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

From: Mathieu Desnoyers
Date: Mon Jul 02 2018 - 20:23:22 EST

----- On Jul 2, 2018, at 8:19 PM, Chris Lameter cl@xxxxxxxxx wrote:

> On Mon, 2 Jul 2018, Mathieu Desnoyers wrote:
>> 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.
> 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.



Mathieu Desnoyers
EfficiOS Inc.