Subject: Re: Undefined behaviour of connect(fd, NULL, 0);

From: Stephen Hemminger <[email-masked]>
Date: Wed, 31 Mar 2010 14:14:45 -0700

On Thu, 1 Apr 2010 07:24:12 +1100
Neil Brown <[email-masked]> wrote:

> On Wed, 31 Mar 2010 11:49:36 -0700
> Stephen Hemminger <[email-masked]> wrote:
> 
> > On Wed, 31 Mar 2010 22:36:37 +1100
> > Neil Brown <[email-masked]> wrote:
> > 
> > > 
> > > Hi Netdev.
> > > 
> > > We have a customer who was reporting strangely unpredictable behaviour of an
> > > in-house application that used networking.
> > > 
> > > It called connect on a non-blocking socket and subsequently called
> > >    connect(fd, NULL, 0)
> > > 
> > > to check if the connection had succeeded.
> > > This would sometime "work" and sometimes close the connection.
> > > 
> > > Looking at the code (sys_connect, move_addr_to_kernel, inet_stream_connect),
> > > it seems that in this case an uninitialised on-stack address is passed
> > > to inet_stream_connect and it makes a decision based on ->sa_family (which is
> > > uninitialised).
> > > 
> > > It seems clear that connect(fd, NULL, 0) is the wrong thing to do in this
> > > circumstance, but I think it would be good if it failed consistently rather
> > > than unpredictably.
> > > 
> > > Would it be appropriate for move_addr_to_kernel to zero out the remainder of
> > > the address?
> > >    memset(kaddr+ulen, 0, MAX_SOCK_ADDR-ulen);
> > > ??
> > > 
> > > Then connect(fd, NULL, 0) would always break the connection.
> > 
> > I think the problem is inet_stream_connect referencing past addr_len.
> > 
> > --- a/net/ipv4/af_inet.c	2010-03-31 11:47:01.952910248 -0700
> > +++ b/net/ipv4/af_inet.c	2010-03-31 11:48:09.852938406 -0700
> > @@ -575,7 +575,7 @@ int inet_stream_connect(struct socket *s
> >  
> >  	lock_sock(sk);
> >  
> > -	if (uaddr->sa_family == AF_UNSPEC) {
> > +	if (addr_len < sizeof(sa_family_t) || uaddr->sa_family == AF_UNSPEC) {
> >  		err = sk->sk_prot->disconnect(sk, flags);
> >  		sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED;
> >  		goto out;
> 
> Thanks for the reply.
> 
> The implication of this patch is that
>    connect(fd, NULL, 0)
> is actually a valid way to check if an in-progress connection has completed.
> 
> Is that the intention?
The rationale is that move_addr_to_kernel, explcitly allow addr=NULL with addr_len=0
so if it is allowed there why not let it through. The implication of this is that
addr_len is the same as AF_UNSPEC.

> Does all other address manipulation code check the addr_len ?? (probably).

Not sure.
Someone ought to check BSD/Solaris to see if there is some standard here.