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From:   David Howells <dhowells@redhat.com>
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To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     dhowells@redhat.com, Ian Kent <raven@themaw.net>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
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Date:   Tue, 03 Mar 2020 09:12:53 +0000
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Miklos Szeredi <miklos@szeredi.hu> wrote:

> I'm doing a patch.   Let's see how it fares in the face of all these
> preconceptions.

Don't forget the efficiency criterion.  One reason for going with fsinfo(2) is
that scanning /proc/mounts when there are a lot of mounts in the system is
slow (not to mention the global lock that is held during the read).

Now, going with sysfs files on top of procfs links might avoid the global
lock, and you can avoid rereading the options string if you export a change
notification, but you're going to end up injecting a whole lot of pathwalk
latency into the system.

On top of that, it isn't going to help with the case that I'm working towards
implementing where a container manager can monitor for mounts taking place
inside the container and supervise them.  What I'm proposing is that during
the action phase (eg. FSCONFIG_CMD_CREATE), fsconfig() would hand an fd
referring to the context under construction to the manager, which would then
be able to call fsinfo() to query it and fsconfig() to adjust it, reject it or
permit it.  Something like:

	fd = receive_context_to_supervise();
	struct fsinfo_params params = {
		.flags		= FSINFO_FLAGS_QUERY_FSCONTEXT,
		.request	= FSINFO_ATTR_SB_OPTIONS,
	};
	fsinfo(fd, NULL, &params, sizeof(params), buffer, sizeof(buffer));
	supervise_parameters(buffer);
	fsconfig(fd, FSCONFIG_SET_FLAG, "hard", NULL, 0);
	fsconfig(fd, FSCONFIG_SET_STRING, "vers", "4.2", 0);
	fsconfig(fd, FSCONFIG_CMD_SUPERVISE_CREATE, NULL, NULL, 0);
	struct fsinfo_params params = {
		.flags		= FSINFO_FLAGS_QUERY_FSCONTEXT,
		.request	= FSINFO_ATTR_SB_NOTIFICATIONS,
	};
	struct fsinfo_sb_notifications sbnotify;
	fsinfo(fd, NULL, &params, sizeof(params), &sbnotify, sizeof(sbnotify));
	watch_super(fd, "", AT_EMPTY_PATH, watch_fd, 0x03);
	fsconfig(fd, FSCONFIG_CMD_SUPERVISE_PERMIT, NULL, NULL, 0);
	close(fd);

However, the supervised mount may be happening in a completely different set
of namespaces, in which case the supervisor presumably wouldn't be able to see
the links in procfs and the relevant portions of sysfs.

David