Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs

From: Peter Zijlstra
Date: Tue Jul 03 2018 - 13:03:15 EST

On Tue, Jul 03, 2018 at 09:40:48AM -0700, Andi Kleen wrote:
> >
> > So I think you're good... But yes, you raise an interresting point.
> So it sounds like architectures that don't have an instruction atomic u64
> *_user need to disable interrupts during the access, and somehow handle that
> case when a page fault happens?

So for 32bit, as Linus already said, a split store is _fine_, because
the top word is always going to be 0 anyway.

So all we really need is native word sized loads / stores. s390 is just
a little weird here (it wouldn't be s390 if it wasn't I suppose) for not
actually using regular loads / stores because of the split address space