take the targets of /proc/*/ns/* symlinks to separate fs

New pseudo-filesystem: nsfs.  Targets of /proc/*/ns/* live there now.
It's not mountable (not even registered, so it's not in /proc/filesystems,
etc.).  Files on it *are* bindable - we explicitly permit that in do_loopback().

This stuff lives in fs/nsfs.c now; proc_ns_fget() moved there as well.
get_proc_ns() is a macro now (it's simply returning ->i_private; would
have been an inline, if not for header ordering headache).
proc_ns_inode() is an ex-parrot.  The interface used in procfs is
ns_get_path(path, task, ops) and ns_get_name(buf, size, task, ops).

Dentries and inodes are never hashed; a non-counting reference to dentry
is stashed in ns_common (removed by ->d_prune()) and reused by ns_get_path()
if present.  See ns_get_path()/ns_prune_dentry/nsfs_evict() for details
of that mechanism.

As the result, proc_ns_follow_link() has stopped poking in nd->path.mnt;
it does nd_jump_link() on a consistent <vfsmount,dentry> pair it gets
from ns_get_path().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 0 deletions

diff --git a/init/main.c b/init/main.c
index 800a0daede7e..bcc75057ea87 100644
--- a/init/main.c
+++ b/init/main.c
@@ -78,6 +78,7 @@
 #include <linux/context_tracking.h>
 #include <linux/random.h>
 #include <linux/list.h>
+#include <linux/proc_ns.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -660,6 +661,7 @@ asmlinkage __visible void __init start_kernel(void)
 	/* rootfs populating might need page-writeback */
 	page_writeback_init();
 	proc_root_init();
+	nsfs_init();
 	cgroup_init();
 	cpuset_init();
 	taskstats_init_early();