[Devel] [RFC][PATCH 10/14] Use pid ns from pid_nrs list

[sukadev at us.ibm.com]

From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
Subject: [RFC][PATCH 10/14] Use pid ns from pid_nrs list

We need to decouple pid namespace from nsproxy to allow getting and
releasing pid namespace independently from other namespaces. This is
required since a process's reference to its pid namespace must exist
even after its references to other namespaces are dropped during
process exit.

As stated in earlier patches, to support containers, a process can
have different pid_t values in different pid namespaces and struct
pid_nr and the pid->pid_nrs list provide this association of pid_t
value with pid namespace for a process.

To find the pid namespace of a process, we can now use the pid namespace
stored in the struct pid_nr rather than the one stored in nsproxy.

This patch defines a function, pid_ns(), which returns the "current" or
"primary" pid namespace of the given struct pid and reimplements the
task_pid_ns() and child_reaper() wrapper functions using the pid_ns()

Changelog:

	- Rewrite from an earlier verison which supported unshare()
	  of pid namespace.

Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
---
 include/linux/pid_namespace.h |    5 +++--
 kernel/pid.c                  |   37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

Index: lx26-21-rc3-mm2/kernel/pid.c
===================================================================
--- lx26-21-rc3-mm2.orig/kernel/pid.c	2007-03-20 15:55:52.000000000 -0700
+++ lx26-21-rc3-mm2/kernel/pid.c	2007-03-20 16:57:14.000000000 -0700
@@ -270,6 +270,43 @@ pid_t pid_nr(struct pid *pid)
 }
 EXPORT_SYMBOL_GPL(pid_nr);
 
+/*
+ * Return the current pid-namespace of this pid.
+ *
+ * Processes initially exist only in the init-pid-ns. For such processes,
+ * their "current pid namespace" is just init-pid-ns.
+ *
+ * When a process in init-pid-ns clones using the CLONE_NEWPID flag, it
+ * creates a new child-pid-ns and a child process. This child process then
+ * exists in two namespaces - i.e it has a pid_t value in init pid namespace
+ * and another, usually different pid_t value in the newly-created child-pid-ns.
+ *
+ * For this child process, we treat child-pid-ns as its "current pid namespace".
+ *
+ * The "current pid namespace" is used:
+ * 	a. to determine which process will reap an orphaned process.
+ * 	b. by find_pid() to select the appropriate pid -  note that since
+ * 	   there can be multiple processes with a given pid_t value (one
+ * 	   in each namespace) find_pid() returns the pid corresponding
+ * 	   to the current pid namespace.
+ *
+ * Processes inherit the current and all ancestor pid namespaces from their
+ * parent.
+ */
+struct pid_namespace *pid_ns(struct pid *pid)
+{
+	struct pid_nr* pid_nr;
+	struct hlist_node *head;
+
+	if (!pid)
+		return NULL;
+
+	head = pid->pid_nrs.first;
+	pid_nr = hlist_entry(head, struct pid_nr, node);
+
+	return pid_nr->pid_ns;
+}
+
 struct pid *alloc_pid(void)
 {
 	struct pid *pid;
Index: lx26-21-rc3-mm2/include/linux/pid_namespace.h
===================================================================
--- lx26-21-rc3-mm2.orig/include/linux/pid_namespace.h	2007-03-20 15:55:52.000000000 -0700
+++ lx26-21-rc3-mm2/include/linux/pid_namespace.h	2007-03-20 15:55:52.000000000 -0700
@@ -31,6 +31,7 @@ static inline void get_pid_ns(struct pid
 
 extern struct pid_namespace *copy_pid_ns(int flags, struct pid_namespace *ns);
 extern void free_pid_ns(struct kref *kref);
+extern struct pid_namespace *pid_ns(struct pid * pid);
 
 static inline void put_pid_ns(struct pid_namespace *ns)
 {
@@ -39,12 +40,12 @@ static inline void put_pid_ns(struct pid
 
 static inline struct pid_namespace *task_pid_ns(struct task_struct *tsk)
 {
-        return tsk->nsproxy->pid_ns;
+	return pid_ns(task_pid(tsk));
 }
 
 static inline struct task_struct *child_reaper(struct task_struct *tsk)
 {
-	return init_pid_ns.child_reaper;
+	return task_pid_ns(tsk)->child_reaper;
 }
 
 #endif /* _LINUX_PID_NS_H */
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