linux-kernel - Re: Is prctl(PR_SET_CHILD_SUBREAPER) going to break my code which checks getppid == 1?

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Date:	Fri, 30 Mar 2012 08:17:02 -0500
From:	Karl Pickett <kjp@...uchicago.edu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kay.sievers@...y.org,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: Is prctl(PR_SET_CHILD_SUBREAPER) going to break my code which checks getppid == 1?

On Mar 30, 2012, at 7:44 AM, Oleg Nesterov wrote:

> On 03/29, Karl Pickett wrote:
>> 
>> Re: http://thread.gmane.org/gmane.linux.kernel/1236479
>> 
>> I'm wondering if this is going to break code that checks getppid() == 1?
>> 
>> I have a TCL/TK GUI app that spawns ssh.  I want the ssh to die
>> immediately if the GUI process crashes, so ssh is launched with a wrapper
>> c program that does:
>> 
>>   prctl(PR_SET_PDEATHSIG, SIGHUP);
>>   if (getppid() == 1) /* parent died already? */
>>       return 0;
> 
> Yes, this won't work if the parent of this app does PR_SET_CHILD_SUBREAPER.
> 
>> So what is getppid() going to return for some user using this new
>> 'session manager reaper'?
> 
> The new parent's pid ;)
> 
> Perhaps you can do something like
> 
> 	ppid_for_child = getpid();
> 
> 	if (!fork()) {
> 		// Child
> 		prctl(PR_SET_PDEATHSIG);
> 		if (getppid() != ppid_for_child)
> 			return;
> 		...
> 	}

There are two problems with that.  1., I don't think TCL/TK lets me access the parent pre-fork env like that - all I can change is the execed code.  2., That has a clear race with pid wrap around.  You really need a prctl(PR_DID_MY_REAL_PARENT_DIE) function to be safe.


> 
> Just in case, I do not know how PR_SET_CHILD_SUBREAPER will be really used.
> I do not know if systemd will run the "normal" user applications under
> PR_SET_CHILD_SUBREAPER. Probably yes, but this is the question to Kay.
> 
> Oleg.
> 

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