linux-kernel - Re: [PATCH v2 1/3] Revert "do_SAK: Don't recursively take the tasklist

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Message-ID: <20180117173415.GA7964@redhat.com>
Date:   Wed, 17 Jan 2018 18:34:15 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>, gregkh@...uxfoundation.org,
        jslaby@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] Revert "do_SAK: Don't recursively take the
 tasklist_lock"

On 01/17, Eric W. Biederman wrote:

> Kirill Tkhai <ktkhai@...tuozzo.com> writes:
>
> > This reverts commit 20ac94378de5.
> >
> > send_sig() does not take tasklist_lock for a long time,
> > so this commit and the problem it solves are not relevant
> > anymore.
> >
> > Also, the problem of force_sig() is it clears SIGNAL_UNKILLABLE
> > flag, thus even global init may be killed by __do_SAK(),
> > which is definitely not the expected behavior.
>
> Actually it is.
>
> SAK should kill everything that has the tty open.  If init opens the tty
> I am so sorry, it can not operate correctly.  init should not have your
> tty open.

OK, but then we need "force" in other places too. __do_SAK() does send_sig(SIGKILL)
in do_each_pid_task(PIDTYPE_SID) and if signal->tty == tty.

Plus force_sig() is not rcu-friendly.

So I personally agree with this change. Whether we want to kill the global init
or not should be discussed, if we want to do this __do_SAK() should use
SEND_SIG_FORCED and this is what Kirill is going to do (iiuc), but this needs
another patch.

Oleg.