[PATCH v3 1/2] ns: Allow ns_entries to have custom symlink content

From: Kirill Tkhai
Date: Wed May 03 2017 - 06:32:49 EST

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

This will be used in next patch.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Acked-by: Andrei Vagin <avagin@xxxxxxxxxxxxx>
fs/nsfs.c | 4 +++-
include/linux/proc_ns.h | 1 +
2 files changed, 4 insertions(+), 1 deletion(-)

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);
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);