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

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

[Anshuman Khandual]

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];
-- 
2.43.0

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

[David Hildenbrand]

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?

-- 
Cheers,

David / dhildenb

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

[Anshuman Khandual]

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.

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

[David Hildenbrand]

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?

-- 
Cheers,

David / dhildenb

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

[Anshuman Khandual]

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

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

[David Hildenbrand]

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@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
> 

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

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

[David Hildenbrand]

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@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
>>
> 
> 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