trying to resurrect discussion about "Cannot signal a process over a channel (rfc 4254, section 6.9)"

[Damien Miller]

FWIW, now that privsep is mandatory I have no objection to including
signal support in sshd.

On Wed, 25 Jul 2018, Yonathan Bleyfuesz wrote:

> Hi all,
> 
> I would like to propose some ideas to revivify this subject.
> 
> -First, we could add support on the client to send signal thanks to the escape characters.
> (code : https://github.com/JawaGL/openssh-portable/commit/5bc9e6bc959b1b0f89d7ca7b4b04d7c37079fef0 ).
> 
> With this, in order to send a message requesting the server to send a SIGTERM to the remote process, you need to type  “~ST” which is not really invasive client-side.
> 
> But this means that the client has to enable TTY.
> 
> 
> -Secondly , server-side, there is a problem with the currently suggested patch : it only works when we do an ‘exec’ request to the server (eg : ssh some-host “some; commands;”).
> 
> This is because in the other possible configuration, a shell is launched by the server. Then when we launch a process, it is forked by this shell and thus it has its own group-id.
> 
> When the user launches a signal-request hoping to reach a blocking process, the pid that is used by the ‘killpg’ function is the one of the shell. So it is this shell that catches the signal resulting in it:
> 	- dying and leaving zombies 
> 	- dying and taking its child with him (SIGHUP and SIGKILL)
> 	- ignoring the signal (SIGINT, SIGTERM, SIGQUIT).
> 
> Example of ID’s when I connect to a server and launch the script test_signal.sh :  
>  PID   PPID  PGID  SID
>  4060  1598  4060  1556 sshd            sshd: root at pts/2
>  4062  4060  4062  4062 bash            -bash
>  4075  4062  4075  4062 sh              sh test_signal.sh
>  4076  4075  4075  4062 sh              sh test_signal.sh
> 
> So in order to take this use case into account we could use the 'tcgetpgrp()’  function from ‘unistd.h’. 
> (code : https://github.com/JawaGL/openssh-portable/commit/3667c0d90688c43ac0729083f73afa65102226b4 )
> 
> Of course this would still work if there are no TTY present since we can still access the PGID of the forked child in the session attributes.
> 
> -Finally, in order to test these functionalities, we could integrate a test case in the regress folder. (code : https://github.com/JawaGL/openssh-portable/commit/02c39b15363c54d0e622e5724c721a474e1cacd6).
> 
> 
> I tested all these features on MacOSX and Ubuntu 18.
> 
> I hope this helps,
> Thanks in advance for your returns,
> 
> Yonathan
> 
> 
> _______________________________________________
> openssh-unix-dev mailing list
> openssh-unix-dev at mindrot.org
> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
>