netlink-socket: Work around upstream kernel Netlink bug.

The upstream kernel net/netlink/af_netlink.c netlink_recvmsg() contains the
following code to refill the Netlink socket buffer with more dump skbs
while a dump is in progress:

	if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
		ret = netlink_dump(sk);
		if (ret) {
			sk->sk_err = ret;
			sk->sk_error_report(sk);
		}
	}

The netlink_dump() function that this calls returns a negative number on
error, the convention used throughout the kernel, and thus sk->sk_err
receives a negative value on error.

However, sk->sk_err is supposed to contain either 0 or a positive errno
value, as one can see from a quick "grep" through net for 'sk_err =', e.g.:

    ipv4/tcp.c:2067:		sk->sk_err = ECONNRESET;
    ipv4/tcp.c:2069:		sk->sk_err = ECONNRESET;
    ipv4/tcp_input.c:4106:		sk->sk_err = ECONNREFUSED;
    ipv4/tcp_input.c:4109:		sk->sk_err = EPIPE;
    ipv4/tcp_input.c:4114:		sk->sk_err = ECONNRESET;
    netlink/af_netlink.c:741:			sk->sk_err = ENOBUFS;
    netlink/af_netlink.c:1796:			sk->sk_err = ENOBUFS;
    packet/af_packet.c:2476:		sk->sk_err = ENETDOWN;
    unix/af_unix.c:341:			other->sk_err = ECONNRESET;
    unix/af_unix.c:407:				skpair->sk_err = ECONNRESET;

The result is that the next attempt to receive from the socket will return
the error to userspace with the wrong sign.

(The root of the error in this case is that multiple threads are attempting
to read a single flow dump from a shared fd.  That should work, but the
kernel has an internal race that can result in one or more of those threads
hitting the EINVAL case at the start of netlink_dump().  The EINVAL is
harmless in this case and userspace should be able to ignore it, but
reporting the EINVAL as if it were a 22-byte message received in userspace
throws a real wrench in the works.)

This bug makes me think that there are probably not many programs doing
multithreaded Netlink dumps.  Maybe it is good that we are considering
other approaches.

VMware-BZ: #1255704
Reported-by: Mihir Gangar <gangarm@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>

diff --git a/AUTHORS b/AUTHORS
index dc37961..fc0d876 100644 (file)
--- a/AUTHORS
+++ b/AUTHORS
@@ -227,6 +227,7 @@ Michael A. Collins      mike.a.collins@ark-net.org
 Michael Hu              mhu@nicira.com
 Michael Mao             mmao@nicira.com
 Michael Shigorin        mike@osdn.org.ua
+Mihir Gangar            gangarm@vmware.com
 Mike Bursell            mike.bursell@citrix.com
 Mike Kruze              mkruze@nicira.com
 Min Chen                ustcer.tonychan@gmail.com

diff --git a/lib/netlink-socket.c b/lib/netlink-socket.c
index 215ddc4..93757cf 100644 (file)
--- a/lib/netlink-socket.c
+++ b/lib/netlink-socket.c
@@ -310,6 +310,7 @@ nl_sock_recv__(struct nl_sock *sock, struct ofpbuf *buf, bool wait)
     struct iovec iov[2];
     struct msghdr msg;
     ssize_t retval;
+    int error;
 
     ovs_assert(buf->allocated >= sizeof *nlmsghdr);
     ofpbuf_clear(buf);
@@ -323,12 +324,23 @@ nl_sock_recv__(struct nl_sock *sock, struct ofpbuf *buf, bool wait)
     msg.msg_iov = iov;
     msg.msg_iovlen = 2;
 
+    /* Receive a Netlink message from the kernel.
+     *
+     * This works around a kernel bug in which the kernel returns an error code
+     * as if it were the number of bytes read.  It doesn't actually modify
+     * anything in the receive buffer in that case, so we can initialize the
+     * Netlink header with an impossible message length and then, upon success,
+     * check whether it changed. */
+    nlmsghdr = ofpbuf_base(buf);
     do {
+        nlmsghdr->nlmsg_len = UINT32_MAX;
         retval = recvmsg(sock->fd, &msg, wait ? 0 : MSG_DONTWAIT);
-    } while (retval < 0 && errno == EINTR);
-
-    if (retval < 0) {
-        int error = errno;
+        error = (retval < 0 ? errno
+                 : retval == 0 ? ECONNRESET /* not possible? */
+                 : nlmsghdr->nlmsg_len != UINT32_MAX ? 0
+                 : -retval);
+    } while (error == EINTR);
+    if (error) {
         if (error == ENOBUFS) {
             /* Socket receive buffer overflow dropped one or more messages that
              * the kernel tried to send to us. */
@@ -343,7 +355,6 @@ nl_sock_recv__(struct nl_sock *sock, struct ofpbuf *buf, bool wait)
         return E2BIG;
     }
 
-    nlmsghdr = ofpbuf_data(buf);
     if (retval < sizeof *nlmsghdr
         || nlmsghdr->nlmsg_len < sizeof *nlmsghdr
         || nlmsghdr->nlmsg_len > retval) {