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

[Christian Brauner]

On Tue, Aug 11, 2020 at 09:05:22AM -0700, Linus Torvalds wrote:
> On Tue, Aug 11, 2020 at 8:30 AM Miklos Szeredi <miklos at szeredi.hu> wrote:
> >
> > What's the disadvantage of doing it with a single lookup WITH an enabling flag?
> >
> > It's definitely not going to break anything, so no backward
> > compatibility issues whatsoever.
> 
> No backwards compatibility issues for existing programs, no.
> 
> But your suggestion is fundamentally ambiguous, and you most
> definitely *can* hit that if people start using this in new programs.
> 
> Where does that "unified" pathname come from? It will be generated
> from "base filename + metadata name" in user space, and
> 
>  (a) the base filename might have double or triple slashes in it for
> whatever reasons.
> 
> This is not some "made-up gotcha" thing - I see double slashes *all*
> the time when we have things like Makefiles doing
> 
>     srctree=../../src/
> 
> and then people do "$(srctree)/". If you haven't seen that kind of
> pattern where the pathname has two (or sometimes more!) slashes in the
> middle, you've led a very sheltered life.
> 
>  (b) even if the new user space were to think about that, and remove
> those (hah! when have you ever seen user space do that?), as Al
> mentioned, the user *filesystem* might have pathnames with double
> slashes as part of symlinks.
> 
> So now we'd have to make sure that when we traverse symlinks, that
> O_ALT gets cleared. Which means that it's not a unified namespace
> after all, because you can't make symlinks point to metadata.
> 
> Or we'd retroactively change the semantics of a symlink, and that _is_
> a backwards compatibility issue. Not with old software, no, but it
> changes the meaning of old symlinks!
> 
> So no, I don't think a unified namespace ends up working.
> 
> And I say that as somebody who actually loves the concept. Ask Al: I
> have a few times pushed for "let's allow directory behavior on regular
> files", so that you could do things like a tar-filesystem, and access
> the contents of a tar-file by just doing
> 
>     cat my-file.tar/inside/the/archive.c
> 
> or similar.
> 
> Al has convinced me it's a horrible idea (and there you have a
> non-ambiguous marker: the slash at the end of a pathname that
> otherwise looks and acts as a non-directory)
> 

Putting my kernel hat down, putting my userspace hat on.

I'm looking at this from a potential user of this interface.
I'm not a huge fan of the metadata fd approach I'd much rather have a
dedicated system call rather than opening a side-channel metadata fd
that I can read binary data from. Maybe I'm alone in this but I was
under the impression that other users including Ian, Lennart, and Karel
have said on-list in some form that they would prefer this approach.
There are even patches for systemd and libmount, I thought?

But if we want to go down a completely different route then I'd prefer
if this metadata fd with "special semantics" did not in any way alter
the meaning of regular paths. This has the potential to cause a lot of
churn for userspace. I think having to play concatenation games in
shared libraries for mount information is a bad plan in addition to all
the issues you raised here.

Christian