From: Israel Keys <ikeys@agile.tv>

Subject: [zebra 18677] zebra initialisation bug and patch

Hi All,

I have found a bug in zebra that prevents its routing table and
interface database from being initialised properly. The problem occurs
when a request is made via the netlink socket but the kernel produces a
EWOULDBLOCK/EAGAIN when the result is trying to be retrieved via a
recvmsg(). Zebra does not do anything about this and continues to
function  (with an empty routing table and interface list) as if nothing
has happened. With no such information the routing protocol dosn't work!

Two functions are called during the initialisation of Zebra:
interface_lookup_netlink() and netlink_route_read() - obtaining the
interfaces and routing table from the kernel respectively. These are the
only time these functions are called.

These functions, interface_lookup_netlink() and netlink_route_read(),
use netlink_parse_info() to recieve the data from the netlink socket.
The problem is, netlink_parse_info() returns (without error) when its
call to recvmsg() results in an errno EWOULDBLOCK/EAGAIN. This behaviour
is expected by other funtions calling netlink_parse_info() -
netlink_parse_info is simply recalled at a later stage. However, on
initialisation it is never recalled.

Since zebra is expected to nothing else during initialisation it was
easiest to temporarily change the netlink socket to BLOCK and wait
indefinently until the kernel responds with the required information.

Attached is a patch with these changes.

Comments and questions are welcome.
Please inform me if this patch is added to the Zebra source.

--israel

1 files changed, 73 insertions, 19 deletions

diff --git a/zebra/rt_netlink.c b/zebra/rt_netlink.c
index a2b678ee..b84cdbac 100644
--- a/zebra/rt_netlink.c
+++ b/zebra/rt_netlink.c
@@ -123,6 +123,39 @@ netlink_socket (struct nlsock *nl, unsigned long groups)
   return ret;
 }
 
+int set_netlink_blocking(struct nlsock *nl, int *flags)
+{
+
+  /* Change socket flags for blocking I/O.  */
+  if((*flags = fcntl(nl->sock, F_GETFL, 0)) < 0) 
+    {
+      zlog (NULL, LOG_ERR, "%s:%i F_GETFL error: %s", 
+              __FUNCTION__, __LINE__, strerror (errno));
+      return -1;
+    }
+  *flags &= ~O_NONBLOCK;
+  if(fcntl(nl->sock, F_SETFL, *flags) < 0) 
+    {
+      zlog (NULL, LOG_ERR, "%s:%i F_SETFL error: %s", 
+              __FUNCTION__, __LINE__, strerror (errno));
+      return -1;
+    }
+  return 0;
+}
+
+int set_netlink_nonblocking(struct nlsock *nl, int *flags)
+{  
+  /* Restore socket flags for nonblocking I/O */
+  *flags |= O_NONBLOCK;
+  if(fcntl(nl->sock, F_SETFL, *flags) < 0) 
+    {
+      zlog (NULL, LOG_ERR, "%s:%i F_SETFL error: %s", 
+              __FUNCTION__, __LINE__, strerror (errno));
+      return -1;
+    }
+  return 0;
+}
+
 /* Get type specified information from netlink. */
 static int
 netlink_request (int family, int type, struct nlsock *nl)
@@ -887,7 +920,19 @@ int
 interface_lookup_netlink ()
 {
   int ret;
-
+  int flags;
+  int snb_ret;
+ 
+  /* 
+   * Change netlink socket flags to blocking to ensure we get 
+   * a reply via nelink_parse_info
+   */ 
+  snb_ret = set_netlink_blocking(&netlink_cmd, &flags);
+  if(snb_ret < 0) 
+     zlog (NULL, LOG_WARNING, 
+             "%s:%i Warning: Could not set netlink socket to blocking.", 
+             __FUNCTION__, __LINE__);
+  
   /* Get interface information. */
   ret = netlink_request (AF_PACKET, RTM_GETLINK, &netlink_cmd);
   if (ret < 0)
@@ -914,6 +959,9 @@ interface_lookup_netlink ()
     return ret;
 #endif /* HAVE_IPV6 */
 
+  /* restore socket flags */   
+  if(snb_ret == 0)
+    set_netlink_nonblocking(&netlink_cmd, &flags);
   return 0;
 }
 
@@ -923,7 +971,19 @@ int
 netlink_route_read ()
 {
   int ret;
-
+  int flags;
+  int snb_ret;
+  
+  /* 
+   * Change netlink socket flags to blocking to ensure we get 
+   * a reply via nelink_parse_info
+   */ 
+  snb_ret = set_netlink_blocking(&netlink_cmd, &flags);
+  if(snb_ret < 0) 
+     zlog (NULL, LOG_WARNING, 
+             "%s:%i Warning: Could not set netlink socket to blocking.", 
+             __FUNCTION__, __LINE__);
+  
   /* Get IPv4 routing table. */
   ret = netlink_request (AF_INET, RTM_GETROUTE, &netlink_cmd);
   if (ret < 0)
@@ -942,6 +1002,9 @@ netlink_route_read ()
     return ret;
 #endif /* HAVE_IPV6 */
 
+  /* restore flags */
+  if(snb_ret == 0)
+    set_netlink_nonblocking(&netlink_cmd, &flags);
   return 0;
 }
 
@@ -1025,6 +1088,7 @@ netlink_talk (struct nlmsghdr *n, struct nlsock *nl)
   struct iovec iov = { (void*) n, n->nlmsg_len };
   struct msghdr msg = {(void*) &snl, sizeof snl, &iov, 1, NULL, 0, 0};
   int flags = 0;
+  int snb_ret;
   
   memset (&snl, 0, sizeof snl);
   snl.nl_family = AF_NETLINK;
@@ -1052,17 +1116,11 @@ netlink_talk (struct nlmsghdr *n, struct nlsock *nl)
    * Change socket flags for blocking I/O. 
    * This ensures we wait for a reply in netlink_parse_info().
    */
-  if((flags = fcntl(nl->sock, F_GETFL, 0)) < 0) 
-    {
-      zlog (NULL, LOG_ERR, "%s:%i F_GETFL error: %s", 
-              __FUNCTION__, __LINE__, strerror (errno));
-    }
-  flags &= ~O_NONBLOCK;
-  if(fcntl(nl->sock, F_SETFL, flags) < 0) 
-    {
-      zlog (NULL, LOG_ERR, "%s:%i F_SETFL error: %s", 
-              __FUNCTION__, __LINE__, strerror (errno));
-    }
+  snb_ret = set_netlink_blocking(nl, &flags);
+  if(snb_ret < 0) 
+     zlog (NULL, LOG_WARNING, 
+             "%s:%i Warning: Could not set netlink socket to blocking.", 
+             __FUNCTION__, __LINE__);
 
   /* 
    * Get reply from netlink socket. 
@@ -1071,12 +1129,8 @@ netlink_talk (struct nlmsghdr *n, struct nlsock *nl)
   status = netlink_parse_info (netlink_talk_filter, nl);
   
   /* Restore socket flags for nonblocking I/O */
-  flags |= O_NONBLOCK;
-  if(fcntl(nl->sock, F_SETFL, flags) < 0) 
-    {
-      zlog (NULL, LOG_ERR, "%s:%i F_SETFL error: %s", 
-              __FUNCTION__, __LINE__, strerror (errno));
-    }
+  if(snb_ret == 0)
+     set_netlink_nonblocking(nl, &flags);
   
   return status;
 }