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

From: Mathieu Desnoyers
Date: Mon Jul 02 2018 - 22:30:15 EST

----- On Jul 2, 2018, at 10:18 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:

> On Mon, Jul 2, 2018 at 7:01 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> One thing to consider is how we will implement the load of that pointer
>> on the kernel side.
> 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).

> Again: there is no point in trying to be atomic in the full 64 bits
> (when you're running on 32-bit). The upper bits don't have to "match"
> the lower bits. They just have to be zero. So doing it as two loads is
> fine - the same way it's perfectly fine to do it as two stores (since
> the store to the upper bits will always be zero).

I'd be fine with two atomic loads, but I'd rather have a strong
confirmation about this, because last time around there were
architectures where it was not true as far as I recall.



> Linus

Mathieu Desnoyers
EfficiOS Inc.