Re: [PATCH v3 32/40] mm/uffd: use predicates for userfaultfd checks

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote:
> Rather than directly checking VMA flags, use the newly introduced
> vma_is_kernel_owned() and vma_is_persistent() helpers in userfaultfd when
> assessing VMA suitability for userfaultfd and UFFDIO_MOVE.
> 
> Update vma_move_compatible() so it's expressed in terms of VMA
> characteristics rather than arbitrary flags.
> 
> Additionally, update the use of the deprecated VMA flag API when checking
> VMA_SHADOW_STACK_BIT.
> 
> A VMA_IO_BIT check is no longer required but that is fine as a hard
> invariant has been established that only kernel-owned mappings may set
> VMA_IO_BIT so the check is now redundant.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  mm/userfaultfd.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 949017e60608..ddf0a4a3d399 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1754,10 +1754,18 @@ static inline bool move_splits_huge_pmd(unsigned long dst_addr,
>  }
>  #endif
>  
> -static inline bool vma_move_compatible(struct vm_area_struct *vma)
> +static inline bool vma_move_compatible(const struct vm_area_struct *vma)
>  {
> -	return !(vma->vm_flags & (VM_PFNMAP | VM_IO |  VM_HUGETLB |
> -				  VM_MIXEDMAP | VM_SHADOW_STACK));
> +	/* uffd is generally incompatible with kernel-owned mappings. */
> +	if (vma_is_kernel_owned(vma))
> +		return false;
> +	/* The shadow stack should not be written to by userspace. */
> +	if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
> +		return false;
> +	/* hugetlb mappings cannot be safely moved. */
> +	if (vma_is_hugetlb(vma))
> +		return false;
> +	return true;

return !vma_is_hugetlb(vma);

>  }
>  
>  static int validate_move_areas(struct userfaultfd_ctx *ctx,
> @@ -2146,10 +2154,11 @@ static bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
>  {
>  	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
>  
> -	if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
> +	/* Non-persistent memory is inherently not controllable by userspace. */
> +	if (!vma_is_persistent(vma))
>  		return false;

Completely confusing. Just avoid the helper and open-code this here.

> -
> -	if (!vma_is_hugetlb(vma) && (vma->vm_flags & VM_SPECIAL))
> +	/* The shadow stack should not be written to by userspace. */
> +	if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
>  		return false;
>  
>  	vm_flags &= __VM_UFFD_FLAGS;
> 

Other stuff looks much better to me.


-- 
Cheers,

David