User-visible context-mount API

[David Howells <dhowells@redhat.com>]

I've been looking at the context-mount API visible to userspace as I need to
adjust the security ops to handle it.  I'm thinking I probably need something
like the following system calls.  Note that:

 topology_flags are MS_PRIVATE, MS_SLAVE, MS_SHARED, MS_UNBINDABLE.

 mount_flags are things like MS_NOSUID, MS_NODEV, MS_NOEXEC that get
 translated to MNT_* flags in the kernel.

 (1) Open a filesystem and create a blank context from it:

	fd = fsopen(const char *fs_name, unsigned int flags, ...);

     where flags includes FSOPEN_CLOEXEC, FSOPEN_CREATE_ONLY (don't reuse
     superblock).

 (2) Access and change the context:

	write(fd, "<command>", ...);
	read(fd, ...);
	ioctl(fd, ...);

 (3) Create and set up a context for an existing mountpoint:

	fd = fspick(int dfd, const char *path, unsigned int flags);

     where flags includes FSPICK_CLOEXEC.

 (4) Create a mountpoint on a path, using a context to supply the superblock
     details:

	mount_create(int fd, int dfd, const char *path,
		     unsigned int topology_flags,
		     unsigned int mount_flags);

 (5) Move a mount:

	mount_move(int from_dfd, const char *frompath,
		   int to_dfd, const char *topath);

     This might want to take new topology flags algo.

 (6) Adjust a mountpoint's topology flags:

	mount_set_topology(int dfd, const char *path,
			   unsigned int topology_flags);

 (7) Reconfigure a mountpoint:

	mount_reconfigure(int dfd, const char *path,
			  unsigned int mount_flags);

 (8) Change R/O protection on a mountpoint:

	mount_protect(int dfd, const char *path,
		      bool read_only);

     This involves changing the R/O protection on the superblock also, but
     might be mergeable with mount_reconfigure().

Note that two things are missing from the list:

 (1) Bind mount.  This is done by:

	fd = fspick("/mnt/a");
	mount_create(fd, ..., "/mnt/b", ...);
	mount_create(fd, ..., "/mnt/c", ...);
	mount_create(fd, ..., "/mnt/d", ...);

 (2) Remount.  Superblock reconfiguration is done by something like:

	fd = fspick("/mnt/a");
	write(fd, "? fs");
	read(fd, filesystem_type);
	write(fd, "o user_xattr"); // Indicate changes to be made
	write(fd, "x reconfigure"); // Perform the reconfiguration

Thinking further on this, maybe I should make a mountpoint-context also, so
that it can be loaded up with target namespace information and other goodies.
This would vastly expand the parameter space for a mountpoint beyond the few
syscall args available.  Creating a new mount might then look like:

	sbfd = fsopen("ext4");
	write(sbfd, "d /dev/sda1");
	write(sbfd, "o user_xattr");
	write(sbfd, "x commit");

	mfd = mount_new();
	write(mfd, "ns mnt 123"); // where fd 123 refers to a mount namesapce
	write(mfd, "o bind=1"); // Set MS_BIND
	write(mfd, "o nosuid=1"); // Set MS_NOSUID

	mount_create(mfd, AT_FDCWD, "/mnt/a", sbfd);

David