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

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Message-ID: <e40651de-3666-4ea7-9bd3-0369f07c979d@redhat.com>
Date: Wed, 9 Apr 2025 12:31:12 +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 12:25, 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?

-- 
Cheers,

David / dhildenb