util: netdevbridge: fall back to ioctl from sysfs

There are certain cases e.g. containers where the sysfs path might
exists, but might fail. Unfortunately the exact restrictions are only
known to libvirt when trying to write to it so we need to try it.

But in case it fails there is no need to fully abort, in those cases try
to fall back to the older ioctl interface which can still work.

That makes setting up a bridge in unprivileged LXD containers work.

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1802906Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NLaine Stump <laine@laine.org>
Reported-by: NBrian Candler <b.candler@pobox.com>

src/util/virnetdevbridge.c

@@ -28,6 +28,7 @@
 #include "virutil.h"
 #include "virfile.h"
 #include "viralloc.h"
+#include "virlog.h"
 #include "intprops.h"
 #include "virstring.h"
 
@@ -74,6 +75,7 @@
 
 #define VIR_FROM_THIS VIR_FROM_NONE
 
+VIR_LOG_INIT("util.netdevbridge");
 
 #if defined(HAVE_BSD_BRIDGE_MGMT)
 static int virNetDevBridgeCmd(const char *brname,
@@ -113,6 +115,8 @@ static int virNetDevBridgeCmd(const char *brname,
  * or by  ioctl on older kernels. Perhaps we could just use
  * ioctl for every kernel, but its not clear what the long
  * term lifespan of the ioctl interface is...
+ * Fall back to ioctl if sysfs interface is not available or
+ * failing (e.g. due to container isolation).
  */
 static int virNetDevBridgeSet(const char *brname,
                               const char *paramname,  /* sysfs param name */
@@ -128,29 +132,29 @@ static int virNetDevBridgeSet(const char *brname,
     if (virFileExists(path)) {
         char valuestr[INT_BUFSIZE_BOUND(value)];
         snprintf(valuestr, sizeof(valuestr), "%lu", value);
-        if (virFileWriteStr(path, valuestr, 0) < 0) {
-            virReportSystemError(errno,
-                                 _("Unable to set bridge %s %s"), brname, paramname);
-            return -1;
-        }
+        if (virFileWriteStr(path, valuestr, 0) >= 0)
+            return 0;
+        VIR_DEBUG("Unable to set bridge %s %s via sysfs", brname, paramname);
+    }
+
+    unsigned long paramid;
+    if (STREQ(paramname, "stp_state")) {
+        paramid = BRCTL_SET_BRIDGE_STP_STATE;
+    } else if (STREQ(paramname, "forward_delay")) {
+        paramid = BRCTL_SET_BRIDGE_FORWARD_DELAY;
     } else {
-        unsigned long paramid;
-        if (STREQ(paramname, "stp_state")) {
-            paramid = BRCTL_SET_BRIDGE_STP_STATE;
-        } else if (STREQ(paramname, "forward_delay")) {
-            paramid = BRCTL_SET_BRIDGE_FORWARD_DELAY;
-        } else {
-            virReportSystemError(EINVAL,
-                                 _("Unable to set bridge %s %s"), brname, paramname);
-            return -1;
-        }
-        unsigned long args[] = { paramid, value, 0, 0 };
-        ifr->ifr_data = (char*)&args;
-        if (ioctl(fd, SIOCDEVPRIVATE, ifr) < 0) {
-            virReportSystemError(errno,
-                                 _("Unable to set bridge %s %s"), brname, paramname);
-            return -1;
-        }
+        virReportSystemError(EINVAL,
+                             _("Unable to set bridge %s %s via ioctl"),
+                             brname, paramname);
+        return -1;
+    }
+    unsigned long args[] = { paramid, value, 0, 0 };
+    ifr->ifr_data = (char*)&args;
+    if (ioctl(fd, SIOCDEVPRIVATE, ifr) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to set bridge %s %s via ioctl"),
+                             brname, paramname);
+        return -1;
     }
 
     return 0;