linux-kernel - Re: [PATCH] rtc: Modify leap year test for more simpler way

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 6 Feb 2013 21:00:38 +0800
From:	Haojian Zhuang <haojian.zhuang@...il.com>
To:	jonghwa3.lee@...sung.com
Cc:	Venu Byravarasu <vbyravarasu@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"a.zummo@...ertech.it" <a.zummo@...ertech.it>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>
Subject: Re: [PATCH] rtc: Modify leap year test for more simpler way

On Wed, Feb 6, 2013 at 8:43 PM,  <jonghwa3.lee@...sung.com> wrote:
> On 2013년 02월 06일 20:42, Venu Byravarasu wrote:
>> By definition, leap year is one, which is a divisible by 4 & 400, excluding multiples of 100s.
>> Hence I feel this patch is not correct.
>
> No, I think you might misunderstood the it's meaning. The former code checks
> whether if it is multiple of 4 or not. Formal mathematical way to verify multiple of 4
> is just checks the last two digits are multiple of 4. This '(!year % 4) && (year % 100)'
> part does it. But with only that checking, it may miss the case of multiple of 400 which
> is also multiple of 4. Then my modification checks in hexadecimal, whether if number
> has any of 1st and 2nd bit with value 1. Because any number which has all bits above
> the 3rd can be divided with 4(2^2).
> (e.g. 44(0b101100) = 2^5+2^3+2^2 = 2^2(2^3 + 2 + 1))
> So It does same things with less instructions.

I still can't understand your logic.

Please check whether 200 year is leap year.

200(decimal) = 2b11001000

!(200 & 0x3) = 1 (Your condition said that 200 year is a leap year.)

According to this logic in below.
 if year mod 4 = 0 and year mod 100 <> 0 or year mod 400 = 0, then
it's a leap year.

This tells us that 200 year isn't a leap year because 200 mod 100 ==
0. So who is wrong?

Regards
Haojian
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/