Restoring block mode on exiting

David Newall openssh at
Fri Apr 23 12:44:41 AEST 2021

Hi Dmitry,

On 23/4/21 1:06 am, Dmitry Belyavskiy wrote:
> OpenSSH incorrectly restores the standard mode (blocking mode) on standard
> output upon exiting.

I agree that SSH should *restore* the blocking mode.  I've only spent a 
amount of timelooking at it and I think SSH currently does not do that, and
your patch also does not.

Your patch sets blocking mode if SSH cleared it.  I think it would be more
correct to restore it tothe state that it was when SSH took over the file
because we shouldn't assume.

In channel_register_fds, don't record if we set non-blocking mode, record
what mode it currentlyhas:

    c->nonblock = (get_nonblock(rfd) << 2) | (get_nonblock(wfd) << 1) |

Restore in channel_close_fds:

    channel_close_fd(ssh, &c->sock, 0);
    if (rfd != sock) channel_close_fd(ssh, &c->rfd, c->nonblock & 4);
    if (wfd != sock) channel_close_fd(ssh, &c->wfd, c->nonblock & 2);
    if (efd != sock) channel_close_fd(ssh, &c->efd, c->nonblock & 1);

In channel_close_fd:

    if (fd != -1) (nonblock?set_nonblock:unset_nonblock)(fd);



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