linux-kernel - Re: [RFD] posix-timers: CRIU woes

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Date:   Thu, 11 May 2023 10:49:37 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andrey Vagin <avagin@...nvz.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Christian Brauner <brauner@...nel.org>,
        Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [RFD] posix-timers: CRIU woes

On Wed, May 10, 2023 at 01:16:26AM -0700, Andrey Vagin wrote:
... 
> Hi Thomas,
> 
> If you give us a new API to create timers with specified id-s, we will
> figure out how to live with it. It isn't good to ask users to update
> CRIU to work on new kernels, but here are reasons and event improvements
> for CRIU, so I think it's worth it.
> 
> As for API, we can use one bit of sigevent.sigev_notify to request a
> timer with a specified id.

Which will do the trick but would look somehow strange I think, since signals
are not some how related to timer's ID. Another option might be to use output
`created_timer_id` parameter as an input cookie.

Say we describe input as

struct {
	u32 magic;
	timer_t timer_id;
};

Then if magic doesn't match we use `created_timer_id` for output only, and
otherwise we read `timer_id` from input and use it. Of course there is a
chance that some unitialized memory passed with existing old programs but
i think false positive gonna be very-very low if ever. Just IMHO.

	Cyrill