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Date:   Tue, 3 Jul 2018 20:10:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.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 Tue, Jul 03, 2018 at 10:59:45AM -0700, Linus Torvalds wrote:
> On Tue, Jul 3, 2018 at 10:49 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > I can simply document that loads/stores from/to all struct rseq fields
> > > should be thread-local then ?
> >
> > I'm not sure that covers things sufficiently. You really want the
> > userspace load/stores to be single instructions.
> 
> Actually, I think we should try very hard to limit even that to _just_
> the rseq pointer itself.

> So I'd suggest that the only part we aim to have any "atomic" behavior
> at all is for the individual fields in "struct rseq" itself. So the
> cpu id and the base pointer and the flags. And even they are
> thread-local, so the atomicity is not about the kernel, but about user
> space needing to read and update them in word-sized chunks.
> 
> End result: absolutely nothing is atomic for the kernel.

Yes, agreed, that is what I meant but very poorly expressed. Only the
rseq bits themselves need this single-copy atomic stuff -- for
userspace.