ns: allow ns_entries to have custom symlink content

Patch series "Expose task pid_ns_for_children to userspace".

pid_ns_for_children set by a task is known only to the task itself, and
it's impossible to identify it from outside.

It's a big problem for checkpoint/restore software like CRIU, because it
can't correctly handle tasks, that do setns(CLONE_NEWPID) in proccess of
their work.  If they have a custom pid_ns_for_children before dump, they
must have the same ns after restore.  Otherwise, restored task bumped
into enviroment it does not expect.

This patchset solves the problem.  It exposes pid_ns_for_children to ns
directory in standard way with the name "pid_for_children":

  ~# ls /proc/5531/ns -l | grep pid
  lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid -> pid:[4026531836]
  lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid_for_children -> pid:[4026532286]

This patch (of 2):

Make possible to have link content prefix yyy different from the link
name xxx:

  $ readlink /proc/[pid]/ns/xxx
  yyy:[4026531838]

This will be used in next patch.

Link: http://lkml.kernel.org/r/149201120318.6007.7362655181033883000.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <[email protected]>
Reviewed-by: Cyrill Gorcunov <[email protected]>
Acked-by: Andrei Vagin <[email protected]>
Cc: Andreas Gruenbacher <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Serge Hallyn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

2 files changed, 4 insertions, 1 deletions

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 323f492e0822..f3db56e83dd2 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -196,9 +196,11 @@ int ns_get_name(char *buf, size_t size, struct task_struct *task,
 {
 	struct ns_common *ns;
 	int res = -ENOENT;
+	const char *name;
 	ns = ns_ops->get(task);
 	if (ns) {
-		res = snprintf(buf, size, "%s:[%u]", ns_ops->name, ns->inum);
+		name = ns_ops->real_ns_name ? : ns_ops->name;
+		res = snprintf(buf, size, "%s:[%u]", name, ns->inum);
 		ns_ops->put(ns);
 	}
 	return res;
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index 12cb8bd81d2d..88dba3b53375 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -14,6 +14,7 @@ struct inode;
 
 struct proc_ns_operations {
 	const char *name;
+	const char *real_ns_name;
 	int type;
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);