linux-kernel - Re: [PATCH] linux/log2.h: Fix roundup_pow_of

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Message-ID: <CAPAsAGzmWjXJmkn+krACt5GWfyNEhEpPJdmgV4x=UkKhUnZH+A@mail.gmail.com>
Date:	Thu, 3 Dec 2015 18:39:44 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0)

2015-12-03 18:30 GMT+03:00 Sasha Levin <sasha.levin@...cle.com>:
> Passing 0 to roundup_pow_of_two would lead to wrapping around and trying to
> find the last set bit on (unsigned long)(-1), which is obviously wrong.
>
> Instead, deal with this case by rounding it up to the closest power of two
> (2 ** 0).
>
> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  include/linux/log2.h |    3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index fd7ff3d..b6bdf0c 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -60,6 +60,9 @@ bool is_power_of_2(unsigned long n)
>  static inline __attribute__((const))
>  unsigned long __roundup_pow_of_two(unsigned long n)
>  {
> +       if (n == 0)
> +               return 1UL << 0;
> +

Perhaps we should fix callers instead?
Comment near roundup_pow_of_two() says that result is undefined when n == 0:

/**
 * roundup_pow_of_two - round the given value up to nearest power of two
 * @n - parameter
 *
 * round the given value up to the nearest power of two
 * - the result is undefined when n == 0
--
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