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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Message-ID: <909a0c61-d151-2d60-1f3c-a2c22d590b33@linuxfoundation.org>
Date:   Fri, 1 Jul 2022 10:59:11 -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 10:27 AM, Nathan Chancellor wrote:
> On Fri, Jul 01, 2022 at 10:09:45AM -0600, Shuah Khan wrote:
>> 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 w hen memory allocation
>> fails.
> 
> Right, if the variable is unconditonally initialized at the top of the
> function, it can never be flagged as uninitialized but that doesn't mean
> the value is correct for every branch within the function.
> 

+1

>> Initializing isn't always the right answer for these kinds of
>> warnings.
> 
> I would say "unconditional initialization", as ret has to be initialized
> somewhere to fix this warning, right? If you are referring to clang's
> 'ret = 0' suggestion, I agree that it can be misleading (especially in
> the case of suggesting initializing dereferenced pointers to NULL...,
> see [1]) but at the end of the day, it is still on the programmer to
> analyze their code fully in light of the warning before implementing a
> fix. Clang cannot know that the return code should be set in the if
> statement rather than at the top of the function.
> 

+1 on "programmer to analyze their code fully in light of the warning"

I am not necessarily referring to the clang warning. I agree that it could
be misleading and suggest or send us towards a wrong fix. It is on us to
be careful - this would be when fixing the problem and reviewing the fix.

thanks,
-- Shuah