Possible bug in netlink_recvmsg()

Possible bug in netlink_recvmsg()

[David Howells]

As I understand it, according to the recvmsg() manual page, if the packet
being returned is larger than the buffer provided, and the protocol does not
support piecemeal reception of data, then:

 (1) the buffer should be filled,

 (2) MSG_TRUNC should be set in msg_flags, and

 (3) the length of the full packet, including the discarded bit should be
     returned.

AF_NETLINK sockets, however, do not do (3).  See this bit in netlink_recvmsg():

	copied = skb->len;
	if (len < copied) {
		msg->msg_flags |= MSG_TRUNC;
		copied = len;
	}

Or is this only true if the caller of recvmsg() passes MSG_TRUNC in?

David

Re: Possible bug in netlink_recvmsg()

[David Miller]

From: David Howells <dhowells@redhat.com>
Date: Fri, 13 Apr 2007 19:08:31 +0100

> AF_NETLINK sockets, however, do not do (3).  See this bit in netlink_recvmsg():
> 
> 	copied = skb->len;
> 	if (len < copied) {
> 		msg->msg_flags |= MSG_TRUNC;
> 		copied = len;
> 	}
> 
> Or is this only true if the caller of recvmsg() passes MSG_TRUNC in?

See this fix in my net-2.6.22 tree:

commit ad495d7b6cfcd1bc2eaf06c42699be0bb5d84234
Author: David S. Miller <davem@sunset.davemloft.net>
Date:   Tue Mar 6 17:02:35 2007 -0800

    [NETLINK]: Mirror UDP MSG_TRUNC semantics.
    
    If the user passes MSG_TRUNC in via msg_flags, return
    the full packet size not the truncated size.
    
    Idea from Herbert Xu and Thomas Graf.
    
    Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index e73d8f5..b31a732 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1242,6 +1242,9 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 
 	scm_recv(sock, msg, siocb->scm, flags);
 
+	if (flags & MSG_TRUNC)
+		copied = skb->len;
+
 out:
 	netlink_rcv_wake(sk);
 	return err ? : copied;

Re: Possible bug in netlink_recvmsg()

[David Howells]

David Miller <davem@davemloft.net> wrote:

> See this fix in my net-2.6.22 tree:
> 
> commit ad495d7b6cfcd1bc2eaf06c42699be0bb5d84234
> Author: David S. Miller <davem@sunset.davemloft.net>
> Date:   Tue Mar 6 17:02:35 2007 -0800

Ummm... That seems to conflict with something in your net-2.6 tree.  Which one
should I use?

David