linux-kernel - Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

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Date:   Tue, 4 Dec 2018 12:05:44 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     mhocko@...nel.org
Cc:     Ingo Molnar <mingo@...nel.org>, pavel@....cz,
        Oleg Nesterov <oleg@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        rafael.j.wysocki@...el.com, chanho.min@....com,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

On Tue, Dec 4, 2018 at 11:49 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> because honestly, the *only* reason we hold on to that lock is for the
> insane and not really interesting case of "somebody tried to use
> ptrace to change the creds in-flight during the exec".

No, sorry, me confused. Not somebody trying to change them, it's just
ptrace_attach() trying to change _our_ state during this sequence, and
relying on it all being atomic.

So taking a ref is unnecessary and pointless. It's not the creds that
change, it's that we really want to delay ptrace_attach().

We could maybe set that "we're busy now" flag, and have
ptrace_attach() do something like

        if (task_is_busy(task)) {
                sched_yield();
                return -ERESTARTSYS;
        }

or something like that.

                  Linus