linux-kernel - Re: [PATCH v3 1/4] lib/string_helpers: change blk_size to u32 for string_get

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:	Tue, 03 Nov 2015 21:57:36 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	James Bottomley <jbottomley@...n.com>
Cc:	"vkuznets\@redhat.com" <vkuznets@...hat.com>,
	"ulf.hansson\@linaro.org" <ulf.hansson@...aro.org>,
	"andriy.shevchenko\@linux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"keescook\@chromium.org" <keescook@...omium.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/4] lib/string_helpers: change blk_size to u32 for string_get_size() interface

On Tue, Nov 03 2015, James Bottomley <jbottomley@...n.com> wrote:

>
> It was a suggestion when I explained what the missing sources of
> precision were, I don't think it's really a suggestion when it comes
> with an exemplary patch.

ex·em·pla·ry
adjective

    1.
    serving as a desirable model; representing the best of its kind.

Said exemplary patch produces "1.10 KiB" for size=2047,
blk_size=1. (This is caused by the introduction of rounding, and is
probably fixable.)

James, I do understand the algorithm you're trying to use. What I don't
understand is why you insist on using the approach of reducing size and
blk_size all the way before multiplying them. It seems much simpler to
just reduce them till they're below U32_MAX (not keeping track of any
remainders at that point), multiply them, and then proceed as usual,
This avoids having to deal with weird cross-multiplication terms, gives
more accurate results (yes, I tested that) and avoids the extra 64/32
division you introduce by decrementing i.

Rasmus

To be precise, the body I suggest is

	while (blk_size > U32_MAX) {
		do_div(blk_size, divisor[units]);
		i++;
	}
	while (size > U32_MAX) {
		do_div(size, divisor[units]);
		i++;
	}
	size *= blk_size;
	while (size > divisor[units]) {
		remainder = do_div(size, divisor[units]);
		i++;
	}

whether the last one should be > or >= is debatable; I think 1024 KiB is
better than 1.00 MiB.
--
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/