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

Mike Rapoport
2021-02-11T23:10:47Z

On Thu, Feb 11, 2021 at 01:07:10PM +0100, David Hildenbrand wrote:
> On 11.02.21 12:27, Mike Rapoport wrote:
> > On Thu, Feb 11, 2021 at 10:01:32AM +0100, David Hildenbrand wrote:
>
> So let's talk about the main user-visible differences to other memfd files
> (especially, other purely virtual files like hugetlbfs). With secretmem:
>
> - File content can only be read/written via memory mappings.
> - File content cannot be swapped out.
>
> I think there are still valid ways to modify file content using syscalls:
> e.g., fallocate(PUNCH_HOLE). Things like truncate also seems to work just
> fine.

These work perfectly with any file, so maybe we should have added
memfd_create as a flag to open(2) back then and now the secretmem file
descriptors?

> > > AFAIKS, we would need MFD_SECRET and disallow
> > > MFD_ALLOW_SEALING and MFD_HUGETLB.
> >
> > So here we start to multiplex.
>
> Yes. And as Michal said, maybe we can support combinations in the future.

Isn't there a general agreement that syscall multiplexing is not a good
thing?
memfd_create already has flags validation that does not look very nice.
Adding there only MFD_SECRET will make it a bit less nice, but when we'll
grow new functionality into secretmem that will become horrible.

--
Sincerely yours,
Mike.