file metadata via fs API (was: [GIT PULL] Filesystem Information)

[Al Viro]

On Wed, Aug 12, 2020 at 07:33:26PM +0100, Al Viro wrote:

> BTW, what would such opened files look like from /proc/*/fd/* POV?  And
> what would happen if you walk _through_ that symlink, with e.g. ".."
> following it?  Or with names of those attributes, for that matter...
> What about a normal open() of such a sucker?  It won't know where to
> look for your ->private_data...
> 
> FWIW, you keep refering to regularity of this stuff from the syscall
> POV, but it looks like you have no real idea of what subset of the
> things available for normal descriptors will be available for those.

Another question: what should happen with that sucker on umount of
the filesystem holding the underlying object?  Should it be counted
as pinning that fs?

Who controls what's in that tree?  If we plan to have xattrs there,
will they be in a flat tree, or should it mirror the hierarchy of
xattrs?  When is it populated?  open() time?  What happens if we
add/remove an xattr after that point?

If we open the same file several times, what should we get?  A full
copy of the tree every time, with all coherency being up to whatever's
putting attributes there?

What are the permissions needed to do lookups in that thing?

All of that is about semantics and the answers are needed before we
start looking into implementations.  "Whatever my implementation
does" is _not_ a good way to go, especially since that'll be cast
in stone as soon as API becomes exposed to userland...