This fixes an old (2007) selinux regression: filesystem labeling for
/proc/sys returned
     -r--r--r-- unknown                          /proc/sys/fs/file-nr
instead of
     -r--r--r-- system_u:object_r:sysctl_fs_t:s0 /proc/sys/fs/file-nr

Events that lead to breaking of /proc/sys/ selinux labeling:

1) sysctl was reimplemented to route all calls through /proc/sys/

    commit 77b14db5
    [PATCH] sysctl: reimplement the sysctl proc support

2) proc_dir_entry was removed from ctl_table:

    commit 3fbfa981
    [PATCH] sysctl: remove the proc_dir_entry member for the sysctl tables

3) selinux still walked the proc_dir_entry tree to apply
   labeling. Because ctl_tables don't have a proc_dir_entry, we did
   not label /proc/sys/ inodes any more. To achieve this the /proc/sys/
   inodes were marked private and private inodes were ignored by
   selinux.

    commit bbaca6c2
    [PATCH] selinux: enhance selinux to always ignore private inodes

    commit 86a71dbd


    [PATCH] sysctl: hide the sysctl proc inodes from selinux

Access control checks have been done by means of a special sysctl hook
that was called for read/write accesses to any /proc/sys/ entry.

We don't have to do this because, instead of walking the
proc_dir_entry tree we can walk the dentry tree (as done in this
patch). With this patch:
* we don't mark /proc/sys/ inodes as private
* we don't need the sysclt security hook
* we walk the dentry tree to find the path to the inode.

We have to strip the PID in /proc/PID/ entries that have a
proc_dir_entry because selinux does not know how to label paths like
'/1/net/rpc/nfsd.fh' (and defaults to 'proc_t' labeling). Selinux does
know of '/net/rpc/nfsd.fh' (and applies the 'sysctl_rpc_t' label).

PID stripping from the path was done implicitly in the previous code
because the proc_dir_entry tree had the root in '/net' in the example
from above. The dentry tree has the root in '/1'.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Lucian Adrian Grijincu <lucian.grijincu@gmail.com>
Signed-off-by: Eric Paris <eparis@redhat.com>