Re: Remote program doesn't terminate when ssh session ends
On 2021-02-25, Catalin Patulea wrote:
> I believe the right way this works it that:
> - ssh client closes session
> - dropbear closes the read end of command's stdout pipe
> - next time command writes to pipe, it receives SIGPIPE and dies
> Thus I don't think dropbear needs to explicitly kill anything. but perhaps
> the mechanism is somehow not working.
>
> I would investigate this by running 'strace' on the remote host and seeing
> if, after closing the session, 1) dropbear closes the pipe and 2) the
> command tries to write to it and receives SIGPIPE.
Thanks for the clue!
The problem is that the executable I ran (ash) doesn't receive
SIGPIPE. What receives the SIGPIPE are the `date` utility and the
`sort` utilities. They die when they try to write to stdout, but the
shell program that invoked them continues to run.
The same thing happens with the openssh server.
In this case I can fix it by setting the `errexit` shell option in the
shellscript I'm running so that the shell will exit when `date` or
`sort` exit with an error. [This would not work if `sort` and `date`
exited with status 0 when receiving SIGPIPE.]
#!/bin/sh
set -o errexit
while true
do
date
cat /proc/[0-9]*/stat |
awk '$23 > 0 {printf "%5d %20s %8d %5d\n", $1, $2, $23, $24}' |
sort -n
sleep 15
done
Another work-aroud is to force allocation of a tty like this:
$ ssh -t root@10.0.0.99 ./showmem.sh
That way when the user presses ctrl-C and ssh client gets a SIGINT,
the SIGINT will also be sent to the members of the tty group at the
server end. However, if the user exits the ssh client by typing "~."
instead of hitting ctrl-C, the program will continue to run as before
Re: Remote program doesn't terminate when ssh session ends
Years ago, I attempted to fix an issue that sounds a lot like this:
https://hg.ucc.asn.au/dropbear/rev/35183e8a7851
I believe the right way this works it that:
- ssh client closes session
- dropbear closes the read end of command's stdout pipe
- next time command writes to pipe, it receives SIGPIPE and dies
Thus I don't think dropbear needs to explicitly kill anything. but perhaps
the mechanism is somehow not working.
I would investigate this by running 'strace' on the remote host and seeing
if, after closing the session, 1) dropbear closes the pipe and 2) the
command tries to write to it and receives SIGPIPE.
On Thu, Feb 25, 2021 at 12:52 PM Grant Edwards
wrote:
> I have a small ash script that prints memory statistics once a second:
>
> #!/bin/sh
>
> while true
> do
> date
> cat /proc/[0-9]*/stat |
> awk '$23 > 0 {printf "%5d %20s %8d %5d\n", $1, $2, $23, $24}' |
> sort -n
> sleep 60
> done
>
> When I run that remotely via dropbea:
>
> $ ssh root@10.0.0.99 ./showmem.sh
>
> Everything works as expected while the ssh session is active, but when
> I end the ssh connection, the shell script never terminates: it
> continues to run (indifinitely, AFAICT). I don't recall this happening
> when we used to use openssh's server. Is there any way to get dropbear
> to terminate a "child" program when the ssh session closes?
>
>
>
>
>
