linux-kernel - Re: [PATCH] ARM: disallow ARM

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Message-ID: <alpine.LFD.2.20.1612182308290.3360@knanqh.ubzr>
Date:   Sun, 18 Dec 2016 23:10:50 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Jonas Jensen <jonas.jensen@...il.com>
Subject: Re: [PATCH] ARM: disallow ARM_THUMB for ARMv4 builds

On Sun, 18 Dec 2016, Russell King - ARM Linux wrote:

> On Sun, Dec 18, 2016 at 03:04:24PM +0000, Ard Biesheuvel wrote:
> > The only issue I spotted is that the kuser_get_tls routine has only
> > two instruction slots for the return sequence, but we can easily work
> > around that by moving the TLS hardware instruction around in the
> > template (and update the memcpy accordingly in kuser_init()
> 
> You can't actually - everything in this page is ABI, and moving
> that breaks the ABI.
> 
> One thing I'm toying with is splitting out the kuser helpers.  That
> means we can build it according to the configuration, and select the
> appropriate version at run time.  Work in progress.

That's the best solution indeed.  In fact there is already some runtime 
patching of the kuser page for how to retrieve the tls value in 
kuser_init().


Nicolas