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

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Message-ID: <1441934529.13610.4.camel@Odin.com>
Date:	Fri, 11 Sep 2015 01:22:43 +0000
From:	James Bottomley <jbottomley@...n.com>
To:	"vkuznets@...hat.com" <vkuznets@...hat.com>
CC:	"linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko@...ux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"kys@...rosoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH] lib/string_helpers.c: fix infinite loop in
 string_get_size()

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

James

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