Re: [PATCH v2 1/2] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity()

[Will Deacon <will-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Mon, Aug 01, 2022 at 11:41:23AM -0400, Waiman Long wrote:
> The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched:
> Introduce task_struct::user_cpus_ptr to track requested affinity"). It
> is currently used only by arm64 arch due to possible asymmetric cpu
> setup. This patch extends its usage to save user provided cpumask when
> sched_setaffinity() is called for all arches.
> 
> To preserve the existing arm64 use case, a new cpus_affinity_set flag is
> added to differentiate if user_cpus_ptr is set up by sched_setaffinity()
> or by force_compatible_cpus_allowed_ptr(). user_cpus_ptr
> set by sched_setaffinity() has priority and won't be
> overwritten by force_compatible_cpus_allowed_ptr() or
> relax_compatible_cpus_allowed_ptr().
> 
> As a call to sched_setaffinity() will no longer clear user_cpus_ptr
> but set it instead, the SCA_USER flag is no longer necessary and can
> be removed.
> 
> Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
>  include/linux/sched.h |  1 +
>  kernel/sched/core.c   | 71 +++++++++++++++++++++++++++++++------------
>  kernel/sched/sched.h  |  1 -
>  3 files changed, 52 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index c46f3a63b758..60ae022fa842 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -815,6 +815,7 @@ struct task_struct {
>  
>  	unsigned int			policy;
>  	int				nr_cpus_allowed;
> +	int				cpus_affinity_set;
>  	const cpumask_t			*cpus_ptr;
>  	cpumask_t			*user_cpus_ptr;
>  	cpumask_t			cpus_mask;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index da0bf6fe9ecd..7757828c7422 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2607,6 +2607,7 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>  		return -ENOMEM;
>  
>  	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +	dst->cpus_affinity_set = src->cpus_affinity_set;

I haven't been through this thorougly, but it looks a bit suspicious to me
to inherit this field directly across fork(). If a 64-bit task with this
flag set forks and then exec's a 32-bit program, arm64 will be in trouble if
we're not able to override the affinity forcefully.

Will