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

[Peter Zijlstra]

On Mon, Jul 02, 2018 at 10:30:09PM -0400, Mathieu Desnoyers wrote:
> > Use "get_user()". It works for 64-bit objects too, and it will be
> > atomic in the 32-bit sub-parts on a 32-bit architecture.
> 
> Is it really ? Last time we had this discussion, not all architectures
> guaranteed that reading a 64-bit integer would happen in two atomic
> 32-bit sub-parts. This was the main motivation for the LINUX_FIELD_u32_u64()
> macro as it stands today (rather than using a union).

Just state, as a requirement for supporting rseq, that the arch
{get,put}_user(u64) on 32bit targets must be exactly 2 u32 loads/stores.

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?