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From: David Howells <dhowells@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: dhowells@redhat.com, Miklos Szeredi <miklos@szeredi.hu>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	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: Wed, 12 Aug 2020 14:54:46 +0100	[thread overview]
Message-ID: <135551.1597240486@warthog.procyon.org.uk> (raw)
In-Reply-To: <CAHk-=wjzLmMRf=QG-n+1HnxWCx4KTQn9+OhVvUSJ=ZCQd6Y1WA@mail.gmail.com>

Linus Torvalds <torvalds@linux-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


  parent reply	other threads:[~2020-08-12 13:54 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
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 [this message]
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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