netlink: make the check for "send from tx_ring" deterministic

As it is, zero msg_iovlen means that the first iovec in the kernel
array of iovecs is left uninitialized, so checking if its ->iov_base
is NULL is random.  Since the real users of that thing are doing
sendto(fd, NULL, 0, ...), they are getting msg_iovlen = 1 and
msg_iov[0] = {NULL, 0}, which is what this test is trying to catch.
As suggested by davem, let's just check that msg_iovlen was 1 and
msg_iov[0].iov_base was NULL - _that_ is well-defined and it catches
what we want to catch.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 5 insertions, 0 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index a36777b7cfb6..4fd38a612ff6 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2298,7 +2298,12 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
                         goto out;
         }
 
+        /* It's a really convoluted way for userland to ask for mmaped
+         * sendmsg(), but that's what we've got...
+         */
         if (netlink_tx_is_mmaped(sk) &&
+            msg->msg_iter.type == ITER_IOVEC &&
+            msg->msg_iter.nr_segs == 1 &&
             msg->msg_iter.iov->iov_base == NULL) {
                 err = netlink_mmap_sendmsg(sk, msg, dst_portid, dst_group,
                                            &scm);