Re: [PATCH] userfaultfd: prevent registration of special VMAs

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

On 6/18/26 11:35, Mike Rapoport wrote:
> On Thu, Jun 18, 2026 at 11:25:31AM +0200, David Hildenbrand (Arm) wrote:
>> On 6/18/26 11:21, Mike Rapoport wrote:
>>>
>>> Cleaner in what sense?
>>> Will be uglier for sure, just take a look at vma_can_userfault().
>>
>> I was thinking of this:
>>
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 180bad42fc79..8a6803618a91 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -2029,7 +2029,10 @@ 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)
>> +       if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
>> +               return false;
>> +
>> +       if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL))
>>                 return false;
> 
> In a way that's an extra check for hugetlb, but it will work.

My point would be that we exclude all special VMAs, except hugetlb (which is
special but supported ... in its special way).

-- 
Cheers,

David