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

[Yonathan Bleyfuesz]

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