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

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Date:   Fri, 24 Mar 2017 16:19:20 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...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

On Fri, 2017-03-24 at 15:34 -0700, Cong Wang wrote:
> (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;

Wont this break bind() ?