linux-kernel - Re: [PATCH] drivers: target: target_core_mod: use div64_u64

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Date:	Wed, 08 Jan 2014 15:18:37 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Hannes Reinecke <hare@...e.de>
Cc:	Chen Gang <gang.chen.5i5j@...il.com>,
	James Hogan <james.hogan@...tec.com>,
	linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] drivers: target: target_core_mod: use div64_u64_rem()
 instead of operator '%' for u64

On Wed, 2014-01-08 at 08:32 +0100, Hannes Reinecke wrote:
> On 12/24/2013 04:35 AM, Chen Gang wrote:
> > On 12/23/2013 02:51 PM, Nicholas A. Bellinger wrote:
> >> On Sun, 2013-12-22 at 17:17 +0800, Chen Gang wrote:

<SNIP>

> >>> The related fix patch changed "start_lba = lba % ..." to "start_lba =
> >>> lba / ...", and also assumed "segment_size * segment_mult" is still
> >>> within u32 (can not cause type over flow).
> >>>
> >>> I guess the original author already knew about them, and intended to do
> >>> like that (if not, please let me know, thanks).
> >>>
> >>
> >> Sorry, your correct that the original code is using modulo division to
> >> calculate start_lba.
> >>
> > 
> > Oh, that's all right, (in fact, don't need sorry), I am not quite
> > familiar with the details, so need related member help check it.  :-)
> > 
> >> Hannes, can you please double check this below..?
> >>
> > 
> > Please help check when have time, thanks.
> > 
> I would even convert segment_size and segment_mult to u64,
> to ensure no overflows occur:
> 
> diff --git a/drivers/target/target_core_alua.c
> b/drivers/target/target_core_alua
> .c
> index 9b1856d..54b1e52 100644
> --- a/drivers/target/target_core_alua.c
> +++ b/drivers/target/target_core_alua.c
> @@ -477,8 +477,7 @@ static inline int core_alua_state_lba_dependent(
>         u8 *alua_ascq)
>  {
>         struct se_device *dev = cmd->se_dev;
> -       u32 segment_size, segment_mult, sectors;
> -       u64 lba;
> +       u64 segment_size, segment_mult, sectors, lba;
> 
>         /* Only need to check for cdb actually containing LBAs */
>         if (!(cmd->se_cmd_flags & SCF_SCSI_DATA_CDB))
> 
> 

Will squash the above into the original patch shortly in for-next..

> Other than that the sector_div() patch is correct.
> 

<nod> Thanks for confirming that sector_div() is correct here vs. the
original code using modulo that Chen had pointed out.

Thanks Hannes!

--nab

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