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Date:   Mon, 27 May 2019 18:24:23 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jean Delvare <jdelvare@...e.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (smsc47m1) fix outside array bounds warnings

On 5/22/19 8:08 AM, Jean Delvare wrote:
> Hi Masahiro,
> 
> On Tue, 21 May 2019 13:44:56 +0900, Masahiro Yamada wrote:
>> Kbuild test robot reports outside array bounds warnings:
>>
>>    CC [M]  drivers/hwmon/smsc47m1.o
>> drivers/hwmon/smsc47m1.c: In function 'fan_div_store':
>> drivers/hwmon/smsc47m1.c:370:49: warning: array subscript [0, 2] is outside array bounds of 'u8[3]' {aka 'unsigned char[3]'} [-Warray-bounds]
>>    tmp = 192 - (old_div * (192 - data->fan_preload[nr])
>>                                  ~~~~~~~~~~~~~~~~~^~~~
>> drivers/hwmon/smsc47m1.c:372:19: warning: array subscript [0, 2] is outside array bounds of 'u8[3]' {aka 'unsigned char[3]'} [-Warray-bounds]
>>    data->fan_preload[nr] = clamp_val(tmp, 0, 191);
>>    ~~~~~~~~~~~~~~~~~^~~~
>> drivers/hwmon/smsc47m1.c:373:53: warning: array subscript [0, 2] is outside array bounds of 'const u8[3]' {aka 'const unsigned char[3]'} [-Warray-bounds]
>>    smsc47m1_write_value(data, SMSC47M1_REG_FAN_PRELOAD[nr],
>>                               ~~~~~~~~~~~~~~~~~~~~~~~~^~~~
> 
> These messages are pretty confusing. Subscript [0, 2] would refer to a
> bi-dimensional array, while these are 1-dimension arrays. If [0, 2]
> means something else, I still don't get it, because both indexes 0 and
> 2 are perfectly within bounds of a 3-element array. So what do these
> messages mean exactly? Looks like a bogus checker to me.
> 
>> The index field in the SENSOR_DEVICE_ATTR_R* defines is 0, 1, or 2.
>> However, the compiler never knows the fact that the default in the
>> switch statement is unreachable.
>>
>> Reported-by: kbuild test robot <lkp@...el.com>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>> ---
>>
>>   drivers/hwmon/smsc47m1.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/hwmon/smsc47m1.c b/drivers/hwmon/smsc47m1.c
>> index 5f92eab24c62..e00102e05666 100644
>> --- a/drivers/hwmon/smsc47m1.c
>> +++ b/drivers/hwmon/smsc47m1.c
>> @@ -364,6 +364,10 @@ static ssize_t fan_div_store(struct device *dev,
>>   		tmp |= data->fan_div[2] << 4;
>>   		smsc47m1_write_value(data, SMSC47M2_REG_FANDIV3, tmp);
>>   		break;
>> +	default:
>> +		WARN_ON(1);
>> +		mutex_unlock(&data->update_lock);
>> +		return -EINVAL;
>>   	}
> 
> So basically the code is fine, the checker (which checker, BTW?)
> incorrectly thinks it isn't, and you propose to add dead code to make
> the checker happy?
> 
> I disagree with this approach. Ideally the checker must be improved to

Me too. I understand and accept that we sometimes initialize variables
to make he compiler happy, but this goes a bit too far. We really should
not add dead code - it creates the impression that it can be reached,
and would live forever for no good reason.

> understand that the code is correct. If that's not possible, we should
> be allowed to annotate the code to skip that specific check on these
> specific lines, because it has been inspected by a knowledgeable human
> and confirmed to be correct.
> 
Agreed.

> And if that it still not "possible", then the least intrusive fix would > be to make one of the valid cases the default. But adding new code
> which will never be executed, but must still be compiled and stored,
> no, thank you. Another code checker could legitimately complain about
> that actually.
> 
> IMHO if code checkers return false positives then they are not helping
> us and should not be used in the first place.
> 
Checkers are always only providing guidelines and should never be taken
at face value.

In summary - NACK.

Guenter

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