netlink: Fix handling of error from netlink_dump().

netlink_dump() returns a negative errno value on error.  Until now,
netlink_recvmsg() directly recorded that negative value in sk->sk_err, but
that's wrong since sk_err takes positive errno values.  (This manifests as
userspace receiving a positive return value from the recv() system call,
falsely indicating success.) This bug was introduced in the commit that
started checking the netlink_dump() return value, commit ab8cfc9 (netlink:
handle errors from netlink_dump()).

Multithreaded Netlink dumps are one way to trigger this behavior in
practice, as described in the commit message for the userspace workaround
posted here:
    http://openvswitch.org/pipermail/dev/2014-June/042339.html

This commit also fixes the same bug in netlink_poll(), introduced in commit
e363adfe8 (netlink: add flow control for memory mapped I/O).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 15c731f03fa664a64f7bf3cdde36cf1a8e4150b6..e6fac7e3db52e5fcb40629a60472ff2c7aa72dcb 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -636,7 +636,7 @@ static unsigned int netlink_poll(struct file *file, struct socket *sock,
                while (nlk->cb_running && netlink_dump_space(nlk)) {
                        err = netlink_dump(sk);
                        if (err < 0) {
-                               sk->sk_err = err;
+                               sk->sk_err = -err;
                                sk->sk_error_report(sk);
                                break;
                        }
@@ -2483,7 +2483,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
            atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
                ret = netlink_dump(sk);
                if (ret) {
-                       sk->sk_err = ret;
+                       sk->sk_err = -ret;
                        sk->sk_error_report(sk);
                }
        }