openssh unix dev - Nov 2004 - patch to fix non-echo tty on scp SIGINT

A long-time missing feature (or bug, depending on how you look at it) is
that a Ctrl-C at the password prompt in scp does not restore the
terminal settings, thus dropping you to the command prompt without any
keyboard echo.  (A "reset" command will fix it.)  This is a pretty
regular occurance for me, and some others I've talked to - usually when
you realize that the scp command you typed has a typo and decide to
abort.

Strangely, ssh handles this correctly, but scp does not.  The single
handler in scp.c is killchild(), which should pass the SIGINT along to
the ssh process, which would restore the terminal.  However this doesn't
work for some reason I couldn't discern - perhaps the ssh process is
invoked in such a way that it doesn't have access to the terminal
settings?

I created the attached patch which fixes the problem by saving the
terminal settings at the start of program execution and restoring them
on SIGINT.  Although this is slightly hacky I think it is good general
practice to leave the terminal in the same state that it began in.  Even
if the ssh process were hardlocked this would still restore the
terminal.

Adam

-------------- next part --------------
--- openssh-3.9p1/scp.c 2004-08-13 04:19:38.000000000 -0700
+++ openssh-3.9p1-ttyfix/scp.c  2004-11-22 00:54:45.679885981 -0800
@@ -105,12 +105,19 @@
 /* This is used to store the pid of ssh_program */
 pid_t do_cmd_pid = -1;

+/* Save terminal state for restore on exit */
+static struct termios _saved_tio;
+
 static void
 killchild(int signo)
 {
        if (do_cmd_pid > 1)
                kill(do_cmd_pid, signo);

+       /* restore the terminal to its original state */
+       if (tcsetattr(fileno(stdin), TCSADRAIN, &_saved_tio) == -1)
+               perror("tcsetattr");
+
        _exit(1);
 }

@@ -218,6 +225,11 @@
        extern char *optarg;
        extern int optind;

+       if (tcgetattr(fileno(stdin), &_saved_tio) == -1) {
+               perror("tcgetattr");
+               return 0;
+       }
+
        __progname = ssh_get_progname(argv[0]);

        args.list = NULL;

Adam Wiggins wrote:
> A long-time missing feature (or bug, depending on how you look at it) is
> that a Ctrl-C at the password prompt in scp does not restore the
> terminal settings, thus dropping you to the command prompt without any
> keyboard echo.  (A "reset" command will fix it.)  This is a
pretty
> regular occurance for me, and some others I've talked to - usually when
> you realize that the scp command you typed has a typo and decide to
> abort.
Yeah, after a while "stty sane" gets old, doesn't it :-?
> Strangely, ssh handles this correctly, but scp does not.  The single
> handler in scp.c is killchild(), which should pass the SIGINT along to
> the ssh process, which would restore the terminal.  However this
doesn't
> work for some reason I couldn't discern - perhaps the ssh process is
> invoked in such a way that it doesn't have access to the terminal
> settings?
I think the problem is fundamentally a race.

scp passes the signal to ssh then immediately exits.  ssh will then get 
the passed signal (eg a SIGINT) plus a SIGPIPE (since the other end of 
its stdin/stdout descriptors have closed).  Same applies to sftp too.

I think that in many cases the SIGPIPE is killing ssh before the SIGINT 
handler has a chance to restore the terminal modes.  The attached patch 
prevents this by waiting for ssh to exit before scp does.

The problem doesn't seem to occur on OpenBSD, however I'm not sure if 
this is simply because the dice didn't come up right, or there's some 
mechanism which prevents it (eg process scheduling or a different signal 
delivery order).

(I vaguely recall someone else mentioning this but a search of the 
archives failed to locate it.  If anyone can point me to a reference 
I'll make sure it's credited appropriately).

-- 
Darren Tucker (dtucker at zip.com.au)
GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
     Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.
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