linux-kernel - Re: [PATCH] prctl: add PR_[GS]ET_PDEATHSIG

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Date:   Wed, 13 Sep 2017 19:26:18 +0200
From:   Jürg Billeter <j@...ron.ch>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH] prctl: add PR_[GS]ET_PDEATHSIG_PROC

On Wed, 2017-09-13 at 19:11 +0200, Oleg Nesterov wrote:
> On 09/12, Jürg Billeter wrote:
> > 
> > On Tue, 2017-09-12 at 19:05 +0200, Oleg Nesterov wrote:
> > > On 09/09, Jürg Billeter wrote:
> > > > Unlike
> > > > PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole
> > > > subtree without race conditions.
> > > 
> > > but I am still not sure this is right... at least I can't understand the
> > > "without race conditions" above.
> > > 
> > > IOW, the child can do prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) right after fork(),
> > > why this is not enough to kill a whole subtree without race conditions?
> > 
> > What if the parent dies between fork() and prctl()?
> 
> The child will be killed? Sorry, can't understand...

If PR_SET_PDEATHSIG_PROC was not inherited across fork and the parent
died between fork() and prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) in the
child, the child would not be killed. It would be reparented to init(1)
or a subreaper, i.e., you end up with a runaway process. It would be
possible to safe guard against this race condition in other ways but
inheriting the setting avoids it nicely, and makes it easy to
apply/enforce PDEATHSIG_PROC for all descendants.

> > > Say, CLONE_PARENT. Should it succeed if ->pdeath_signal_proc != 0 ?
> > 
> > Yes, I don't see an issue with that. The new process will be a sibling
> > and inheriting pdeath_signal_proc seems sensible to me for this.
> 
> I meant, the process created by clone(CLONE_PARENT) won't be killed by
> pdeath_signal if the creator process exits, exactly because it won't be
> its child. Not that I think this is wrong.

Right, creator and parent won't be the same.

Jürg