netdev - Re: EINVAL when using connect() for udp sockets

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Date:   Fri, 24 Mar 2017 15:34:51 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daurnimator <quae@...rnimator.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        William Ahern <william@...handclement.com>,
        "Santi T." <santitm99@...il.com>, mtk.manpages@...il.com
Subject: Re: EINVAL when using connect() for udp sockets

(Cc'ing Michael Kerrisk)

On Wed, Mar 22, 2017 at 10:18 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2017-03-23 at 13:22 +1100, Daurnimator wrote:
>> On 9 March 2017 at 14:10, Daurnimator <quae@...rnimator.com> wrote:
>> > When debugging https://github.com/daurnimator/lua-http/issues/73 which
>> > uses https://github.com/wahern/dns we ran into an issue where modern
>> > linux kernels return EINVAL if you try and re-use a udp socket.
>> > The issue seems to occur if you go from a local destination ip to a
>> > non-local one.
>>
>> Did anyone get a chance to look into this issue?
>
> I believe man page is not complete.
>
> A disconnect is needed before another connect()

Is it? Making connect() reentrant is reasonable for connection-less
protocol like UDP, but I don't dig POSIX for the details. If so we need
something like below...

--- a/net/ipv4/datagram.c
+++ b/net/ipv4/datagram.c
@@ -40,7 +40,7 @@ int __ip4_datagram_connect(struct sock *sk, struct
sockaddr *uaddr, int addr_len
        sk_dst_reset(sk);

        oif = sk->sk_bound_dev_if;
-       saddr = inet->inet_saddr;
+       saddr = inet->inet_saddr = 0;
        if (ipv4_is_multicast(usin->sin_addr.s_addr)) {
                if (!oif)
                        oif = inet->mc_index;