linux-kernel - Re: [PATCH] lib/string_helpers.c: fix infinite loop in string_get

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Date:	Mon, 14 Sep 2015 14:43:30 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	James Bottomley <jbottomley@...n.com>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"linux\@rasmusvillemoes.dk" <linux@...musvillemoes.dk>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"kys\@microsoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH] lib/string_helpers.c: fix infinite loop in string_get_size()

Andy Shevchenko <andriy.shevchenko@...ux.intel.com> writes:

> On Fri, 2015-09-11 at 18:31 +0000, James Bottomley wrote:
>> On Thu, 2015-09-10 at 16:08 -0700, Andrew Morton wrote:
>> > On Fri,  4 Sep 2015 14:56:33 +0200 Vitaly Kuznetsov <
>> > vkuznets@...hat.com> wrote:
>> > 
>> > > string_get_size(1, 512, 0, ..., ...) call results in an infinite 
>> > > loop. The
>> > > problem is that if size == 0 when we start calculating sf_cap 
>> > > this loop
>> > > will never end.
>> > > 
>> > > The caller causing the issue is sd_read_capacity(), the problem 
>> > > was noticed
>> > > on Hyper-V.
>> > 
>> > When fixing bugs, please provide enough info for others to be able 
>> > to
>> > understand which kernel version(s) need the fix.  In this case: 
>> > what
>> > end-user action triggers this bug?  (iow, how does sdkp->capacity
>> > become zero?)
>> 
>> Any more details.  The attached programme, which is cut straight out 
>> of
>> the algorithm in string_helpers.c and modified for a C environment
>> slightly (only in do_div and the typedefs) produces this
>> 
>> hello
>> STRING IS 512 B
>> 
>> With your input, so I don't think the problem is where you think it 
>> is.
>> 
>> James
>> 
>
> Vitaly, it might make sense to extend test-string_helpers.c to what you
> are trying to do right.

The issue is that string_get_size() enters an infinite loop on some
inputs so if we add a test for such inputs we'll hang our kernel...

-- 
  Vitaly
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