Subject: Re: [PATCH v7 10/22] sched: Reject CPU affinity changes based on task_cpu_possible_mask()

From: Peter Zijlstra <[email-masked]>
Date: Wed, 26 May 2021 17:15:51 +0200

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 <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  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.

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