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

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> > > We're piece-wise enabling rseq across architectures anyway, and when the
> > > relevant maintains do this, they can have a look at their
> > > {get,put}_user() implementations and fix them.
> > > 
> > > If you rely on get_user(u64) working, that means microblaze is already
> > > broken, but I suppose it already was, since their rseq enablement patch
> > > is extremely dodgy. Michal?
> > 
> > s390 uses the mvcos instruction to implement get_user(). That instruction
> > is not defined to be atomic, but may copy bytes piecemeal.. I had the
> > impression that the rseq fields are supposed to be updated within the
> > context of a single thread (user + kernel space).
> > 
> > However if another user space thread is allowed to do this as well, then
> > the get_user() approach won't fly on s390.
> > 
> > That leaves the question: does it even make sense for a thread to update
> > the rseq structure of a different thread?
> 
> The problem is interrupts; we need interrupts on the CPU doing the store
> to observe either the old or the new value, not a mix.
> 
> If mvcos does not guarantee that, we're having problems. Is there a
> reason get_user() cannot use a 'regular' load?

Well, that's single instruction semantics. This is something we actually
can guarantee, since the mvcos instruction itself won't be interrupted and
copies all 1/2/4/8 bytes in a row.

So we are talking about that single instructions are required and not
atomic accesses?