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

Peter Zijlstra
2021-05-26T16:32:56Z

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@google.com>
> Signed-off-by: Will Deacon <will@kernel.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.