linux-kernel - Re: [PATCH v7 10/22] sched: Reject CPU affinity changes based on task_cpu_possible

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Date:   Wed, 26 May 2021 17:15:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 10/22] sched: Reject CPU affinity changes based on
 task_cpu_possible_mask()

On Tue, May 25, 2021 at 04:14:20PM +0100, Will Deacon wrote:
> Reject explicit requests to change the affinity mask of a task via
> set_cpus_allowed_ptr() if the requested mask is not a subset of the
> mask returned by task_cpu_possible_mask(). This ensures that the
> 'cpus_mask' for a given task cannot contain CPUs which are incapable of
> executing it, except in cases where the affinity is forced.
> 
> Reviewed-by: Quentin Perret <qperret@...gle.com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>  kernel/sched/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 00ed51528c70..8ca7854747f1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2346,6 +2346,7 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>  				  u32 flags)
>  {
>  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
> +	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
>  	unsigned int dest_cpu;
>  	struct rq_flags rf;
>  	struct rq *rq;
> @@ -2366,6 +2367,9 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>  		 * set_cpus_allowed_common() and actually reset p->cpus_ptr.
>  		 */
>  		cpu_valid_mask = cpu_online_mask;
> +	} else if (!cpumask_subset(new_mask, cpu_allowed_mask)) {
> +		ret = -EINVAL;
> +		goto out;
>  	}

So what about the case where the 32bit task is in-kernel and in
migrate-disable ? surely we ought to still validate the new mask against
task_cpu_possible_mask.