Re: Sealed memfd & no-fault mmap

[Matthew Wilcox]

On Thu, Jun 03, 2021 at 01:14:47PM +0000, Simon Ser wrote:
> 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?

This is going to operate at a page boundary, so the example you gave
will work.  How about this:

 - The client sets up a 1KiB shm file and sends it to the compositor.
 - The client expands the file to 5KiB
 - The compositor sees the new data up to 4KiB but zeroes past the 4KiB
   mark.

Does that still make userspace happy?