From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755834AbbINPQM (ORCPT ); Mon, 14 Sep 2015 11:16:12 -0400 Received: from mx2.parallels.com ([199.115.105.18]:54444 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932092AbbINPQJ (ORCPT ); Mon, 14 Sep 2015 11:16:09 -0400 From: James Bottomley To: "vkuznets@redhat.com" CC: "linux@rasmusvillemoes.dk" , "andriy.shevchenko@linux.intel.com" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "kys@microsoft.com" Subject: Re: [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size() Thread-Topic: [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size() Thread-Index: AQHQ7uz/KdEga8uOFkq9koNAM0dI1J48mAIA Date: Mon, 14 Sep 2015 15:15:58 +0000 Message-ID: <1442243758.2214.9.camel@Odin.com> References: <1442235476-31837-1-git-send-email-vkuznets@redhat.com> In-Reply-To: <1442235476-31837-1-git-send-email-vkuznets@redhat.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Evolution 3.12.11 x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [184.11.141.41] Content-Type: text/plain; charset="utf-8" Content-ID: <7B6B18B2CE5AE74D83A64FEB07000C06@sw.swsoft.com> MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from base64 to 8bit by mail.home.local id t8EFGGBp019725 On Mon, 2015-09-14 at 14:57 +0200, Vitaly Kuznetsov wrote: > Some string_get_size() calls (e.g.: > string_get_size(1, 512, STRING_UNITS_10, ..., ...) > string_get_size(15, 64, STRING_UNITS_10, ..., ...) > ) result in an infinite loop. The problem is that if size is equal to > divisor[units]/blk_size and is smaller than divisor[units] we'll end > up with size == 0 when we start doing sf_cap calculations: > > For string_get_size(1, 512, STRING_UNITS_10, ..., ...) case: > ... > remainder = do_div(size, divisor[units]); -> size is 0, remainder is 1 > remainder *= blk_size; -> remainder is 512 > ... > size *= blk_size; -> size is still 0 > size += remainder / divisor[units]; -> size is still 0 > > The caller causing the issue is sd_read_capacity(), the problem was noticed > on Hyper-V, such weird size was reported by host when scanning collides > with device removal. This is probably a separate issue worth fixing, this > patch is intended to prevent the library routing from infinite looping. > > Signed-off-by: Vitaly Kuznetsov > --- > lib/string_helpers.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/lib/string_helpers.c b/lib/string_helpers.c > index c98ae81..d0e607c 100644 > --- a/lib/string_helpers.c > +++ b/lib/string_helpers.c > @@ -59,7 +59,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units, > } > > exp = divisor[units] / (u32)blk_size; Add a comment here explaining to those who come after why strict greater than is necessary. Something like: /* size must be strictly greater than exp here to ensure * that remainder is greater than divisor[units] coming out of the if */ > - if (size >= exp) { > + if (size > exp) { > remainder = do_div(size, divisor[units]); > remainder *= blk_size; > i++; So when pushed, you finally found the algorithmic problem. Well done. Acked-by: James Bottomley James {.n++%ݶw{.n+{G{ayʇڙ,jfhz_(階ݢj"mG?&~iOzv^m ?I