Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

David Hildenbrand
2022-08-23T07:37:15Z

On 18.08.22 01:41, Kirill A. Shutemov wrote:
> On Fri, Aug 05, 2022 at 07:55:38PM +0200, Paolo Bonzini wrote:
>> On 7/21/22 11:44, David Hildenbrand wrote:
>>>
>>> Also, I*think* you can place pages via userfaultfd into shmem. Not
>>> sure if that would count "auto alloc", but it would certainly bypass
>>> fallocate().
>>
>> Yeah, userfaultfd_register would probably have to forbid this for
>> F_SEAL_AUTO_ALLOCATE vmas. Maybe the memfile_node can be reused for this,
>> adding a new MEMFILE_F_NO_AUTO_ALLOCATE flags? Then userfault_register
>> would do something like memfile_node_get_flags(vma->vm_file) and check the
>> result.
>
> I donno, memory allocation with userfaultfd looks pretty intentional to
> me. Why would F_SEAL_AUTO_ALLOCATE prevent it?
>

Can't we say the same about a write()?

> Maybe we would need it in the future for post-copy migration or something?
>
> Or existing practises around userfaultfd touch memory randomly and
> therefore incompatible with F_SEAL_AUTO_ALLOCATE intent?
>
> Note, that userfaultfd is only relevant for shared memory as it requires
> VMA which we don't have for MFD_INACCESSIBLE.

This feature (F_SEAL_AUTO_ALLOCATE) is independent of all the lovely
encrypted VM stuff, so it doesn't matter how it relates to MFD_INACCESSIBLE.

--
Thanks,

David / dhildenb