linux-kernel - Re: [char-misc:char-misc-linus 3/3] drivers/misc/cardreader/rtsx_usb.c:639:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is true

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Date:   Fri, 1 Jul 2022 10:09:45 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [char-misc:char-misc-linus 3/3]
 drivers/misc/cardreader/rtsx_usb.c:639:6: warning: variable 'ret' is used
 uninitialized whenever 'if' condition is true

On 7/1/22 9:52 AM, Shuah Khan wrote:
> On 7/1/22 9:39 AM, Nathan Chancellor wrote:
>> On Fri, Jul 01, 2022 at 08:48:11AM -0600, Shuah Khan wrote:
>>> On 7/1/22 2:59 AM, Greg Kroah-Hartman wrote:

>>>>> All warnings (new ones prefixed by >>):
>>>>>
>>>>>>> drivers/misc/cardreader/rtsx_usb.c:639:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
>>>>>              if (!ucr->rsp_buf)
>>>>>                  ^~~~~~~~~~~~~
>>>>>      drivers/misc/cardreader/rtsx_usb.c:678:9: note: uninitialized use occurs here
>>>>>              return ret;
>>>>>                     ^~~
>>>>>      drivers/misc/cardreader/rtsx_usb.c:639:2: note: remove the 'if' if its condition is always false
>>>>>              if (!ucr->rsp_buf)
>>>>>              ^~~~~~~~~~~~~~~~~~
>>>>>      drivers/misc/cardreader/rtsx_usb.c:622:9: note: initialize the variable 'ret' to silence this warning
>>>>>              int ret;
>>>>>                     ^
>>>>>                      = 0
>>>>>      1 warning generated.
>>>>
>>>> Odd, gcc doesn't show this for me.  Shuah, can you send a follow-on
>>>> patch to fix this?  The warning does look correct.
>>>>
>>>
>>> gcc didn't complain when I compiled either. I will send a follow-on patch.
>>
>> Unfortunately, GCC won't warn for most uninitialized variables by
>> default after 5.7, which included commit 78a5255ffb6a ("Stop the ad-hoc
>> games with -Wno-maybe-initialized"). They will potentially show up at
>> W=2 or with an explicit KCFLAGS=-Wmaybe-uninitialized (it does in this
>> case):
>>
> 
> Thank you.
> 
>> | drivers/misc/cardreader/rtsx_usb.c: In function ‘rtsx_usb_probe’:
>> | drivers/misc/cardreader/rtsx_usb.c:678:16: error: ‘ret’ may be used uninitialized [-Werror=maybe-uninitialized]
>> |   678 |         return ret;
>> |       |                ^~~
>> | drivers/misc/cardreader/rtsx_usb.c:622:13: note: ‘ret’ was declared here
>> |   622 |         int ret;
>> |       |             ^~~
>> | cc1: all warnings being treated as errors
>>
> 
> This is a bug and a good find. ret should have been set
> in the if (!ucr->rsp_buf) before going to error handling.
> 
> I wonder if it would have been flagged if ret were to be
> initialized to 0. Something to experiment.
> 

I had to try. As I suspected initializing ret will mask this bug.

KCFLAGS=-Wmaybe-uninitialized will not flag it even though
the bug still exists. It will return 0 when memory allocation
fails.

Initializing isn't always the right answer for these kinds of
warnings.

thanks,
-- Shuah