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

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Date:	Thu, 29 Oct 2015 22:27:58 +0000
From:	James Bottomley <jbottomley@...n.com>
To:	"vkuznets@...hat.com" <vkuznets@...hat.com>
CC:	"ulf.hansson@...aro.org" <ulf.hansson@...aro.org>,
	"linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko@...ux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-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

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.

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.