linux-kernel - Re: [PATCH] selftests/mm: Fix compiler -Wmaybe-uninitialized warning

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Message-ID: <12ac3d80-5d12-49f2-9a13-633fec78ad98@redhat.com>
Date: Wed, 9 Apr 2025 12:36:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/mm: Fix compiler -Wmaybe-uninitialized warning

On 09.04.25 12:31, David Hildenbrand wrote:
> On 09.04.25 12:25, Anshuman Khandual wrote:
>>
>>
>> On 4/9/25 15:51, David Hildenbrand wrote:
>>> On 09.04.25 12:09, Anshuman Khandual wrote:
>>>>
>>>>
>>>> On 4/9/25 15:27, David Hildenbrand wrote:
>>>>> On 09.04.25 11:50, Anshuman Khandual wrote:
>>>>>> Following build warning comes up for cow test as 'transferred' variable has
>>>>>> not been initialized. Fix the warning via zero init for the variable.
>>>>>>
>>>>>>       CC       cow
>>>>>> cow.c: In function ‘do_test_vmsplice_in_parent’:
>>>>>> cow.c:365:61: warning: ‘transferred’ may be used uninitialized [-Wmaybe-uninitialized]
>>>>>>       365 |                 cur = read(fds[0], new + total, transferred - total);
>>>>>>           |                                                 ~~~~~~~~~~~~^~~~~~~
>>>>>> cow.c:296:29: note: ‘transferred’ was declared here
>>>>>>       296 |         ssize_t cur, total, transferred;
>>>>>>           |                             ^~~~~~~~~~~
>>>>>>       CC       compaction_test
>>>>>>       CC       gup_longterm
>>>>>>
>>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>>>> Cc: Shuah Khan <shuah@...nel.org>
>>>>>> Cc: linux-mm@...ck.org
>>>>>> Cc: linux-kselftest@...r.kernel.org
>>>>>> Cc: linux-kernel@...r.kernel.org
>>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>>>>> ---
>>>>>>      tools/testing/selftests/mm/cow.c | 2 +-
>>>>>>      1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
>>>>>> index f0cb14ea8608..b6cfe0a4b7df 100644
>>>>>> --- a/tools/testing/selftests/mm/cow.c
>>>>>> +++ b/tools/testing/selftests/mm/cow.c
>>>>>> @@ -293,7 +293,7 @@ static void do_test_vmsplice_in_parent(char *mem, size_t size,
>>>>>>              .iov_base = mem,
>>>>>>              .iov_len = size,
>>>>>>          };
>>>>>> -    ssize_t cur, total, transferred;
>>>>>> +    ssize_t cur, total, transferred = 0;
>>>>>>          struct comm_pipes comm_pipes;
>>>>>>          char *old, *new;
>>>>>>          int ret, fds[2];
>>>>>
>>>>>
>>>>> if (before_fork) {
>>>>>        transferred = vmsplice(fds[1], &iov, 1, 0);
>>>>> ...
>>>>>
>>>>> if (!before_fork) {
>>>>>        transferred = vmsplice(fds[1], &iov, 1, 0);
>>>>> ...
>>>>>
>>>>> for (total = 0; total < transferred; total += cur) {
>>>>> ...
>>>>>
>>>>>
>>>>> And I don't see any jump label that could jump to code that would ve using transferred.
>>>>>
>>>>> What am I missing?
>>>>
>>>> Probably because both those conditional statements are not mutually
>>>> exclusive above with an if-else construct. Hence compiler flags it
>>>> rather as a false positive ? Initializing with 0 just works around
>>>> that false positive.
>>>
>>> This is something the compiler should clearly be able to verify. before_fork is never changed in that function.
>>>
>>> We should not work around wrong compilers.
>>>
>>> Which compiler are you using such that you run into this issue?
>>
>> gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
>>
> 
> gcc (GCC) 14.2.1 20250110 (Red Hat 14.2.1-7)
> 
> Seems to be fine, just like all other compilers people used with this
> over the years.
> 
> Maybe something about that compiler is shaky that was fixed in the meantime?
> 

Reading

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105562#c29

"Note that sanitizers tend to increase the rate of false positive 
warnings, most notably those around -Wmaybe-uninitialized. We recommend 
against combining -Werror and [the use of] sanitizers."

Is this maybe related to the use of sanitizers?

-- 
Cheers,

David / dhildenb