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

[Will Deacon]

On Wed, May 26, 2021 at 05:15:51PM +0200, Peter Zijlstra wrote:
> 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@xxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> >  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.

That's a good question.

Given that 32-bit tasks in the kernel are running in 64-bit mode, we can
actually tolerate them moving around arbitrarily as long as they _never_ try
to return to userspace on a 64-bit-only CPU. I think this should be the case
as long as we don't try to return to userspace with migration disabled, no?

Will