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Posted Nov 23, 2014 16:37 UTC (Sun) by flussence (guest, #85566)
In reply to: Thanks by mgb
Parent article: Today's Debian technical committee resignation: Ian Jackson

Reliably upgrading sshd doesn't have much in common with socket activation. You can still do the normal "restart daemon, log in on new sshd to verify it works, log out of old sshd when it's safe" routine, though that of course renders this micro-optimization pointless.

On the other hand, a system using cgroups to indiscriminately purge entire process ancestries will create exactly the problem you're describing... by design!

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Posted Nov 23, 2014 16:57 UTC (Sun) by mgb (guest, #3226) [Link] (5 responses)

> On the other hand, a system using cgroups to indiscriminately purge entire process ancestries will create exactly the problem you're describing... by design!

Exactly.

A hypothetical modular portable dependency-based init could have been a useful tool but systemd is the opposite of a tool - it is a restrictive imposition. Systemd's short-sighted design is extraordinarily counter-productive and this becomes glaringly obvious as systemd attempts to move from its desktop niche to the critical server and embedded spaces.

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Posted Nov 23, 2014 17:53 UTC (Sun) by zuki (subscriber, #41808) [Link] (4 responses)

>> Reliably upgrading sshd doesn't have much in common with socket
>> activation. You can still do the normal "restart daemon, log in on new
>> sshd to verify it works, log out of old sshd when it's safe" routine,
>> though that of course renders this micro-optimization pointless.
Not if exit-on-idle is implemented. If it is, you can do a test login after upgrade, and still have the process go away after while.

I agree that for a server this is pointless, but let's say that you are running containers or lightweight VMs, in multiple instances. Then avoiding starting the process can be a nice optimization.

>> On the other hand, a system using cgroups to indiscriminately purge
>> entire process ancestries will create exactly the problem you're
>> describing... by design!
> Exactly.
I don't grok. When an SSH session is started, the ssh instance and user process forked off it are moved to a separate scope unit, which of course also means a different subtree in the cgroup hierarchy. So individual sessions are completely independent of the ssh service. As for the sshd service itself, yes, all processes in it are stopped during restart. That pretty much describes the basic functionality of systemd achieved with cgroups. If you think something is wrong with "purging" all processes of a service when it is stopped, please explain.

[snip the rant]

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Posted Nov 23, 2014 18:06 UTC (Sun) by mgb (guest, #3226) [Link] (3 responses)

> If you think something is wrong with "purging" all processes of a service when it is stopped, please explain.

You should try pre-systemd Debian Stable which doesn't kill existing sshd connections during upgrades. It's great.

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Posted Nov 23, 2014 18:15 UTC (Sun) by zuki (subscriber, #41808) [Link] (2 responses)

>> If you think something is wrong with "purging" all processes of a
>> service when it is stopped, please explain.
> You should try pre-systemd Debian Stable which doesn't kill existing sshd
> connections during upgrades. It's great.

Once again: existing sshd connections *are* *not* *part* of sshd.service.
After they are established they are independent and are not touched when sshd.service is stopped or restarted.

(It is possible that there's a bug in your Debian package or setup or whatever... I can only say that it works for me and apparently for most people, and of course is *designed* to work this way. If it doesn't work for you, please provide the details and we'll work on a fix. Probably best to do this on the distribution bugtracker rather than here though.)

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Posted Nov 23, 2014 18:43 UTC (Sun) by mpr22 (subscriber, #60784) [Link] (1 responses)

Asking mgb for details about specific failures of systemd-based systems is a waste of electrons; they have explicitly stated on this site a refusal to use systemd or help debug systemd.

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Posted Nov 25, 2014 16:31 UTC (Tue) by nix (subscriber, #2304) [Link]

It's also clear from this subthread that mgb doesn't bother to actually read posts to which it responds: rather, it scans them for things that can be attacked, and ignores everything else, including inconvenient facts that might contradict its rants.

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Posted Nov 23, 2014 23:28 UTC (Sun) by mchapman (subscriber, #66589) [Link] (1 responses)

> On the other hand, a system using cgroups to indiscriminately purge entire process ancestries will create exactly the problem you're describing... by design!

Except it doesn't. There are two reasons for this.

First, pam_systemd will move the SSH child process into a different cgroup.

Second, even if it didn't do this, the sshd.service contains KillMode=process, which means only the main process is killed upon stop or restart. Other processes in the sshd.service cgroup are unaffected.

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Posted Nov 24, 2014 20:16 UTC (Mon) by smurf (subscriber, #17840) [Link]

This did happen, once upon a time. It was a bug. It was fixed rather quickly, for obvious reasons.

Too bad (from the point of view of the anti-systemd crowd, that is).