linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Date:   Wed, 12 Aug 2020 14:54:46 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> IOW, if you do something more along the lines of
> 
>        fd = open(""foo/bar", O_PATH);
>        metadatafd = openat(fd, "metadataname", O_ALT);
> 
> it might be workable.

What is it going to walk through?  You need to end up with an inode and dentry
from somewhere.

It sounds like this would have to open up a procfs-like magic filesystem, and
walk into it.  But how would that actually work?  Would you create a new
superblock each time you do this, labelled with the starting object (say the
dentry for "foo/bar" in this case), and then walk from the root?

An alternative, maybe, could be to make a new dentry type, say, and include it
in the superblock of the object being queried - and let the filesystems deal
with it.  That would mean that non-dir dentries would then have virtual
children.  You could then even use this to implement resource forks...

Another alternative would be to note O_ALT and then skip pathwalk entirely,
but just use the name as a key to the attribute, creating an anonfd to read
it.  But then why use openat() at all?  You could instead do:

	metadatafd = openmeta(fd, "metadataname");

and save the page flag.  You could even merge the two opens and do:

	metadatafd = openmeta("foo/bar", "metadataname");

Why not even combine this with Miklos's readfile() idea:

	readmeta(AT_FDCWD, "foo/bar", "metadataname", buf, sizeof(buf));

and we're now down to one syscall and no fds and you don't even need a magic
filesystem to make it work.

There's another consideration too: Paths are not unique handles to mounts.
It's entirely possible to have colocated mounts.  We need to be able to query
all the mounts on a mountpoint.

David