linux-kernel - Re: [PATCH 1/4] gpio: Remove VLA from gpiolib

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Message-ID: <CAHp75VeJ5WQeEFaoExCCrFBuuKUX+17vC5JOdMW8V4KNvniT-Q@mail.gmail.com>
Date:   Mon, 19 Mar 2018 17:09:54 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Laura Abbott <labbott@...hat.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Mathias Duckeck <m.duckeck@...bus.de>,
        Nandor Han <nandor.han@...com>,
        Semi Malinen <semi.malinen@...com>,
        Patrice Chotard <patrice.chotard@...com>
Subject: Re: [PATCH 1/4] gpio: Remove VLA from gpiolib

On Mon, Mar 19, 2018 at 9:00 AM, Lukas Wunner <lukas@...ner.de> wrote:
> On Sun, Mar 18, 2018 at 09:34:12PM +0100, Rasmus Villemoes wrote:
>> On 2018-03-18 15:23, Lukas Wunner wrote:

>> > Actually, scratch that.  If ngpio is usually smallish, we can just
>> > allocate reasonably sized space for mask and bits on the stack,
>> > and fall back to the kcalloc slowpath only if chip->ngpio exceeds
>> > that limit.

>> Well, I'd suggest not adding that fallback code now, but simply add a
>> check in gpiochip_add_data_with_key to ensure ngpio is sane (and refuse
>> to register the chip otherwise), at least if we know that every
>> currently supported/known chip is covered by the 256 (?).
>
> The number 256 was arbitrarily chosen.  I really wouldn't be surprised
> if gpiochips with more pins exist, but they're probably rare, so using
> the slowpath seems fine, but dropping support for them completely would
> be a regression.

All modern Intel SoCs have GPIO count in between of ~230-380.
Though, few of them are split to communities by (much) less than 256
pin in each when there is a 1:1 mapping between community and
gpiochip.

OTOH, the function you are fixing is most likely is not used together
with the drivers for x86.

-- 
With Best Regards,
Andy Shevchenko