svn commit: r324405 - head/sys/kern

[Jason Eggleston]

In tcp_input.c, this is where a RST is handled. so_error gets ECONNRESET,
then tcp_close() is called.

                                case TCPS_ESTABLISHED:
                                case TCPS_FIN_WAIT_1:
                                case TCPS_FIN_WAIT_2:
                                case TCPS_CLOSE_WAIT:
                                case TCPS_CLOSING:
                                case TCPS_LAST_ACK:
                                        so->so_error = ECONNRESET;
                                close:
                                        /* FALLTHROUGH */
                                default:
                                        tp = tcp_close(tp);

tcp_close() calls soisdisconnected() which sets this flag:

so->so_state |= SS_ISDISCONNECTED;

Followed by:

                ...
                socantrcvmore_locked(so);
                ...
                socantsendmore_locked(so);
                ...

Those two functions set these flags:

so->so_rcv.sb_state |= SBS_CANTRCVMORE;
so->so_snd.sb_state |= SBS_CANTSENDMORE;

A TCP session cannot survive a RST, and i/o calls will not work afterword,
no matter how many times you try.

Having said that, there's a race condition between when so_error is set and
when the socket and socket buffer flags are set, and I agree with your
strategy of:

- have sendfile() match send()
- The current send() behavior is probably fine as is

I agree sendfile() should match write() and send(), by checking in this
order:

SBS_CANTSENDMORE
so_error
SS_ISCONNECTED

and will prep a new review with your patch.

On Mon, Oct 9, 2017 at 4:20 PM, Gleb Smirnoff <glebius at freebsd.org> wrote:

>   Sean & Jason,
>
> On Sat, Oct 07, 2017 at 11:30:57PM +0000, Sean Bruno wrote:
> S> Author: sbruno
> S> Date: Sat Oct  7 23:30:57 2017
> S> New Revision: 324405
> S> URL: https://svnweb.freebsd.org/changeset/base/324405
> S>
> S> Log:
> S>   Check so_error early in sendfile() call.  Prior to this patch, if a
> S>   connection was reset by the remote end, sendfile() would just report
> S>   ENOTCONN instead of ECONNRESET.
> S>
> S>   Submitted by:      Jason Eggleston <jason at eggnet.com>
> S>   Reviewed by:       glebius
> S>   Sponsored by:      Limelight Networks
> S>   Differential Revision:     https://reviews.freebsd.org/D12575
> S>
> S> Modified:
> S>   head/sys/kern/kern_sendfile.c
> S>
> S> Modified: head/sys/kern/kern_sendfile.c
> S> ============================================================
> ==================
> S> --- head/sys/kern/kern_sendfile.c    Sat Oct  7 23:10:16 2017
> (r324404)
> S> +++ head/sys/kern/kern_sendfile.c    Sat Oct  7 23:30:57 2017
> (r324405)
> S> @@ -514,6 +514,11 @@ sendfile_getsock(struct thread *td, int s, struct
> file
> S>      *so = (*sock_fp)->f_data;
> S>      if ((*so)->so_type != SOCK_STREAM)
> S>              return (EINVAL);
> S> +    if ((*so)->so_error) {
> S> +            error = (*so)->so_error;
> S> +            (*so)->so_error = 0;
> S> +            return (error);
> S> +    }
> S>      if (((*so)->so_state & SS_ISCONNECTED) == 0)
> S>              return (ENOTCONN);
> S>      return (0);
>
> Despite my initial positive review, now I am quite unsure on that.
>
> Problem is that sendfile(2) isn't defined by SUS, so there is no
> distinctive final answer on that. Should we match other OSes?
> Should we match our historic behaviour? Or should we match
> write(2)/send(2) to socket, which are closest analogy. I probably
> believe in the latter: sendfile(2) belongs to write(2)/send(2)
> family.
>
> SUS specifies that write may return ECONNRESET. It also documents
> EPIPE. However, our manual page documents only EPIPE for both
> send(2) and write(2). For write we have:
>
>         SOCKBUF_LOCK(&so->so_snd);
>         if (so->so_snd.sb_state & SBS_CANTSENDMORE) {
>                 SOCKBUF_UNLOCK(&so->so_snd);
>                 error = EPIPE;
>                 goto out;
>         }
>         if (so->so_error) {
>                 error = so->so_error;
>                 so->so_error = 0;
>                 SOCKBUF_UNLOCK(&so->so_snd);
>                 goto out;
>         }
>
> Indeed, EPIPE will be returned prior to return/clear of so_error,
> which supposedly is ECONNRESET.
>
> In the sendfile(2) implementation we see exactly same code:
>
>                 if (so->so_snd.sb_state & SBS_CANTSENDMORE) {
>                         error = EPIPE;
>                         SOCKBUF_UNLOCK(&so->so_snd);
>                         goto done;
>                 } else if (so->so_error) {
>                         error = so->so_error;
>                         so->so_error = 0;
>                         SOCKBUF_UNLOCK(&so->so_snd);
>                         goto done;
>                 }
>
> But it isn't reached. Before due to SS_ISCONNECTED check, now
> due to your change. Now we got two spots where so_error is
> returned/cleared.
>
> For the receive family (read(2) and recv(2)), SUS specifies
> ECONNRESET and our code does that. It also clears so_error.
>
> So, after your change at least one standard case is broken: an
> application that polls/selects for both readability and
> writeability of a socket. It discovers that socket is available
> for writing, does sendfile(2), receives ECONNRESET. Then it does
> read on the socket, and blocks(?) or returns a bogus error(?),
> not sure. But definitely not ECONNRESET, since it is now cleared.
>
> Now, where does this ENOTCONN come from? These two lines:
>
>         if (((*so)->so_state & SS_ISCONNECTED) == 0)
>                 return (ENOTCONN);
>
> can be traced back all the way to original 1998 commit of sendfile.
>
> Here comes difference between sendfile(2) and send(2). In send(2),
> this check is done _after_ so_error check, so your change is correct
> in placing of so_error check wrt so_state check, but is incorrect
> in placing wrt socket buffer state check.
>
> After all this considerations, I would suggest to revert your change,
> and apply this patch. It would make behavior of sendfile(2) to match
> send(2). I would appreciate more reviews around it.
>
> --
> Gleb Smirnoff
>