ptrace in production systems

[Valery Reznic]

--- On Sun, 2/1/09, Shachar Shemesh <shachar at shemesh.biz> wrote:

> From: Shachar Shemesh <shachar at shemesh.biz>
> Subject: ptrace in production systems
> To: "linux-il" <linux-il at cs.huji.ac.il>
> Date: Sunday, February 1, 2009, 3:42 PM
> Hi all,
> 
> I've been bad. I know I have. This goes against any
> instinct that I have, but I am failing to see a good reason
> WHY.
> 
> The setup - I have an embedded system that is composed of
> several daemons. The situation is that one of the daemons
> has to restart another daemon. The restarted daemon (an SNMP
> agent) is outward facing, and therefor the time it takes it
> to restart should be minimized, if possible.
> 
> Then again, it is a daemon. It has no parent (well, init is
> its parent). I can find it easily enough using its pid file,
> but I cannot get a notification when it has actually quit.
> There is an option to add to the SNMP the commands to send a
> notification to the other daemon when it exists, but this
> has several disadvantages I will not go into right now. I
> can also poll for the exit (i.e. - kill( pid, 0) every
> second until it says there is no such process), but that
> adds latency until I begin the restart process.
> 
> So what I did was to use ptrace. The controlling daemon
> connects to the SNMP agent as a debugger, and this way gets
> notified with the usual "wait" interface when the
> agent exits. I am not doing any fancy register manipulation
> or any such stuff.
> 
> Using ptrace as part of a production system feels wrong,
> but I cannot say exactly why. The small amount of
> experimentation I did with this system, as well as my
> extensive experience with ptrace when working on fakeroot-ng
> tell me that the interface is stable enough. Still, it feels
> wrong.
> 
> Any feedback would be welcome.
> 
What about following:
#!/bin/sh

while :; do
   snmp_agen
done

Valery.

> Shachar
> 
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