linux-kernel - Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW

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Message-ID: <20161005214729.GA27121@remoulade>
Date:   Wed, 5 Oct 2016 22:47:40 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Fredrik Markström <fredrik.markstrom@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nicolas Pitre <nico@...aro.org>,
        Will Deacon <will.deacon@....com>, kristina.martsenko@....com,
        linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Chris Brandt <chris.brandt@...esas.com>,
        Michal Marek <mmarek@...e.com>,
        Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Jonathan Austin <jonathan.austin@....com>
Subject: Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW

On Wed, Oct 05, 2016 at 10:01:38PM +0100, Russell King - ARM Linux wrote:
> On Wed, Oct 05, 2016 at 09:44:53PM +0100, Mark Rutland wrote:
> > The zeroing case is similar to the restartable sequences design. So that's
> > probably worth looking into.
> 
> You're sending mixed messages: in your previous message, you said:
> 
>   Arguably, someone could have (ab)used TPIDRURW between commits 6a1c531
>   and a4780ad to detect context switches, but in practice they don't
>   appear to have, and we know of an established user relying on the
>   current behaviour.
> 
>   For better or worse, the current behaviour is ABI.
> 
> Now you're suggesting that we could go back to the case where the
> register is zeroed.

Sorry; clumsy wording on my behalf.

I meant that functionality-wise, restartable sequences had similar behaviour to
the zeroing case (without touching TPIDRURW at all) and were probably worth
looking at. I did not intend to suggest that we should go pack to case where
TPIDRURW was zeroed.

> Well, the fact is that we _can_ change the TPIDRURW behaviour - we just
> need to be careful about how we change it.  Eg, we _could_ introduce a
> per-process flag which indicates that we want some other behaviour from
> TPIDRURW such as zeroing it on context switches.  The default would be
> to preserve the existing behaviour as doing anything else breaks
> existing programs.  The problem there is finding an acceptable way to
> control such a flag from userspace (eg, prctl, syscall, etc).

Sure. Something like that could work.

Thanks,
Mark.