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Date:   Tue, 3 Jul 2018 13:38:59 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Peter Zijlstra <peterz@infradead.org>
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Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs
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Thread-Topic: rseq: use __u64 for rseq_cs fields, validate user inputs
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----- On Jul 3, 2018, at 1:34 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Jul 03, 2018 at 10:10:37AM -0700, Linus Torvalds wrote:
>> On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <andi@firstfloor.org> wrote:
>> >
>> > So it sounds like architectures that don't have an instruction atomic u64
>> > *_user need to disable interrupts during the access, and somehow handle that
>> > case when a page fault happens?
>> 
>> No. It's actually the store by *user* space that is the critical one.
>> Not the whole 64-bit value, just the low pointer part.
>> 
>> The kernel could do it as a byte-by-byte load, really. It's
>> per-thread, and once the kernel is running, it's not going to change.
>> The kernel never changes the value, it just loads it from user space.
> 
> The kernel doesn't change _this_ value, but the kernel does change other
> values, like for instance rseq->cpu_id. But even there, it could use
> byte stores and it is again the userspace load of that field that is
> critical again and needs to be a single op.

I can simply document that loads/stores from/to all struct rseq fields
should be thread-local then ?

Thanks,

Mathieu


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