Re: Sealed memfd & no-fault mmap

[Simon Ser]

On Saturday, May 29th, 2021 at 10:15 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:

> And IIUC it would have to be the recipient (Wayland compositor) doing
> the NOFAULT business, because (going back to the original mail) we are
> only considering this so that Wayland might satisfy clients who predate
> or refuse Linux-only APIs.  So, an ioctl (or fcntl, as sealing chose)
> at the client end cannot be expected; and could not be relied on anyway.

Yes, that is correct.

> NOFAULT? Does BSD use "fault" differently, and in Linux terms we
> would say NOSIGBUS to mean the same?
>
> Can someone point to a specification of BSD's __MAP_NOFAULT?
> Searching just found me references to bugs.

__MAP_NOFAULT isn't documented, sadly. The commit that introduces the
flag [1] is the best we're going to get, I think.

> What mainly worries me about the suggestion is: what happens to the
> zero page inserted into NOFAULT mappings, when later a page for that
> offset is created and added to page cache?

Not 100% sure exactly this means what I think it means, but from my PoV,
it's fine if the contents of an expanded shm file aren't visible from the
process that has mapped it with MAP_NOFAULT/MAP_NOSIGBUS. In other words,
it's fine if:

- The client sets up a 1KiB shm file and sends it to the compositor.
- The compositor maps it with MAP_NOFAULT/MAP_NOSIGBUS.
- The client expands the file to 2KiB and writes interesting data in it.
- The compositor still sees zeros past the 1KiB mark. The compositor needs
  to unmap and re-map the file to see the data past the 1KiB mark.

If the MAP_NOFAULT/MAP_NOSIGBUS flag only affects the mapping itself and
nothing else, this should be fine?

> Treating it as an opaque blob of zeroes, that stays there ever after,
> hiding the subsequent data: easy to implement, but a hack that we would
> probably regret.  (And I notice that even the quote from David Herrmann
> in the original post allows for the possibility that client may want to
> expand the object.)
>
> I believe the correct behaviour would be to unmap the nofault page
> then, allowing the proper page to be faulted in after.  That is
> certainly doable (the old mm/filemap_xip.c used to do so), but might
> get into some awkward race territory, with filesystem dependence
> (reminiscent of hole punch, in reverse).  shmem could operate that
> way, and be the better for it: but I wouldn't want to add that,
> without also cleaning away all the shmem_recalc_inode() stuff.

[1]: https://github.com/openbsd/src/commit/37f480c7e4870332b7ffb802fa6578f547c8a19f