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

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Date:	Mon, 14 Sep 2015 11:19:26 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	James Bottomley <jbottomley@...n.com>
Cc:	"linux\@rasmusvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko\@linux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"kys\@microsoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH] lib/string_helpers.c: fix infinite loop in string_get_size()

James Bottomley <jbottomley@...n.com> writes:

> On Fri, 2015-09-04 at 14:56 +0200, Vitaly Kuznetsov 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.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  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..a155c7b 100644
>> --- a/lib/string_helpers.c
>> +++ b/lib/string_helpers.c
>> @@ -76,7 +76,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
>>  		i++;
>>  	}
>>  
>> -	sf_cap = size;
>> +	sf_cap = size ? size : 1;
>
> If size can become zero after the scale adjustment, then there's a fault
> in the algorithm, and this probably isn't the right fix.  However, I did
> a brief calculation, and I can't see how size becomes zero ...

... but it does ...

> it might be that I haven't looked at this long enough (I am on holiday).

The function itself looks over complicated to me but you're probably
right and I'll try to find the root cause of the issue in the algorythm.

Thanks,

>
> James
>
>>  	for (j = 0; sf_cap*10 < 1000; j++)
>>  		sf_cap *= 10;
>>  

-- 
  Vitaly
--
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