Re: [PATCH v3 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

[Will Deacon <will@kernel.org>]

On Thu, Nov 19, 2020 at 09:24:07AM +0000, Quentin Perret wrote:
> On Friday 13 Nov 2020 at 09:37:13 (+0000), Will Deacon wrote:
> > When exec'ing a 32-bit task on a system with mismatched support for
> > 32-bit EL0, try to ensure that it starts life on a CPU that can actually
> > run it.
> > 
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/kernel/process.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 1540ab0fbf23..17b94007fed4 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp)
> >  	return sp & ~0xf;
> >  }
> >  
> > +static void adjust_compat_task_affinity(struct task_struct *p)
> > +{
> > +	const struct cpumask *mask = system_32bit_el0_cpumask();
> > +
> > +	if (restrict_cpus_allowed_ptr(p, mask))
> > +		set_cpus_allowed_ptr(p, mask);
> 
> My understanding of this call to set_cpus_allowed_ptr() is that you're
> mimicking the hotplug vs affinity case behaviour in some ways. That is,
> if a task is pinned to a CPU and userspace hotplugs that CPU, then the
> kernel will reset the affinity of the task to the remaining online CPUs.

Correct. It looks to the 32-bit application like all the 64-bit-only CPUs
were hotplugged off at the point of the execve().

> I guess that is a sensible fallback path when userspace gives
> contradictory commands to the kernel, but that most certainly deserves a
> comment :)

Good point, I'll add this:

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index dba94af1b840..687d6acf2f81 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -619,6 +619,16 @@ static void adjust_compat_task_affinity(struct task_struct *p)
 {
 	const struct cpumask *mask = system_32bit_el0_cpumask();
 
+	/*
+	 * Restrict the CPU affinity mask for a 32-bit task so that it contains
+	 * only the 32-bit-capable subset of its original CPU mask. If this is
+	 * empty, then forcefully override it with the set of all
+	 * 32-bit-capable CPUs that we know about.
+	 *
+	 * From the perspective of the task, this looks similar to what would
+	 * happen if the 64-bit-only CPUs were hot-unplugged at the point of
+	 * execve().
+	 */
 	if (restrict_cpus_allowed_ptr(p, mask))
 		set_cpus_allowed_ptr(p, mask);
 }

Will