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Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem
 Information)
From:   Ian Kent <raven@themaw.net>
To:     Christian Brauner <christian.brauner@ubuntu.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Karel Zak <kzak@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Wed, 12 Aug 2020 08:53:42 +0800
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On Tue, 2020-08-11 at 21:39 +0200, Christian Brauner wrote:
> 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@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?

Not quite sure what you mean here.

Karel (with some contributions by me) has implemented the interfaces
for David's mount notifications and fsinfo() call in libmount. We
still have a little more to do on that.

I also have a systemd implementation that uses these libmount features
for mount table handling that works quite well, with a couple more
things to do to complete it, that Lennart has done an initial review
for.

It's no secret that I don't like the proc file system in general
but it is really useful for many things, that's just the way it
is.

Ian