From: Christian Brauner <christian.brauner@ubuntu.com>
To: 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>,
Ian Kent <raven@themaw.net>,
LSM <linux-security-module@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
Date: Tue, 11 Aug 2020 21:39:16 +0200 [thread overview]
Message-ID: <20200811193916.zcwebstmbyvushau@wittgenstein> (raw)
In-Reply-To: <CAHk-=wg_bfVf5eazwH2uXTG-auCYZUpq-xb1kDeNjY7yaXS7bw@mail.gmail.com>
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?
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
next prev parent reply other threads:[~2020-08-11 19:39 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-03 15:27 [GIT PULL] Mount notifications David Howells
2020-08-03 15:49 ` [GIT PULL] Filesystem Information David Howells
2020-08-03 16:42 ` Miklos Szeredi
2020-08-04 2:15 ` Ian Kent
2020-08-04 14:36 ` Miklos Szeredi
2020-08-05 1:33 ` Ian Kent
2020-08-05 8:00 ` Miklos Szeredi
2020-08-05 11:13 ` Ian Kent
2020-08-05 8:24 ` file metadata via fs API (was: [GIT PULL] Filesystem Information) Miklos Szeredi
2020-08-11 13:54 ` Miklos Szeredi
2020-08-11 14:08 ` Al Viro
2020-08-11 14:22 ` Miklos Szeredi
2020-08-11 14:31 ` Al Viro
[not found] ` <CAAgocE07=vVKpQhG+rjEGO=NEBKZ02gjg4TRPxECAc+RKrzn=Q@mail.gmail.com>
2020-08-11 14:36 ` Al Viro
2020-08-11 14:36 ` Miklos Szeredi
2020-08-11 14:42 ` Al Viro
2020-08-11 14:47 ` Miklos Szeredi
2020-08-11 15:20 ` Linus Torvalds
2020-08-11 15:30 ` Miklos Szeredi
2020-08-11 16:05 ` Linus Torvalds
2020-08-11 18:49 ` Miklos Szeredi
2020-08-11 19:31 ` Lennart Poettering
2020-08-11 19:50 ` Christian Brauner
2020-08-11 19:39 ` Christian Brauner [this message]
2020-08-12 0:53 ` Ian Kent
2020-08-11 15:39 ` Andy Lutomirski
2020-08-11 16:17 ` Casey Schaufler
2020-08-11 16:30 ` Linus Torvalds
2020-08-11 20:28 ` Miklos Szeredi
2020-08-11 20:36 ` Jann Horn
2020-08-11 20:56 ` Miklos Szeredi
2020-08-11 21:17 ` Andy Lutomirski
2020-08-11 21:18 ` Linus Torvalds
2020-08-12 7:23 ` Miklos Szeredi
2020-08-12 14:39 ` Al Viro
2020-08-12 14:46 ` Miklos Szeredi
2020-08-12 15:08 ` Al Viro
2020-08-12 15:13 ` Miklos Szeredi
2020-08-12 16:33 ` Al Viro
2020-08-12 17:16 ` Miklos Szeredi
2020-08-12 17:39 ` Al Viro
2020-08-12 18:33 ` Al Viro
2020-08-12 21:30 ` Al Viro
2020-08-18 9:41 ` Miklos Szeredi
2020-08-18 9:30 ` Miklos Szeredi
2020-08-12 15:22 ` David Howells
2020-08-11 21:20 ` Al Viro
2020-08-11 21:35 ` Casey Schaufler
2020-08-11 16:05 ` Al Viro
2020-08-11 16:09 ` Linus Torvalds
2020-08-11 16:39 ` Al Viro
2020-08-12 10:14 ` Karel Zak
2020-08-12 13:09 ` Miklos Szeredi
2020-08-12 13:33 ` David Howells
2020-08-12 13:54 ` Miklos Szeredi
2020-08-12 0:05 ` David Howells
2020-08-12 7:55 ` Miklos Szeredi
2020-08-12 8:29 ` David Howells
2020-08-12 8:37 ` Miklos Szeredi
2020-08-12 9:43 ` file metadata via fs API Steven Whitehouse
2020-08-12 10:04 ` Miklos Szeredi
2020-08-12 11:28 ` Karel Zak
2020-08-12 12:43 ` Miklos Szeredi
2020-08-13 8:52 ` Karel Zak
2020-08-12 13:06 ` David Howells
2020-08-13 1:01 ` Ian Kent
2020-08-12 18:18 ` file metadata via fs API (was: [GIT PULL] Filesystem Information) Linus Torvalds
2020-08-12 19:34 ` file metadata via fs API Steven Whitehouse
2020-08-12 19:50 ` Linus Torvalds
2020-08-13 3:44 ` Ian Kent
2020-08-13 10:36 ` Karel Zak
2020-08-14 7:58 ` Lennart Poettering
2020-08-17 11:32 ` Steven Whitehouse
2020-08-17 17:15 ` Linus Torvalds
2020-08-17 22:44 ` Linus Torvalds
2020-08-18 12:50 ` Miklos Szeredi
2020-08-18 18:51 ` Linus Torvalds
2020-08-18 20:18 ` Miklos Szeredi
2020-08-18 20:53 ` Linus Torvalds
2020-08-21 13:17 ` Miklos Szeredi
2020-08-19 2:29 ` Al Viro
2020-08-13 3:53 ` file metadata via fs API (was: [GIT PULL] Filesystem Information) Jeffrey E Altman
2020-08-14 17:05 ` Linus Torvalds
2020-08-18 15:01 ` Jeffrey E Altman
2020-08-14 8:06 ` Lennart Poettering
2020-08-12 13:54 ` David Howells
2020-08-12 14:10 ` Miklos Szeredi
2020-08-12 14:23 ` David Howells
2020-08-03 22:48 ` [GIT PULL] Mount notifications Ian Kent
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