linux-kernel - Re: [3/3] Use __fls for fls64 on 64-bit archs

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]

Message-id: <1215276997.7167.12.camel@localhost>
Date:	Sat, 05 Jul 2008 17:56:37 +0100
From:	"Ricardo M. Correia" <Ricardo.M.Correia@....COM>
To:	Alexander van Heukelum <heukelum@...lshack.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>, heukelum@...tmail.fm
Subject: Re: [3/3] Use __fls for fls64 on 64-bit archs

(Sorry, sending this again as I screwed up the previous mail).

Hi,

I have a question about fls64() which I hope you or someone else could
clarify, please see below.

On Sáb, 2008-03-15 at 18:32 +0100, Alexander van Heukelum wrote: 
> +#elif BITS_PER_LONG == 64
> +static inline int fls64(__u64 x)
> +{
> +	if (x == 0)
> +		return 0;
> +	return __fls(x) + 1;
> +}

It seems fls64() is implemented on top of __fls(), however the __fls()
implementation on the x86-64 architecture states that the result is
undefined if the argument does not have any zero bits.
So if I understand correctly, the statement "fls64(~0ULL)" would return
an undefined result on x64-64 instead of 64 as one would expect.

Wouldn't it make sense to check for ~0ULL in fls64()?

Thanks,
Ricardo

--
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/