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

Mike Rapoport
2021-02-14T09:21:30Z

On Fri, Feb 12, 2021 at 10:18:19AM +0100, David Hildenbrand wrote:
> On 12.02.21 00:09, Mike Rapoport wrote:
> > 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?
>
> I think open() vs memfd_create() makes sense: for open, the path specifies
> main properties (tmpfs, hugetlbfs, filesystem). On memfd, there is no such
> path and the "type" has to be specified differently.
>
> Also, open() might open existing files - memfd always creates new files.

Yes, but still open() returns a handle to a file and memfd_create() returns
a handle to a file. The differences may be well hidden by e.g. O_MEMORY and
than features unique to memfd files will have their set of O_SOMETHING
flags.

It's the same logic that says "we already have an interface that's close
enough and it's fine to add a bunch of new flags there".

And here we come to the question "what are the differences that justify a
new system call?" and the answer to this is very subjective. And as such we
can continue bikeshedding forever.

--
Sincerely yours,
Mike.