linux-kernel - Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk

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Message-ID: <CAGsJ_4ySwMuKGYxywY+RH_FkNvjsThhvFQr+d1++KykOqjxarg@mail.gmail.com>
Date: Tue, 17 Jun 2025 14:24:26 +1200
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, Liam.Howlett@...cle.com, 
	vbabka@...e.cz, jannh@...gle.com, lorenzo.stoakes@...cle.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk_vmas

On Tue, Jun 17, 2025 at 2:05 PM Lance Yang <ioworker0@...il.com> wrote:
>
> From: Lance Yang <lance.yang@...ux.dev>
>
> The prev pointer was uninitialized, which could lead to undefined behavior
> where its address is taken and passed to the visit() callback without being
> assigned a value.
>
> Initializing it to NULL makes the code safer and prevents potential bugs
> if a future callback function attempts to read from it.

Is there any read-before-write case here? I haven't found one.

It also looks like we're assuming that *prev == NULL implies
a specific condition:

*prev = NULL;   /* tell sys_madvise we drop mmap_lock */

*prev = NULL; /* mmap_lock has been dropped, prev is stale */

>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>  mm/madvise.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 267d8e4adf31..c87325000303 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
>                                    struct vm_area_struct **prev, unsigned long start,
>                                    unsigned long end, void *arg))
>  {
> +       struct vm_area_struct *prev = NULL;
>         struct vm_area_struct *vma;
> -       struct vm_area_struct *prev;
> -       unsigned long tmp;
>         int unmapped_error = 0;
> +       unsigned long tmp;
>         int error;
>
>         /*
> --
> 2.49.0
>

Thanks
Barry