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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <b4be11f6-8235-1ee1-9a01-c09eb9b88404@I-love.SAKURA.ne.jp>
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To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     dhowells@redhat.com, viro@zeniv.linux.org.uk,
        tomoyo-dev-en@lists.sourceforge.jp,
        linux-security-module@vger.kernel.org,
        torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, miklos@szeredi.hu,
        kent.overstreet@gmail.com
Subject: Re: [PATCH 13/38] tomoyo: Implement security hooks for the new mount API [ver #10]
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Date:   Mon, 30 Jul 2018 11:49:16 +0100
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Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Would you provide examples of each possible combination as a C program?
> For example, if one mount point from multiple sources with different
> options are possible, please describe such pattern using syscall so that
> LSM modules can run it to see whether they are working as expected. 

One example could be overlayfs.  So you might do, say:

	ufd = open("/overlay", O_PATH);
	fsfd = fsopen("overlay", 0);
	fsconfig(fsfd, fsconfig_set_path, "lowerdir", "/src", AT_FDCWD);
	fsconfig(fsfd, fsconfig_set_path, "upperdir", "upper", ufd);
	fsconfig(fsfd, fsconfig_set_path, "workdir", "scratch", ufd);
	mfd = fsmount(fsfd, 0, 0);
	move_mount(fsfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

which would allow you to specify the "sources" using dirfds.

Another possibility is could be ext4 with separate journal:

	fsfd = fsopen("ext4", 0);
	fsconfig(fsfd, fsconfig_set_path, "source", "/dev/sda1", AT_FDCWD);
	fsconfig(fsfd, fsconfig_set_path, "journal_path", "/dev/sda2", AT_FDCWD);
	mfd = fsmount(fsfd, 0, 0);
	move_mount(fsfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

And then there's bcachefs which suggests on the webpage:

	mount -t bcachefs /dev/sda1:/dev/sdb1 /mnt

but you could then do:

	fsfd = fsopen("bcachefs", 0);
	fsconfig(fsfd, fsconfig_set_path, "source", "/dev/sda1", AT_FDCWD);
	fsconfig(fsfd, fsconfig_set_path, "source", "/dev/sdb2", AT_FDCWD);
	mfd = fsmount(fsfd, 0, 0);
	move_mount(fsfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

One thing I'm not certain of is whether I should allow multiple values to the
same key name, or whether I should require that each key be labelled
differently, possibly something like:

	fsconfig(fsfd, fsconfig_set_path, "source", "/dev/sda1", AT_FDCWD);
	fsconfig(fsfd, fsconfig_set_path, "source.1", "/dev/sdb2", AT_FDCWD);

David