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

[Anshuman Khandual <anshuman.khandual@arm.com>]

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@linux-foundation.org>
>>>> Cc: Shuah Khan <shuah@kernel.org>
>>>> Cc: linux-mm@kvack.org
>>>> Cc: linux-kselftest@vger.kernel.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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