Re: interesting claims

[Steve Litt <slitt@troubleshooters.com>]

On Mon, 29 Apr 2019 21:19:58 +0200
Jeff <sysinit@yandex.com> wrote:

> i came across some interesting claims recently. on
> http://skarnet.org/software/s6/
> it reads
> 
> "suckless init is incorrect, because it has no supervision
> capabilities, and thus, killing all processes but init can brick the
> machine."

Oh, that.

First of all, Suckless Init is a PID1 that forks an rc script and then
hangs around reaping zombies. You could use that rc file to run
supervision systems such as daemontools-encore and the supervision part
of runit and s6. So Suckless Init is the PID1 part of an init system.
It's 83 lines of C.

There are three philosophies: 

1) The supervision should be done by PID1: Supported by Laurent Bercot

2) The supervision should be done outside of PID1: Perhaps supported by
   Rich Felker in his http://ewontfix.com/about/

> 
> a rather bold claim IMO !
> where was the "correct" init behaviour specified ?
> where can i learn how a "correct" init has to operate ?
> or is it true since s6-svscan already provides such respawn
> capabilities ? ;-)
> 
> there is actually NO need for a "correct" working init implementation
> to provide respawn capabilities at all IMO.
> this can easily done in/by a subprocess and has 2 advantages:
> 
> - it simplyfies the init implementation
> 
> - process #1 is the default subprocess reaper on any unix
>   implementation and hence a lot of terminated zombie subprocesses
>   get assigned to it, subprocesses that were not started by it.
>   if it has respawn capabilities it has to find out if any of this
> recently assigned but elsewhere terminated subprocesses is one of its
>   own childs to be respawned. if it has lots of services to respawn
>   this means lots of unnecessary work that could be also done
>   in/by a suprocess aswell.
> 
> when do you kill a non supvervised process running with UID 0
> "accidently" ? when calling kill ( -1, SIGTERM ) ?
> the kernel protects special/important processes in this case from
> being killed "accidently", that's true.
> but where do we usually see that ? in the shutdown stage, i guess.
> and that's exactly where one wants to kill all process with PID > 1
> (sometimes excluding the calling process since it has to complete
> more tasks). or when going into single user mode.
> 
> so this looks like a rather artificial and constructed argument for
> the necessity of respawn functionality in an init implementation IMO.
> 



-- 
SteveT

Steve Litt 
January 2019 featured book: Troubleshooting: Just the Facts
http://www.troubleshooters.com/tjust