Re: [PATCH] mm/vma: Add VM_WARN_ON for commit_merge

[Andrew Morton]

On Thu, 16 Jan 2025 10:50:05 +0800 Liu Ye <liuye@xxxxxxxxxx> wrote:

>     Add VM_WARN_ON to prevent 'adjust' from accessing NULL pointers
>     when 'adjust' is NULL and 'expanded' is false or 'adj_start' is
>     not zero.
>
> ...
>
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -641,6 +641,7 @@ static int commit_merge(struct vma_merge_struct *vmg,
>  
>  	init_multi_vma_prep(&vp, vmg->vma, adjust, remove, remove2);
>  
> +	VM_WARN_ON(!adjust && (!expanded || adj_start));
>  	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
>  		   vp.anon_vma != adjust->anon_vma);
>  

This won't prevent a null deref.  It will emit a warning which
duplicates all the information which we're about to emit from the oops
handler.

Are there any reports of an oops from a NULL deref of `adjust'?