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Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd
From: |
David Hildenbrand |
Subject: |
Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd |
Date: |
Tue, 23 Aug 2022 09:36:57 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 |
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