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

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Message-ID: <56606825.4000105@oracle.com>
Date:	Thu, 3 Dec 2015 11:04:53 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.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)

On 12/03/2015 10:39 AM, Andrey Ryabinin wrote:
> 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:

That's how I've started doing it, but when it showed up with 3 different callers
I figured it's better to fix it at the source.

This fix would return a valid value and is working fine with the callers.


Thanks,
Sasha
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