linux-kernel - Re: [resend RFC 2/3] sched/prctl: add PR_SCHED_CORE

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Date:   Tue, 25 Jan 2022 13:15:45 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Chris Hyser <chris.hyser@...cle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Balbir Singh <sblbir@...zon.com>
Subject: Re: [resend RFC 2/3] sched/prctl: add PR_SCHED_CORE_SHARE command

On Mon, Jan 24, 2022 at 04:31:05PM -0800, Josh Don wrote:
> Hey Christian,
> 
> This seems like a reasonable extension of the interface to me.
> 
> > @@ -200,6 +212,20 @@ int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
> >                 __sched_core_set(current, cookie);
> >                 goto out;
> >
> > +       case PR_SCHED_CORE_SHARE:
> > +               rcu_read_lock();
> > +               p = task_by_pid(pid_share);
> > +               if (!p)
> > +                       err = -ESRCH;
> > +               else if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
> > +                       err = -EPERM;
> > +               if (!err)
> > +                       cookie = sched_core_clone_cookie(p);
> > +               rcu_read_unlock();
> > +               if (err)
> > +                       goto out;
> > +               break;
> > +
> 
> Did you consider folding this into SCHED_CORE_SHARE_TO? SHARE_TO isn't
> using the last arg right now; it could use it as an override for the
> task we copy the cookie from instead of always choosing 'current'.
> Since the code currently rejects any SCHED_CORE prctl calls with a
> non-zero last arg for commands other than SCHED_CORE_GET, this would
> be a safe change for userspace.

Yeah, that sounds good to me too. I can certainly rework the patch to do
that!