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From: Miklos Szeredi <miklos@szeredi.hu>
To: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Stefan Metzmacher <metze@samba.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-ext4@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>,
	Trond Myklebust <trond.myklebust@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Linux NFS list <linux-nfs@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Ian Kent <raven@themaw.net>, Miklos Szeredi <mszeredi@redhat.com>,
	Christian Brauner <christian@brauner.io>,
	Jann Horn <jannh@google.com>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Karel Zak <kzak@redhat.com>, Jeff Layton <jlayton@redhat.com>,
	linux-fsdevel@vger.kernel.org,
	LSM <linux-security-module@vger.kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/14] VFS: Filesystem information [ver #18]
Date: Tue, 10 Mar 2020 10:18:08 +0100	[thread overview]
Message-ID: <CAJfpegt9TqfyJuk0G-OJdWLiKuxSeY0cQKK=1GVf1fStA9COBw@mail.gmail.com> (raw)
In-Reply-To: <537182.1583794373@warthog.procyon.org.uk>

On Mon, Mar 9, 2020 at 11:53 PM David Howells <dhowells@redhat.com> wrote:
>
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > >  (1) It can be targetted.  It makes it easy to query directly by path or
> > >      fd, but can also query by mount ID or fscontext fd.  procfs and sysfs
> > >      cannot do three of these things easily.
> >
> > See above: with the addition of open(path, O_PATH) it can do all of these.
>
> That's a horrible interface.  To query a file by path, you have to do:
>
>         fd = open(path, O_PATH);
>         sprintf(procpath, "/proc/self/fdmount/%u/<attr>");
>         fd2 = open(procpath, O_RDONLY);
>         read(fd2, ...);
>         close(fd2);
>         close(fd);
>
> See point (3) about efficiency also.  You're having to open *two* files.

I completely agree, opening two files is surely going to kill
performance of application needing to retrieve a billion mount
attributes per second.</sarcasm>

> > >  (2) Easier to provide LSM oversight.  Is the accessing process allowed to
> > >      query information pertinent to a particular file?
> >
> > Not quite sure why this would be easier for a new ad-hoc interface than for
> > the well established filesystem API.
>
> You're right.  That's why fsinfo() uses standard pathwalk where possible,
> e.g.:
>
>         fsinfo(AT_FDCWD, "/path/to/file", ...);
>
> or a fairly standard fd-querying interface:
>
>         fsinfo(fd, "", { resolve_flags = RESOLVE_EMPTY_PATH },  ...);
>
> to query an open file descriptor.  These are well-established filesystem APIs.

Yes.  The problem is with the "..." part where you pass random
structures to a function.   That's useful sometimes, but at the very
least it breaks type safety, and not what I would call a "clean" API.

> > Now onto the advantages of a filesystem based API:
> >
> >  - immediately usable from all programming languages, including scripts
>
> This is not true.  You can't open O_PATH from shell scripts, so you can't
> query things by path that you can't or shouldn't open (dev file paths, for
> example; symlinks).

Yes.  However, you just wrote the core of a utility that could do this
(in 6 lines, no less).  Now try that feat with fsinfo(2)!

Thanks,
Miklos

      reply	other threads:[~2020-03-10  9:18 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 14:00 David Howells
2020-03-09 14:00 ` [PATCH 01/14] VFS: Add additional RESOLVE_* flags " David Howells
2020-03-09 20:56   ` Stefan Metzmacher
2020-03-09 21:13   ` David Howells
2020-03-10  0:55   ` Aleksa Sarai
2020-03-10  1:14     ` Linus Torvalds
2020-03-10  7:25     ` David Howells
2020-03-11 17:59       ` Linus Torvalds
2020-03-12  9:08         ` Stefan Metzmacher
2020-03-12 16:24           ` Linus Torvalds
2020-03-12 17:11             ` Stefan Metzmacher
2020-03-12 19:37               ` Al Viro
2020-03-12 21:48               ` Jeremy Allison
2020-03-13  9:59               ` Aleksa Sarai
2020-03-13 10:00                 ` Aleksa Sarai
2020-03-13 16:48                 ` Jeremy Allison
2020-03-13 18:28                 ` Al Viro
2020-03-13 18:35                   ` Jeremy Allison
2020-03-16 14:21                   ` Aleksa Sarai
2020-03-16 14:20                     ` Aleksa Sarai
2020-03-12 19:25             ` Al Viro
2020-03-12 16:56           ` David Howells
2020-03-12 18:09             ` Linus Torvalds
2020-03-13  9:53               ` Aleksa Sarai
2020-03-13  9:50         ` Aleksa Sarai
2020-03-09 14:01 ` [PATCH 02/14] fsinfo: Add fsinfo() syscall to query filesystem information " David Howells
2020-03-10  9:31   ` Christian Brauner
2020-03-10  9:32     ` [PATCH v19 01/14] fsinfo: Add fsinfo() syscall to query filesystem information Christian Brauner
2020-03-10  9:32       ` [PATCH v19 14/14] arch: wire up fsinfo syscall Christian Brauner
2020-03-09 14:01 ` [PATCH 03/14] fsinfo: Provide a bitmap of supported features [ver #18] David Howells
2020-03-09 14:01 ` [PATCH 04/14] fsinfo: Allow retrieval of superblock devname, options and stats " David Howells
2020-03-09 14:01 ` [PATCH 05/14] fsinfo: Allow fsinfo() to look up a mount object by ID " David Howells
2020-03-09 14:01 ` [PATCH 06/14] fsinfo: Add a uniquifier ID to struct mount " David Howells
2020-03-09 14:01 ` [PATCH 07/14] fsinfo: Allow mount information to be queried " David Howells
2020-03-10  9:04   ` Miklos Szeredi
2020-03-09 14:02 ` [PATCH 08/14] fsinfo: Allow the mount topology propogation flags to be retrieved " David Howells
2020-03-10  8:42   ` Christian Brauner
2020-03-09 14:02 ` [PATCH 09/14] fsinfo: Provide notification overrun handling support " David Howells
2020-03-09 14:02 ` [PATCH 10/14] fsinfo: sample: Mount listing program " David Howells
2020-03-09 14:02 ` [PATCH 11/14] fsinfo: Add API documentation " David Howells
2020-03-09 14:02 ` [PATCH 12/14] fsinfo: Add support for AFS " David Howells
2020-03-09 14:02 ` [PATCH 13/14] fsinfo: Example support for Ext4 " David Howells
2020-03-09 14:02 ` [PATCH 14/14] fsinfo: Example support for NFS " David Howells
2020-03-09 17:50 ` [PATCH 00/14] VFS: Filesystem information " Jeff Layton
2020-03-09 19:22   ` Andres Freund
2020-03-09 22:49     ` Jeff Layton
2020-03-10  0:18       ` Andres Freund
2020-03-09 20:02 ` Miklos Szeredi
2020-03-09 22:52 ` David Howells
2020-03-10  9:18   ` Miklos Szeredi [this message]

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