Subject: Re: [PATCH v3 11/14] sched: Reject CPU affinity changes based on arch_cpu_allowed_mask()

From: Quentin Perret <[email-masked]>
Date: Thu, 19 Nov 2020 09:47:44 +0000

On Friday 13 Nov 2020 at 09:37:16 (+0000), 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 arch_cpu_allowed_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.
> 
> 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 8df38ebfe769..13bdb2ae4d3f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1877,6 +1877,7 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
>  					 struct rq_flags *rf)
>  {
>  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
> +	const struct cpumask *cpu_allowed_mask = arch_cpu_allowed_mask(p);
>  	unsigned int dest_cpu;
>  	int ret = 0;
>  
> @@ -1887,6 +1888,9 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
>  		 * Kernel threads are allowed on online && !active CPUs
>  		 */
>  		cpu_valid_mask = cpu_online_mask;
> +	} else if (!cpumask_subset(new_mask, cpu_allowed_mask)) {
> +		ret = -EINVAL;
> +		goto out;

So, IIUC, this should make the sched_setaffinity() syscall fail and
return -EINVAL to userspace if it tries to put 64bits CPUs in the
affinity mask of a 32 bits task, which I think makes sense.

But what about affinity change via cpusets? e.g., if a 32 bit task is
migrated to a cpuset with 64 bit CPUs, then the migration will be
'successful' and the task will appear to be in the destination cgroup,
but the actual affinity of the task will be something completely
different?

That's a bit yuck, but I'm not sure what else can be done here :/

Thanks,
Quentin

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