Re: [PATCH v3 05/40] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma

[Gregory Price <gourry@gourry.net>]

On Thu, Sep 17, 2026 at 05:22:14PM +0100, Lorenzo Stoakes (ARM) wrote:
> When a user requests an mmap_action be performed in mmap_prepare, this
> involves populating the VMA range with data.
> 
> However, if the VMA is mergeable, it might then mistakenly be merged with
> another VMA without having populated the range.
> 
> Every mmap action currently available sets VMA flags such that the VMA
> cannot be merged.
> 
> However, to ensure that no future mmap action falls foul of this, assert
> that this is the case upon mmap_prepare validation.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  mm/vma.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index d6ed10cefc8f..62f2ce1ad5a1 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2809,6 +2809,15 @@ static int mmap_validate(unsigned long prev_start, unsigned long prev_end,
>  int mmap_prepare_validate(const struct vm_area_desc *prev_desc,
>  			  const struct vm_area_desc *desc)
>  {
> +	/*
> +	 * It is not valid to execute mmap actions for VMAs which can be merged,
> +	 * as any such merge would leave portions of the mapping incorrectly
> +	 * unmapped.
> +	 */
> +	if (vma_flags_can_merge(&desc->vma_flags) &&
> +	    WARN_ON_ONCE(desc->action.type != MMAP_NOTHING))
> +		return -EINVAL;
> +

If you wanted to make this unit-testable, you could pull it out into a
separate function:

static bool mmap_action_is_valid(const struct vm_area_desc *desc)
{
	return desc->action.type == MMAP_NOTHING ||
		!vma_flags_can_merge(&desc->vma_flags);
}

then write:

if (WARN_ON_ONCE(!mmap_action_is_valid(desc)))
	return -EINVAL;

And you can write a unit test directly against mmap_action_is_valid

otherwise

Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>