netdev - Re: Fw: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3)

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Date:	Wed, 21 Oct 2015 04:28:52 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alan Burlison <Alan.Burlison@...cle.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Fw: [Bug 106241] New: shutdown(3)/close(3) behaviour is
 incorrect for sockets in accept(3)

On Wed, 2015-10-21 at 11:49 +0100, Alan Burlison wrote:
> On 21/10/2015 11:25, David Laight wrote:
> 
> >> The problem with poll() is that it returns immediately when passed a FD
> >> that is in the listening state. rather than waiting until there's an
> >> incoming connection to handle. As I said, that means you can't use
> >> poll() to multiplex between read/write/accept sockets.
> >
> > That seems to work for me...
> 
> In my test case I was setting all the available event bits in 
> pollfd.events to see what came back. With poll() on a listen() socket 
> you get an immediate return with bits set in revents indicating the 
> socket is available for output, which of course it isn't. Indeed an 
> attempt to write to it fails. If you remove the output event bits in 
> pollfd.events then the poll() waits as expected until there's an 
> incoming connection on the socket.
> 
> I suppose one answer is "Well, don't do that then" but returning an 
> output indication on a socket that's in listen() seems rather odd.
> 
> With POLLOUT|POLLWRNORM|POLLWRBAND:
> 
> main: polling #1
> [returns immediately]
> main: poll #1: Success
> poll fd: 0 revents: POLLOUT POLLWRBAND
> main: write #1: Transport endpoint is not connected
> 
> Without POLLOUT|POLLWRNORM|POLLWRBAND:
> 
> main: polling #1
> [waits for connection]
> main: poll #1: Success
> poll fd: 0 revents: POLLIN POLLRDNORM
> 

This works for me. Please double check your programs

242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1, 4294967295 <unfinished ...>
242046 <... poll resumed> )             = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35888), inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 4
242046 close(4)                         = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1, 4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35890), inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 4
242046 close(4)                         = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1, 4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35892), inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 4
242046 close(4)                         = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1, 4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35894), inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 4
242046 close(4)                         = 0

poll() only cares about POLLIN | POLLRDNORM for a listener.


static inline unsigned int inet_csk_listen_poll(const struct sock *sk)
{
        return !reqsk_queue_empty(&inet_csk(sk)->icsk_accept_queue) ?
                        (POLLIN | POLLRDNORM) : 0;
}

if (sk->sk_state == TCP_LISTEN)
    return inet_csk_listen_poll(sk);



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