linux-kernel - Re: drivers/hwmon/pmbus/ibm-cffps.c:184:60: warning: '%04X' directive output may be truncated writing between 4 and 8 bytes into a region of size 5

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Message-ID: <ddf5066a-8bf3-4686-a39d-a921452c41fb@roeck-us.net>
Date: Fri, 20 Dec 2024 09:56:26 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: David Laight <david.laight.linux@...il.com>
Cc: kernel test robot <lkp@...el.com>, Eddie James <eajames@...ux.ibm.com>,
 oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: drivers/hwmon/pmbus/ibm-cffps.c:184:60: warning: '%04X' directive
 output may be truncated writing between 4 and 8 bytes into a region of size 5

On 12/20/24 09:29, David Laight wrote:
> On Fri, 20 Dec 2024 09:03:51 -0800
> Guenter Roeck <linux@...ck-us.net> wrote:
> 
>> On 12/20/24 08:30, David Laight wrote:
> ...
>>>>   
>>>>>       drivers/hwmon/pmbus/ibm-cffps.c:171:60: warning: '%02X' directive output may be truncated writing between 2 and 8 bytes into a region of size 3 [-Wformat-truncation=]
>>>>>         171 |                                 snprintf(&data[i * 2], 3, "%02X", rc);
>>>
>>> The only ways I know are to bound check the value or to use
>>> OPTIMISER_HIDE_VAR() on the length.

Actually, that doesn't work.

-                               snprintf(&data[i * 2], 3, "%02X", rc);
+                               snprintf(&data[i * 2], OPTIMIZER_HIDE_VAR(3), "%02X", rc);

triggers:

drivers/hwmon/pmbus/ibm-cffps.c: In function ‘ibm_cffps_debugfs_read’:
./include/linux/compiler.h:186:9: error: expected expression before ‘__asm__’

>>>    
>>
>> -                               snprintf(&data[i * 2], 3, "%02X", rc);
>> +                               snprintf(&data[i * 2], 3, "%02X", rc & 0xff);
>>
>> works as well, at least with gcc 11 and 12, but I really dislike that kind of nonsense.
> 
> Ditto.
> Using the result in some ways can also remove the warning.
> But you have to try quite hard, a simple (void) really ought to be
> enough to show you 'just don't care' but even 'if (snprintf(...)) {}'
> isn't enough (it does silence 'warn-unused-result').
> 
> I mean, the whole point of snprintf() is that it truncates.
> 

Using scnprintf() "fixes" the problem as well, but there are other patches
suggesting that snprintf() should be used if the return value is not
needed/checked because it is slightly simpler.

Guenter