linux-kernel - Re: ptrace.2: PTRACE_KILL needs a stopped process too

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Date:	Wed, 9 May 2012 12:41:56 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Pedro Alves <palves@...hat.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	pacman@...h.dhis.org, "linux-man" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	Jan Kratochvil <jan.kratochvil@...hat.com>
Subject: Re: ptrace.2: PTRACE_KILL needs a stopped process too

On Wednesday 09 May 2012 16:12:19 Oleg Nesterov wrote:
> On 05/09, Pedro Alves wrote:
> > On 05/09/2012 04:09 PM, Mike Frysinger wrote:
> > > probably not that big of a deal, but the reason i like using
> > > ptrace(PTRACE_KILL) over a raw kill() is that you are less likely to
> > > kill the wrong process by accident.  maybe not that big of a deal in
> > > practice though.
> > 
> > And you can do tgkill instead.  It was specifically invented to handle
> > the reuse case.
> 
> tgkill() can kill the wrong process/thread too, although it lessens the
> risk.
> 
> But I don't really understand the problem. The traced thread can't go away
> until the tracer does wait/detach, and thus its pid can't be reused?

or the process has received a SIGKILL for some reason

> May be, "by accident" above means something else, not pid reuse...

i like to assume that my code isn't going to be bug free, so the more 
mechanisms i have in place to protect innocent bystanders the better :)
-mike

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