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

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Message-ID: <8d8bfc81-b584-20f4-65ae-d626f019cdf4@csgroup.eu>
Date:   Wed, 24 Aug 2022 06:10:30 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>,
        "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 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 ?

>   
>   	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);