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

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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?