linux-kernel - Re: [PATCH 9/10] drivers/mmc: Move a dereference below a NULL test

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Message-ID: <c656a4d20907200533t1cbe1476l62d8a51fdb796c70@mail.gmail.com>
Date:	Mon, 20 Jul 2009 18:03:40 +0530
From:	arun c <arun.edarath@...il.com>
To:	Adrian Hunter <adrian.hunter@...ia.com>
Cc:	Julia Lawall <julia@...u.dk>,
	"Lavinen Jarkko (Nokia-D/Helsinki)" <jarkko.lavinen@...ia.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH 9/10] drivers/mmc: Move a dereference below a NULL test

On Mon, Jul 20, 2009 at 1:31 PM, Adrian Hunter<adrian.hunter@...ia.com> wrote:
> Julia Lawall wrote:
>>
>> From: Julia Lawall <julia@...u.dk>
>>
>> If the NULL test is necessary, then the dereference should be moved below
>> the NULL test.
>>
>> The semantic patch that makes this change is as follows:
>> (http://www.emn.fr/x-info/coccinelle/)
>>
>> // <smpl>
>> @@
>> type T;
>> expression E,E1;
>> identifier i,fld;
>> statement S;
>> @@
>>
>> - T i = E->fld;
>> + T i;
>>  ... when != E=E1
>>      when != i
>>  BUG_ON (E == NULL||
>> -     i
>> +     E->fld
>>       == NULL || ...);
>> + i = E->fld;
>> // </smpl>
>>
>> Signed-off-by: Julia Lawall <julia@...u.dk>
>>
>> ---
>>  drivers/mmc/host/omap.c             |    5 +++--
>>  1 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/mmc/host/omap.c b/drivers/mmc/host/omap.c
>> index e7a331d..89281ab 100644
>> --- a/drivers/mmc/host/omap.c
>> +++ b/drivers/mmc/host/omap.c
>> @@ -255,11 +255,12 @@ static void mmc_omap_slot_release_work(struct
>> work_struct *work)
>>   static void mmc_omap_release_slot(struct mmc_omap_slot *slot, int
>> clk_enabled)
>>  {
>> -       struct mmc_omap_host *host = slot->host;
>> +       struct mmc_omap_host *host;
>>        unsigned long flags;
>>        int i;
>>  -       BUG_ON(slot == NULL || host->mmc == NULL);
>> +       BUG_ON(slot == NULL || slot->host->mmc == NULL);
>> +       host = slot->host;
>>          if (clk_enabled)
>>                /* Keeps clock running for at least 8 cycles on valid freq
>> */
>> --
>
> If slot is NULL it will oops anyway, so the following is better IMHO:
>
> static void mmc_omap_release_slot(struct mmc_omap_slot *slot, int
> clk_enabled)
> {
>        struct mmc_omap_host *host = slot->host;
>        unsigned long flags;
>        int i;
>
>> -       BUG_ON(slot == NULL || host->mmc == NULL);
>> +       BUG_ON(host->mmc == NULL);
>
>        if (clk_enabled)
>                /* Keeps clock running for at least 8 cycles on valid freq */
> --

According to http://isc.sans.org/diary.html?storyid=6820&rss "NULL check"
must be done before assigning the values. Does Julia Lawall's version
is the more correct one??

Arun
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