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

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Message-ID: <CAABZP2w7jw4OHXPDANNabaD1L1BbUG8ymu9zLGEt5xA8YgZ3ow@mail.gmail.com>
Date:   Wed, 24 Aug 2022 14:41:08 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
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

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.
>
> >
> >       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);