linux-kernel - Re: [PATCH linux-next] mm: fix used but uninitialized variable

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Message-ID: <66f0770d-83ee-34cb-2dea-b8d957613c7f@csgroup.eu>
Date:   Wed, 24 Aug 2022 06:49:52 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "nathan@...nel.org" <nathan@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "trix@...hat.com" <trix@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [PATCH linux-next] mm: fix used but uninitialized variable



Le 24/08/2022 à 08:41, Zhouyi Zhou a écrit :
> Thank Christophe for reviewing my patch
> 
> On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>>
>>
>> Le 24/08/2022 à 07:56, Zhouyi Zhou a écrit :
>>> In function walk_hugetlb_range, the local variable err may
>>> be used uninitialzed when:
>>> ops->pte_hole in side of "else if (ops->pte_hole)" is false.
>>>
>>> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
>>> ---
>>> Dear mm Developers:
>>>
>>> When I build kernel using "make CC=clang-14"
>>> the compiler complains following:
>>>
>>> CC      mm/pagewalk.o
>>> mm/pagewalk.c:318:12: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>>>                   else if (ops->pte_hole)
>>>                            ^~~~~~~~~~~~~
>>> mm/pagewalk.c:321:7: note: uninitialized use occurs here
>>>                   if (err)
>>>                       ^~~
>>> mm/pagewalk.c:318:8: note: remove the 'if' if its condition is always true
>>>                   else if (ops->pte_hole)
>>>                        ^~~~~~~~~~~~~~~~~~
>>> mm/pagewalk.c:311:10: note: initialize the variable 'err' to silence this warning
>>>                   int err;
>>>                          ^
>>>                           = 0
>>> 1 error generated.
>>> make[1]: *** [scripts/Makefile.build:250: mm/pagewalk.o] Error 1
>>> make: *** [Makefile:2006: mm] Error 2
>>>
>>> I initialize that variable outside of the for loop because we can assign 0 to err
>>> only once in this function.
>>>
>>> After my fix, I can compile the kernel.
>>>
>>> Many Thanks
>>> Zhouyi
>>> --
>>>    mm/pagewalk.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>>> index 54b2a1beeeb3..b6eb330e8ecd 100644
>>> --- a/mm/pagewalk.c
>>> +++ b/mm/pagewalk.c
>>> @@ -306,9 +306,9 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
>>>        unsigned long hmask = huge_page_mask(h);
>>>        unsigned long sz = huge_page_size(h);
>>>        const struct mm_walk_ops *ops = walk->ops;
>>> +     int err = 0;
>>
>> Why do you move it back outside of the for loop allthough it is
>> exclusively used inside the loop ?
> I move it outside of the for loop for performance consideration. Because
> if we initialize err inside, there will be an assignment statement
> every iteration.

I think GCC is smart enough to do the assignment only when necessary, 
maybe have a look at the generated assembly in order to confirm.

>>
>>>
>>>        for (; addr < end; addr = next) {
>>> -             int err;
>>
>> Another solution would be to add an explicit else, setting err = 0 in
>> the if/else if sequence.
> Thank Christophe for your valuable advice, I am going to prepare a 2nd version

Don't spend too much time on that, there are already other people 
looking at it, see 
https://lore.kernel.org/linux-mm/20220823153055.2517764-1-nathan@kernel.org/T/

> 
> Thanks
> Zhouyi
>>
>>>                pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
>>>
>>>                next = hugetlb_entry_end(h, addr, end);