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Date:   Tue, 3 Jul 2018 14:15:34 -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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----- On Jul 3, 2018, at 2:11 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Jul 03, 2018 at 01:58:37PM -0400, Mathieu Desnoyers wrote:
>> I can modify the ABI to put the cpu_id_start and cpu_id fields inside
>> a union, and update it with a single store.
>> 
>> Thoughts ?
> 
> Let's keep them for now, we can always frob this later, they are aligned
> and proper, no need to expose that union to userspace.

Isn't it weird to change the API of an exposed public uapi header ? What
if userspace chooses to do sizeof(__rseq_abi.cpu_id) ? We would break
this unless we use a transparent union, which puts constraints I would
hope not to have on compilers supporting transparent unions (I recall
C++ had issues with this).

I'd prefer to expose the union right away if it's fine with you.

Thanks,

Mathieu


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