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

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Message-ID: <ab78f4ee-dfa4-4c49-8944-a67546a739ca@redhat.com>
Date: Tue, 17 Jun 2025 09:54:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: 21cnbao@...il.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 17.06.25 04:05, Lance Yang 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.

So, we are passing the pointer value to visit(), which is not undefined 
behavior.

The issue would be if anybody takes a look at the value stored at that 
pointer. Because, already passing an uninitialized value to a 
(non-inlined) function is undefined behavior according to C.

In madvise_update_vma()->vma_modify_flags_name() we do exactly that, 
correct?

	vma = vma_modify_flags_name(&vmi, *prev, ...

We should use Fixes: then.


Acked-by: David Hildenbrand <david@...hat.com>

> 
> Initializing it to NULL makes the code safer and prevents potential bugs
> if a future callback function attempts to read from it.
> 
> 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;
>   
>   	/*


-- 
Cheers,

David / dhildenb