Tetsuo Handa [off-list ref] 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
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