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

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Date:	Wed, 9 May 2012 23:14:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mike Frysinger <vapier@...too.org>
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 05/09, Mike Frysinger wrote:
>
> 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

And? In this case it will be killed, yes. But this zombie can't go away
until the tracer does do_wait().

OK, the multi-threaded exec adds more fun.

> > 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

>From this pov PTRACE_KILL is safer, I agree ;)

Oleg.

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