netdev - Re: 2.6.34: Problem with UDP traffic on lo + poll(?)

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Date:	Tue, 07 Sep 2010 21:26:09 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Krzysztof Olędzki <ole@....pl>,
	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: 2.6.34: Problem with UDP traffic on lo + poll(?)

Le mardi 07 septembre 2010 à 18:36 +0200, Eric Dumazet a écrit :

> Hmm, I have a pretty good idea of what the problem is, and will post a
> fix soon ;)

David, if you feel this is too invasive for stable, we can make UDP
rehash the socket in case we dont want to change ip4_datagram_connect()

[PATCH] inet: Dont set inet_rcv_saddr in connect()

So the problem is that the sequence :

socket(PF_INET, SOCK_DGRAM, IPPROTO_IP)

connect(fd, {sa_family=AF_INET, sin_port=htons(xx),
sin_addr=inet_addr("1.2.3.4")}, 28)

1) Does an implicit inet_autobind()
  (using an INADDR_ANY address, and selecting a random port).

2) Then does an ip4_datagram_connect() to specify the address/port of
remote end point.

Problem is ip4_datagram_connect() also sets inet->inet_rcv_saddr (from
INADDR_ANY to IP source address, given the current route to remote end
point). Only the first connect() on the socket does this. Following ones
dont change the (possibly wrong) source address.

This breaks the secondary UDP hash, based on (ADDRESS, port), that was
computed by inet_autobind().

This also potentially breaks multiple connect() to change remote
endpoints, because old source address might be non usable for packets to
new destination.

If route happens to change, then we should automatically change our
source address too, at next sendmsg() call, and UDP code deals with this
just fine.

If an application needs to specify a precise source address, it must use
bind() system call. connect() man page only refers to remote address,
not local one.

Reported-by: Krzysztof Olędzki <ole@....pl>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/ipv4/datagram.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ipv4/datagram.c b/net/ipv4/datagram.c
index f055094..8a17241 100644
--- a/net/ipv4/datagram.c
+++ b/net/ipv4/datagram.c
@@ -60,10 +60,19 @@ int ip4_datagram_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 		ip_rt_put(rt);
 		return -EACCES;
 	}
+/*
+ * Should connect() change inet_rcv_saddr ?
+ * It should not IMHO, because we want to specify the peer to which
+ * datagrams are to be sent, regardless of our source address that might
+ * change in the future, after a route change.
+ * To specify our source address, bind() is the right API.
+ */
+#if 0
 	if (!inet->inet_saddr)
 		inet->inet_saddr = rt->rt_src;	/* Update source address */
 	if (!inet->inet_rcv_saddr)
 		inet->inet_rcv_saddr = rt->rt_src;
+#endif
 	inet->inet_daddr = rt->rt_dst;
 	inet->inet_dport = usin->sin_port;
 	sk->sk_state = TCP_ESTABLISHED;

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