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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 11 Aug 2020 20:49:35 +0200
Message-ID: <CAJfpeguo5nAWcmduX4frknQGiRJeaj9Rdj018xUBrwqOJhVufw@mail.gmail.com>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     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>
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On Tue, Aug 11, 2020 at 6:05 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:

> 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.

Oh, I have.   That's why I opted for triple slashes, since that should
work most of the time even in those concatenated cases.  And yes, I
know, most is not always, and this might just be hiding bugs, etc...
I think the pragmatic approach would be to try this and see how many
triple slash hits a normal workload gets and if it's reasonably low,
then hopefully that together with warnings for O_ALT would be enough.

>  (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.

That's exactly what I implemented in the proof of concept patch.

> Which means that it's not a unified namespace
> after all, because you can't make symlinks point to metadata.

I don't think that's a great deal.  Also I think other limitations
would make sense:

 - no mounts allowed under ///
 - no ./.. resolution after ///
 - no hardlinks
 - no special files, just regular and directory
 - no seeking (regular or dir)

>     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)

Umm, can you remind me what's so horrible about that?  Yeah, hard
linked directories are a no-no.  But it doesn't have to be implemented
in a way to actually be a problem with hard links.

Thanks,
Miklos