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

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Date:   Wed, 13 Sep 2017 19:48:04 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jürg Billeter <j@...ron.ch>
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 09/13, Jürg Billeter wrote:
>
> 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.

Aah, sorry. I forgot about another oddity of pdeath_signal API...

Somehow I misread this patch as if reparent_leader() looks at
current->signal->pdeath_signal_proc, not child->signal->pdeath_signal_proc.
And to me the former makes more sense. But I won't insist.

Oleg.