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Date:   Mon, 2 Jul 2018 21:17:24 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Chris Lameter <cl@linux.com>
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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 2, 2018, at 8:35 PM, Chris Lameter cl@linux.com wrote:

> On Mon, 2 Jul 2018, Mathieu Desnoyers wrote:
> 
>> >
>> > Platforms with 32 bit word size only guarantee atomicity of a 32 bit
>> > write or RMV instruction.
>> >
>> > Special instructions may exist on a platform to perform 64 bit atomic
>> > updates. We use cmpxchg64 f.e. on Intel 32 bit platforms to guarantee
>> > atomicity8.
>> >
>> > So use the macros that we have to guarantee 64 bit ops and you should be
>> > fine. See linux/arch/x86/include/asm/atomic64_32.h
>>
>> We are talking about user-space here. What we need is a single instruction
>> atomic store, similar to what WRITE_ONCE() does in the kernel. The discussion
>> is about whether doing the user-space equivalent of a WRITE_ONCE() to a u64
>> on a 32-bit architecture should be considered to provide single-copy atomicity
>> on the low 32 bits.
> 
> Right. You would need to make this work for userspace. atomic64_32.h is a
> good reference as to which instructions provide 64 bit atomicity on 32
> bit platforms.

We only need to update a pointer, so we don't need 64-bit atomicity on
32-bit processes.

What we need is to ensure single-copy atomicity of the 32-bit pointer update
on the 32-bit process in a field read from the kernel as a __u64.

Thanks,

Mathieu


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