[RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu

[Catalin Marinas]

On Fri, Oct 09, 2020 at 11:25:59AM +0200, Peter Zijlstra wrote:
> On Fri, Oct 09, 2020 at 10:13:12AM +0200, Morten Rasmussen wrote:
> > On Fri, Oct 09, 2020 at 09:29:43AM +0200, Peter Zijlstra wrote:
> 
> > > Fundamentally, you're not supposed to change the userspace provided
> > > affinity mask. If we want to do something like this, we'll have to teach
> > > the scheduler about this second mask such that it can compute an
> > > effective mask as the intersection between the 'feature' and user mask.
> > 
> > I agree that we shouldn't mess wit the user-space mask directly. Would it
> > be unthinkable to go down the route of maintaining a new mask which is
> > the intersection of the feature mask (controlled and updated by arch
> > code) and the user-space mask?
> > 
> > It shouldn't add overhead in the scheduler as it would use the
> > intersection mask instead of the user-space mask, the main complexity
> > would be around making sure the intersection mask is updated correctly
> > (cpusets, hotplug, ...).
> 
> IFF we _need_ to go there, then yes that was the plan, compose the
> intersection when either the (arch) feature(set) mask or the userspace
> mask changes.

Another thought: should the arm64 compat_elf_check_arch() and
sys_arm64_personality(PER_LINUX32) validate the user cpumask before
returning success/failure? It won't prevent the user from changing the
affinity at run-time but at least it won't randomly get killed just
because the kernel advertises 32-bit support.

-- 
Catalin