This patch adds FUSE core.

This contains the following files:

 o inode.c
    - superblock operations (alloc_inode, destroy_inode, read_inode,
      clear_inode, put_super, show_options)
    - registers FUSE filesystem

 o fuse_i.h
    - private header file

Requirements
============

 The most important difference between orinary filesystems and FUSE is
 the fact, that the filesystem data/metadata is provided by a userspace
 process run with the privileges of the mount "owner" instead of the
 kernel, or some remote entity usually running with elevated
 privileges.

 The security implication of this is that a non-privileged user must
 not be able to use this capability to compromise the system.  Obvious
 requirements arising from this are:

  - mount owner should not be able to get elevated privileges with the
    help of the mounted filesystem

  - mount owner should not be able to induce undesired behavior in
    other users' or the super user's processes

  - mount owner should not get illegitimate access to information from
    other users' and the super user's processes

 These are currently ensured with the following constraints:

  1) mount is only allowed to directory or file which the mount owner
    can modify without limitation (write access + no sticky bit for
    directories)

  2) nosuid,nodev mount options are forced

  3) any process running with fsuid different from the owner is denied
     all access to the filesystem

 1) and 2) are ensured by the "fusermount" mount utility which is a
    setuid root application doing the actual mount operation.

 3) is ensured by a check in the permission() method in kernel

 I started thinking about doing 3) in a different way because Christoph
 H. made a big deal out of it, saying that FUSE is unacceptable into
 mainline in this form.

 The suggested use of private namespaces would be OK, but in their
 current form have many limitations that make their use impractical (as
 discussed in this thread).

 Suggested improvements that would address these limitations:

   - implement shared subtrees

   - allow a process to join an existing namespace (make namespaces
     first-class objects)

   - implement the namespace creation/joining in a PAM module

 With all that in place the check of owner against current->fsuid may
 be removed from the FUSE kernel module, without compromising the
 security requirements.

 Suid programs still interesting questions, since they get access even
 to the private namespace causing some information leak (exact
 order/timing of filesystem operations performed), giving some
 ptrace-like capabilities to unprivileged users.  BTW this problem is
 not strictly limited to the namespace approach, since suid programs
 setting fsuid and accessing users' files will succeed with the current
 approach too.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>