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

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Date:   Wed, 12 Aug 2020 16:10:56 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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)

On Wed, Aug 12, 2020 at 3:54 PM David Howells <dhowells@...hat.com> wrote:
>
> 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 writemeta() and createmeta() and readdirmeta() and ...

The point is that generic operations already exist and no need to add
new, specialized ones to access metadata.

Thanks,
Miklos