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Message-ID: <067d073f-556c-9df0-37f8-fd63fcdc7eb9@redhat.com>
Date: Mon, 14 May 2018 15:49:21 -0700
From: Laura Abbott <labbott@...hat.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
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
Subject: Re: [PATCHv5] gpio: Remove VLA from gpiolib
On 04/20/2018 02:02 AM, Geert Uytterhoeven wrote:
> Hi Laura,
>
> Thanks for your patch!
>
> On Fri, Apr 13, 2018 at 11:24 PM, 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.
>
> Blindly replacing VLAs by allocated arrays is IMHO not such a good solution.
> On the largest systems, NR_GPIOS is 2048, so that makes 2 arrays of 256
> bytes. That's an uppper limit, and assumes they are all on the same gpiochip,
> which they probably aren't.
>
> Still, 2 x 256 bytes is a lot, so I agree it should be fixed.
>
> So, wouldn't it make more sense to not allocate memory, but just process
> the request in chunks (say, at most 128 gpios per chunk, i.e. 2 x
> 16 bytes)? The code already caters for handling chunks due to not all gpios
> belonging to the same gpiochip. That will probably also be faster than
> allocating memory, which is the main idea behind this API.
>
>> 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>
>
>> --- 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",
>> + chip->ngpio, FASTPATH_NGPIO);
>
> FWIW, can this warning be triggered from userspace?
>
>> @@ -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)];
>
> Hence just use a fixed size here...
>
>> + 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);
>> + }
>> +
>> if (!can_sleep)
>> WARN_ON(chip->can_sleep);
>>
>> /* collect all inputs belonging to the same chip */
>> first = i;
>> - memset(mask, 0, sizeof(mask));
>> do {
>> const struct gpio_desc *desc = desc_array[i];
>> int hwgpio = gpio_chip_hwgpio(desc);
>
> Out-of-context, the code does:
>
> | __set_bit(hwgpio, mask);
> | i++;
> | } while ((i < array_size) &&
>
> ... and change this limit to "(i < min(array_size, first +
> ARRAY_SIZE(mask) * BITS_PER_BYTE))"
>
> | (desc_array[i]->gdev->chip == chip));
>
> ... and you're done?
>
I don't think this approach will work since gpio_chip_{get,set}_multiple
expect to be working on arrays for the entire chip. There doesn't seem
to be a nice way to work on a subset of GPIOs without defeating the point
of the multiple API.
is 2*256 = 512 bytes really too much stack space? I guess we could
switch to a Kconfig to allow for better bounds.
Thanks,
Laura
>> @@ -2878,7 +2904,7 @@ static void gpio_chip_set_multiple(struct gpio_chip *chip,
>> }
>> }
>>
>> -void gpiod_set_array_value_complex(bool raw, bool can_sleep,
>> +int gpiod_set_array_value_complex(bool raw, bool can_sleep,
>
> Same here.
>
> Gr{oetje,eeting}s,
>
> Geert
>
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