kernel-hardening - Re: [PATCHv7] gpio: Remove VLA from gpiolib

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Date: Fri, 18 May 2018 08:51:09 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Laura Abbott <labbott@...hat.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, Kees Cook <keescook@...omium.org>, 
	Lukas Wunner <lukas@...ner.de>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	"open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, kernel-hardening@...ts.openwall.com, 
	Phil Reid <preid@...ctromag.com.au>
Subject: Re: [PATCHv7] gpio: Remove VLA from gpiolib

Hi Laura,

On Fri, May 18, 2018 at 12:32 AM, Laura Abbott <labbott@...hat.com> wrote:
> The new challenge is to remove VLAs from the kernel
> (see https://lkml.org/lkml/2018/3/7/621) to eventually
> turn on -Wvla.
>
> Using a kmalloc array is the easy way to fix this but kmalloc is still
> more expensive than stack allocation. Introduce a fast path with a
> fixed size stack array to cover most chip with gpios below some fixed
> amount. The slow path dynamically allocates an array to cover those
> chips with a large number of gpios.
>
> Reviewed-by: Phil Reid <preid@...ctromag.com.au>
> Reviewed-and-tested-by: Lukas Wunner <lukas@...ner.de>
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> ---
> v7: Tweaked the Kconfig text to clarify the wording. Also fixed a few
> other comments from Geert, including the earlier suggestion to reduce
> the zeroing since I was wrong about that.

Thanks for the update!

With the minor nit below resolved:
Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>

> --- a/drivers/gpio/Kconfig
> +++ b/drivers/gpio/Kconfig
> @@ -22,6 +22,18 @@ menuconfig GPIOLIB
>
>  if GPIOLIB
>
> +config GPIOLIB_FASTPATH_LIMIT
> +       int "Maximum number of GPIOs for fast path"
> +       range 16 512

While you can indeed fit 2 * 16 bits in a long, a lower limit of 16 doesn't
make much sense, as you will use 2 longs (mask + bits) anyway.
So please increase it to 32.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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