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

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

[Zhouyi Zhou]

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@gmail.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;
 
 	for (; addr < end; addr = next) {
-		int err;
 		pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
 
 		next = hugetlb_entry_end(h, addr, end);
-- 
2.34.1

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

[Christophe Leroy]

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@gmail.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 ?

>   
>   	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.

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

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

[Zhouyi Zhou]

Thank Christophe for reviewing my patch

On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy
<christophe.leroy@csgroup.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@gmail.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.
>
> >
> >       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

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

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

[Christophe Leroy]

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@csgroup.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@gmail.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);

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

[Zhouyi Zhou]

On Wed, Aug 24, 2022 at 2:49 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> 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@csgroup.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@gmail.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.
Thank Christophe for your guidance, I disassembled the GCC generated
code, it optimized away int err = 0 on both X86 and PowerPC platforms.
The disassembled instructions are quite long, so I prefer not to paste
in my email unless someone asks me to do so ;-)
>
> >>
> >>>
> >>>        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
>
Thank Christophe for your reminder, I only subscribed to the RCU
mailing list currently, so I submitted duplicated work.  I should be
more devoted to the Linux community. I am sorry.
> 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
Many thanks
Zhouyi
> >>
> >>>                pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
> >>>
> >>>                next = hugetlb_entry_end(h, addr, end);