                              "to bridge '%s' : %s"),
                            net->ifname, bridge, virStrerror(err, ebuf, sizeof ebuf));
         }
+        if (template_ifname)
+            VIR_FREE(net->ifname);
         goto error;
     }
     close(tapfd);