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

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Date:   Thu, 30 Mar 2017 16:36:27 -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>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: EINVAL when using connect() for udp sockets

On Tue, Mar 28, 2017 at 5:52 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-03-28 at 16:11 -0700, Eric Dumazet wrote:
>
>> Yes, this looks better.
>>
>> Although you probably need to change a bit later this part :
>>
>> if (!inet->inet_saddr)
>>       inet->inet_saddr = fl4->saddr;  /* Update source address */
>>
>
> I came up with the following tested patch for IPv4
>
> diff --git a/net/ipv4/datagram.c b/net/ipv4/datagram.c
> index f915abff1350a86af8d5bb89725b751c061b0fb5..1454b6191e0d38ffae0ae260578858285bc5f77b 100644
> --- 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 = (sk->sk_userlocks & SOCK_BINDADDR_LOCK) ? inet->inet_saddr : 0;
>         if (ipv4_is_multicast(usin->sin_addr.s_addr)) {
>                 if (!oif)
>                         oif = inet->mc_index;
> @@ -64,9 +64,8 @@ int __ip4_datagram_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len
>                 err = -EACCES;
>                 goto out;
>         }
> -       if (!inet->inet_saddr)
> +       if (!(sk->sk_userlocks & SOCK_BINDADDR_LOCK)) {
>                 inet->inet_saddr = fl4->saddr;  /* Update source address */
> -       if (!inet->inet_rcv_saddr) {
>                 inet->inet_rcv_saddr = fl4->saddr;
>                 if (sk->sk_prot->rehash)
>                         sk->sk_prot->rehash(sk);

Why do we need this here? If you mean bind() INADDR_ANY is bound,
then it is totally a different problem?

BTW, I am still not sure about what POSIX says about the connect()
behavior, I can only find this [1]:

"
If the initiating socket is not connection-mode, then connect() shall set the
socket's peer address, and no connection is made. For SOCK_DGRAM
sockets, the peer address identifies where all datagrams are sent on
subsequent send() functions, and limits the remote sender for subsequent
recv() functions.
"

It doesn't say anything about source address. But the man page [2] says:

"
When
       connect(2) is called on an unbound socket, the socket is
       automatically bound to a random free port or to a usable shared port
       with the local address set to INADDR_ANY.
"

Seems the last part is inaccurate, kernel actually picks a source address
from route instead of just using INADDR_ANY for connect(2).

So, for me, I think the following behaviors make sense for UDP:

1) When a bind() is called before connect()'s, aka:

bind();
connect(addr1); // should not change source addr
connect(addr2); // should fail is the source addr can not reach peer addr

2) No bind() before connect()'s, aka:

connect(addr1); // Free to bind a source addr
connect(addr2); // Free to bind a new source addr and change peer addr

Thoughts?

1. http://pubs.opengroup.org/onlinepubs/9699919799/functions/connect.html
2. http://man7.org/linux/man-pages/man7/ip.7.html