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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 2, 2018, at 7:06 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

> On Mon, Jul 2, 2018 at 4:00 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
>> with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
>> the padding with 0, and only update the low bits with single-copy atomicity.
> 
> Well... It's actually still single-copy atomicity as a 64-bit value.
> 
> Why? Because it doesn't matter how you write the upper bits. You'll be
> writing the same value to them (zero) anyway.
> 
> So who cares if the write ends up being two instructions, because the
> write to the upper bits doesn't actually *do* anything.
> 
> Hmm?

Are there any kind of guarantees that a __u64 update on a 32-bit architecture
won't be torn into something daft like byte-per-byte stores when performed
from C code ?

I don't worry whether the upper bits get updated or how, but I really care
about not having store tearing of the low bits update.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com