Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

David Hildenbrand
2021-02-11T09:15:19Z

On 11.02.21 09:39, Michal Hocko wrote:
> On Thu 11-02-21 09:13:19, Mike Rapoport wrote:
>> On Tue, Feb 09, 2021 at 02:17:11PM +0100, Michal Hocko wrote:
>>> On Tue 09-02-21 11:09:38, Mike Rapoport wrote:
> [...]
>>>> Citing my older email:
>>>>
>>>> I've hesitated whether to continue to use new flags to memfd_create() or to
>>>> add a new system call and I've decided to use a new system call after I've
>>>> started to look into man pages update. There would have been two completely
>>>> independent descriptions and I think it would have been very confusing.
>>>
>>> Could you elaborate? Unmapping from the kernel address space can work
>>> both for sealed or hugetlb memfds, no? Those features are completely
>>> orthogonal AFAICS. With a dedicated syscall you will need to introduce
>>> this functionality on top if that is required. Have you considered that?
>>> I mean hugetlb pages are used to back guest memory very often. Is this
>>> something that will be a secret memory usecase?
>>>
>>> Please be really specific when giving arguments to back a new syscall
>>> decision.
>>
>> Isn't "syscalls have completely independent description" specific enough?
>
> No, it's not as you can see from questions I've had above. More on that
> below.
>
>> We are talking about API here, not the implementation details whether
>> secretmem supports large pages or not.
>>
>> The purpose of memfd_create() is to create a file-like access to memory.
>> The purpose of memfd_secret() is to create a way to access memory hidden
>> from the kernel.
>>
>> I don't think overloading memfd_create() with the secretmem flags because
>> they happen to return a file descriptor will be better for users, but
>> rather will be more confusing.
>
> This is quite a subjective conclusion. I could very well argue that it
> would be much better to have a single syscall to get a fd backed memory
> with spedific requirements (sealing, unmapping from the kernel address
> space). Neither of us would be clearly right or wrong. A more important
> point is a future extensibility and usability, though. So let's just
> think of few usecases I have outlined above. Is it unrealistic to expect
> that secret memory should be sealable? What about hugetlb? Because if
> the answer is no then a new API is a clear win as the combination of
> flags would never work and then we would just suffer from the syscall
> multiplexing without much gain. On the other hand if combination of the
> functionality is to be expected then you will have to jam it into
> memfd_create and copy the interface likely causing more confusion. See
> what I mean?
>
> I by no means do not insist one way or the other but from what I have
> seen so far I have a feeling that the interface hasn't been thought
> through enough. Sure you have landed with fd based approach and that
> seems fair. But how to get that fd seems to still have some gaps IMHO.
>

I agree with Michal. This has been raised by different
people already, including on LWN (https://lwn.net/Articles/835342/).

I can follow Mike's reasoning (man page), and I am also fine if there is
a valid reason. However, IMHO the basic description seems to match quite good:

memfd_create() creates an anonymous file and returns a file descriptor that refers to it. The
file behaves like a regular file, and so can be modified, truncated, memory-mapped, and so on.
However, unlike a regular file, it lives in RAM and has a volatile backing storage. Once all
references to the file are dropped, it is automatically released. Anonymous memory is used
for all backing pages of the file. Therefore, files created by memfd_create() have the same
semantics as other anonymous memory allocations such as those allocated using mmap(2) with the
MAP_ANONYMOUS flag.

AFAIKS, we would need MFD_SECRET and disallow
MFD_ALLOW_SEALING and MFD_HUGETLB.

In addition, we could add MFD_SECRET_NEVER_MAP, which could disallow any kind of
temporary mappings (eor migration). TBC.

---

Some random thoughts regarding files.

What is the page size of secretmem memory? Sometimes we use huge pages,
sometimes we fallback to 4k pages. So I assume huge pages in general?

What are semantics of MADV()/FALLOCATE() etc on such files?
I assume PUNCH_HOLE fails in a nice way? does it work?

Does mremap()/mremap(FIXED) work/is it blocked?

Does mprotect() fail in a nice way?

Is userfaultfd() properly fenced? Or does it even work (doubt)?

How does it behave if I mmap(FIXED) something in between?
In which granularity can I do that (->page-size?)?

What are other granularity restrictions (->page size)?

Don't want to open a big discussion here, just some random thoughts.
Maybe it has all been already figured out and most of the answers
above are "Fails with -EINVAL".

--
Thanks,

David / dhildenb