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

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Message-ID: <52CCFF0C.7070704@suse.de>
Date:	Wed, 08 Jan 2014 08:32:28 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Chen Gang <gang.chen.5i5j@...il.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	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 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:
>>> On 12/22/2013 10:56 AM, Nicholas A. Bellinger wrote:
>>>> Hi Chen,
>>>>
>>>> On Sat, 2013-12-21 at 10:08 +0800, Chen Gang wrote:
>>>>> In kernel, need use div64_u64_rem() instead of operator '%' for u64, or
>>>>> can not pass compiling (with allmodconfig under metag):
>>>>>
>>>>>     MODPOST 2909 modules
>>>>>   ERROR: "__umoddi3" [drivers/target/target_core_mod.ko] undefined!
>>>>>
>>>>> Also need u64 type cast for u32 variable multiply u32 variable, or will
>>>>> cause type overflow issue.
>>>>>
>>>>>
>>>>> Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
>>>>> ---
>>>>>  drivers/target/target_core_alua.c | 3 ++-
>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>
>>>> FYI, this unsigned long long division in core_alua_state_lba_dependent()
>>>> was fixed for 32-bit in linux-next >= 12192013 code.
>>>>
>>>
>>> OK, thanks.
>>>
>>> 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))


Other than that the sector_div() patch is correct.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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