linux-kernel - Re: [PATCH v3 1/4] lib/string_helpers: change blk_size to u32 for string_get

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Date:	Fri, 30 Oct 2015 11:46:29 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	James Bottomley <jbottomley@...n.com>
Cc:	"ulf.hansson\@linaro.org" <ulf.hansson@...aro.org>,
	"linux\@rasmusvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko\@linux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"keescook\@chromium.org" <keescook@...omium.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/4] lib/string_helpers: change blk_size to u32 for string_get_size() interface

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

> On Thu, 2015-10-29 at 17:30 +0100, Vitaly Kuznetsov wrote:
>> string_get_size() can't really handle huge block sizes, especially
>> blk_size > U32_MAX but string_get_size() interface states the opposite.
>> Change blk_size from u64 to u32 to reflect the reality.
>
> What is the actual evidence for this?  The calculation is designed to be
> a symmetric 128 bit multiply.  When I wrote and tested it, it worked
> fine for huge block sizes.

We have 'u32 remainder' and then we do:

exp = divisor[units] / (u32)blk_size;
...
remainder = do_div(size, divisor[units]);
remainder *= blk_size;

I'm pretty sure it will overflow for some inputs.

>
> James
>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  include/linux/string_helpers.h | 2 +-
>>  lib/string_helpers.c           | 4 ++--
>>  2 files changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/include/linux/string_helpers.h b/include/linux/string_helpers.h
>> index dabe643..1223e80 100644
>> --- a/include/linux/string_helpers.h
>> +++ b/include/linux/string_helpers.h
>> @@ -10,7 +10,7 @@ enum string_size_units {
>>  	STRING_UNITS_2,		/* use binary powers of 2^10 */
>>  };
>>  
>> -void string_get_size(u64 size, u64 blk_size, enum string_size_units units,
>> +void string_get_size(u64 size, u32 blk_size, enum string_size_units units,
>>  		     char *buf, int len);
>>  
>>  #define UNESCAPE_SPACE		0x01
>> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
>> index 5939f63..f6c27dc 100644
>> --- a/lib/string_helpers.c
>> +++ b/lib/string_helpers.c
>> @@ -26,7 +26,7 @@
>>   * at least 9 bytes and will always be zero terminated.
>>   *
>>   */
>> -void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
>> +void string_get_size(u64 size, u32 blk_size, const enum string_size_units units,
>>  		     char *buf, int len)
>>  {
>>  	static const char *const units_10[] = {
>> @@ -58,7 +58,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
>>  		i++;
>>  	}
>>  
>> -	exp = divisor[units] / (u32)blk_size;
>> +	exp = divisor[units] / blk_size;
>>  	/*
>>  	 * size must be strictly greater than exp here to ensure that remainder
>>  	 * is greater than divisor[units] coming out of the if below.

-- 
  Vitaly
--
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