linux-kernel - Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int

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:   Mon, 18 Dec 2017 17:44:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Crt Mori <cmo@...exis.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ian Abbott <abbotti@....co.uk>,
        Larry Finger <Larry.Finger@...inger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@...natech.se>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int_sqrt

On Mon, Dec 18, 2017 at 04:05:44PM +0100, Crt Mori wrote:
> There is no option to perform 64bit integer sqrt on 32bit platform.
> Added stronger typed int_sqrt64 enables the 64bit calculations to
> be performed on 32bit platforms. Although int_sqrt() is a rough
> approximation, the same algorithm is used in int_sqrt64() as good
> enough on 32bit platform.

You clearly haven't read a recent version of the file you're patching.
Please take a moment to do so now.

> +/**
> + * int_sqrt64 - strongly typed int_sqrt function
> + * @x: 64bit integer of which to calculate the sqrt
> + */
> +u64 int_sqrt64(u64 x)

Please explain how the result of sqrt(u64) can be larger than u32.

Also, I expect that this fact could be exploited to optimize this for
32bit archs if one were so inclined.

> +{
> +	u64 b, m, y = 0;
> +
> +	if (x <= 1)
> +		return x;
> +
> +	m = 1ULL << (64 - 2);
> +	while (m != 0) {
> +		b = y + m;
> +		y >>= 1;
> +
> +		if (x >= b) {
> +			x -= b;
> +			y += m;
> +		}
> +		m >>= 2;
> +	}
> +	return y;
> +}

so yeah, no, please try again after reading the current file.