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

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Date:	Mon, 14 Sep 2015 17:33:41 +0300
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	James Bottomley <jbottomley@...n.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kys@...rosoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH] lib/string_helpers.c: fix infinite loop in
 string_get_size()

On Mon, 2015-09-14 at 14:43 +0200, Vitaly Kuznetsov wrote:
> 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...
> 

I didn't see a problem to add this test case. Since test module is
dedicated for that (tests).

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy
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