From: David Howells <dhowells@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: dhowells@redhat.com, torvalds@linux-foundation.org,
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@vger.kernel.org, linux-api@vger.kernel.org,
raven@themaw.net, mszeredi@redhat.com, christian@brauner.io,
jannh@google.com, darrick.wong@oracle.com, kzak@redhat.com,
jlayton@redhat.com, linux-fsdevel@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/14] VFS: Filesystem information [ver #18]
Date: Mon, 09 Mar 2020 22:52:53 +0000 [thread overview]
Message-ID: <537182.1583794373@warthog.procyon.org.uk> (raw)
In-Reply-To: <20200309200238.GB28467@miu.piliscsaba.redhat.com>
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.
> > (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.
Where I vary from this is allowing direct specification of a mount ID also,
with a special flag to say that's what I'm doing:
fsinfo(AT_FDCWD, "23", { flags = FSINFO_QUERY_FLAGS_MOUNT }, ...);
> > (7) Don't have to create/delete a bunch of sysfs/procfs nodes each time a
> > mount happens or is removed - and since systemd makes much use of
> > mount namespaces and mount propagation, this will create a lot of
> > nodes.
>
> This patch creates a single struct mountfs_entry per mount, which is 48bytes.
fsinfo() doesn't create any. Furthermore, it seems that mounts get multiplied
8-10 times by systemd - though, as you say, it's not necessarily a great deal
of memory.
> 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).
I imagine you're thinking of something like:
{
id=`cat /proc/self/fdmount/5/parent_mount`
} 5</my/path/to/my/file
but what if /my/path/to/my/file is actually /dev/foobar?
I've had a grep through the bash sources, but can't seem to find anywhere that
uses O_PATH.
> - same goes for future extensions: no need to update libc, utils, language
> bindings, strace, etc...
Applications and libraries using these attributes would have to change anyway
to make use of additional information.
But it's not a good argument since you now have to have text parsers that
change over time.
David
next prev parent reply other threads:[~2020-03-09 22:53 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-09 14:00 [PATCH 00/14] VFS: Filesystem information [ver #18] 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
[not found] ` <20200310005549.adrn3yf4mbljc5f6@yavin>
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
[not found] ` <20200313095901.tdv4vl7envypgqfz@yavin>
2020-03-13 16:48 ` Jeremy Allison
2020-03-13 18:28 ` Al Viro
2020-03-13 18:35 ` Jeremy Allison
2020-03-16 14:20 ` Aleksa Sarai
2020-03-12 19:25 ` Al Viro
2020-03-12 16:56 ` David Howells
2020-03-12 18:09 ` Linus Torvalds
2020-03-09 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 [this message]
2020-03-10 9:18 ` Miklos Szeredi
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