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Subject: Re: [PATCH] staging: most: don't access hdm_ch before checking it
 valid
To:     Dan Carpenter <dan.carpenter@oracle.com>
References: <20200928104838.189639-1-jingxiangfeng@huawei.com>
 <20200928114857.GR4282@kadam>
CC:     <gregkh@linuxfoundation.org>, <christian.gromm@microchip.com>,
        <masahiroy@kernel.org>, <tglx@linutronix.de>,
        <keescook@chromium.org>, <devel@driverdev.osuosl.org>,
        <linux-kernel@vger.kernel.org>
From:   Jing Xiangfeng <jingxiangfeng@huawei.com>
Message-ID: <5F72875A.8020900@huawei.com>
Date:   Tue, 29 Sep 2020 09:01:14 +0800
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On 2020/9/28 19:48, Dan Carpenter wrote:
> On Mon, Sep 28, 2020 at 06:48:38PM +0800, Jing Xiangfeng wrote:
>> In try_start_dim_transfer(), pointer hdm_ch is accessed before checking.
>> This may lead to a potential null pointer dereference. Fix this by
>> dereferencing hdm_ch after calling BUG_ON().
>>
>> Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
>> ---
>>   drivers/staging/most/dim2/dim2.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/staging/most/dim2/dim2.c b/drivers/staging/most/dim2/dim2.c
>> index 509c8012d20b..ccd7cc7545e4 100644
>> --- a/drivers/staging/most/dim2/dim2.c
>> +++ b/drivers/staging/most/dim2/dim2.c
>> @@ -148,7 +148,7 @@ void dimcb_on_error(u8 error_id, const char *error_message)
>>   static int try_start_dim_transfer(struct hdm_channel *hdm_ch)
>>   {
>>   	u16 buf_size;
>> -	struct list_head *head = &hdm_ch->pending_list;
> This is not a dereference, it's just pointer math.  In other words:
>
> 	struct list_head *head = hdm_ch + offsetof(struct hdm_channel, pending_list);
Thanks for correcting!

>
> So the commit message is wrong because this cannot lead to a NULL
> dereference.  It's better to just delete the BUG_ON().  We don't really
> like BUG_ON().  Checkpatch will complain about them.  An Oops gives
> basically the same information as a BUG_ON() without completely killing
> the kernel so just dereferencing a NULL is preferable.  Finally, we can
> see from the callers that "hdm_ch" is never NULL.
>
> regards,
> dan carpenter
>
> .
>