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From:   Josh Don <joshdon@google.com>
Date:   Mon, 24 Jan 2022 16:31:05 -0800
Message-ID: <CABk29NtFnswO3iaQobbijV1-FwCJd06prm2UMq7S8Tt736hYMA@mail.gmail.com>
Subject: Re: [resend RFC 2/3] sched/prctl: add PR_SCHED_CORE_SHARE command
To:     Christian Brauner <brauner@kernel.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Chris Hyser <chris.hyser@oracle.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@suse.de>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Balbir Singh <sblbir@amazon.com>
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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.