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Message-ID: <CAMuHMdXiopVBM+N+wYMY6JP7_sOJ2nncNkhpFq+aMc0pdfPtrQ@mail.gmail.com>
Date: Thu, 17 May 2018 09:58:26 +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: [PATCHv6] gpio: Remove VLA from gpiolib
Hi Laura,
On Thu, May 17, 2018 at 2:00 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-and-tested-by: Lukas Wunner <lukas@...ner.de>
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> Signed-off-by: Laura Abbott <labbott@...hat.com>
Thanks for your patch!
> Also to other points: I don't think the warning should be triggerable
> from userspace, it should only happen on probe. I also think only
> memsetting half the array is more likely to be error prone. We can
> change it if there is significant overhead.
With the default of 512, that's a memset of 128 bytes. Not so insignificant
on embedded 32 bit.
> --- a/drivers/gpio/Kconfig
> +++ b/drivers/gpio/Kconfig
> @@ -22,6 +22,16 @@ menuconfig GPIOLIB
>
> if GPIOLIB
>
> +config GPIOLIB_FASTPATH_LIMIT
> + int "Maximum number of GPIOs for fast path"
> + default 512
I think you need a range here. Else someone will pick a too large value,
causing stack overflow.
512 (128 bytes for each recursion level) sounds like a safe maximum to me.
> + help
> + This adjusts the point at which certain APIs will switch from
> + using a statically allocated fixed size buffer to a dynamically
The fast path doesn't use a statically allocated buffer (it cannot, due to
recursion), but a buffer on the stack. I think you need to make that very
clear in the help text, as this has the potential of causing random crashes.
> + allocated buffer. This is a trade-off in stackspace vs. speed.
> + You shouldn't need to change this unless you really need to
> + optimize one of those two.
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -1192,6 +1196,10 @@ int gpiochip_add_data_with_key(struct gpio_chip *chip, void *data,
> goto err_free_descs;
> }
>
> + if (chip->ngpio > FASTPATH_NGPIO)
> + chip_warn(chip, "line cnt %d is greater than fast path cnt %d\n",
%u (twice)
> + chip->ngpio, FASTPATH_NGPIO);
> +
> gdev->label = kstrdup_const(chip->label ?: "unknown", GFP_KERNEL);
> if (!gdev->label) {
> status = -ENOMEM;
> @@ -2662,16 +2670,28 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
>
> while (i < array_size) {
> struct gpio_chip *chip = desc_array[i]->gdev->chip;
> - unsigned long mask[BITS_TO_LONGS(chip->ngpio)];
> - unsigned long bits[BITS_TO_LONGS(chip->ngpio)];
> + unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
> + unsigned long *mask, *bits;
> int first, j, ret;
>
> + if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
> + memset(fastpath, 0, sizeof(fastpath));
> + mask = fastpath;
> + bits = fastpath + BITS_TO_LONGS(FASTPATH_NGPIO);
> + } else {
> + mask = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> + sizeof(*mask),
> + can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> + if (!mask)
> + return -ENOMEM;
> + bits = mask + BITS_TO_LONGS(chip->ngpio);
> + }
The assignment to bits could be made common, and moved out of the if/else.
Likewise for the memset, which means you would usually clear a single word
again, instead of 128 bytes (or more).
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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