[PATCH 2/2] pid_ns: refactor the buggy pid_ns_release_proc() logic

pid_namespace holds ns->proc_mnt, while this vfsmount has a referene to
the namespace via PROC_I(sb->s_root->d_inode)->pid. To break this circle
/sbin/init does mntput() in pid_ns_release_proc(). See 6f4e6433.

But we have the following problems:

	- Nobody does mntput() if copy_process() fails after
	  pid_ns_prepare_proc().

	- proc_flush_task() checks upid->nr == 1 to verify we are init,
	  this is wrong if a multi-threaded init does exec.

	- As Louis pointed out, this namespace can have the detached
	  EXIT_DEAD tasks which can use ns->proc_mnt after this mntput().

With this patch only pid_namespace has a reference to ns->proc_mnt, and
mntput(ns->proc_mnt) is called by destroy_pid_namespace() paths when we
know that this ns must not have any references (in particular, there are
no pids in this namespace).

Changes:

	- kill proc_flush_task()->pid_ns_release_proc()

	- change fs/proc/root.c so that we don't create the "artificial"
	  references to the namespace or its pid==1.

	- change destroy_pid_namespace() to call pid_ns_release_proc().

	- change pid_ns_release_proc() to clear s_root->d_inode->pid and
	  sb->s_fs_info. The caller is destroy_pid_namespace(), both pid
	  and ns must not have any reference.

	- change proc_self_readlink() and proc_pid_lookup() to check
	  sb->s_fs_info != NULL to detect the case when the proc fs is
	  kept mounted after pid_ns_release_proc().

Reported-by:  Louis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 kernel/pid_namespace.c |    2 ++
 fs/proc/base.c         |   20 ++++++++++++--------
 fs/proc/root.c         |   11 +++++++----
 3 files changed, 21 insertions(+), 12 deletions(-)

--- 35-rc3/kernel/pid_namespace.c~PNS_6_BREAK_CIRCLE	2010-06-24 15:17:46.000000000 +0200
+++ 35-rc3/kernel/pid_namespace.c	2010-06-24 20:48:18.000000000 +0200
@@ -128,6 +128,8 @@ static void destroy_pid_namespace(struct
 {
 	int i;
 
+	pid_ns_release_proc(ns);
+
 	for (i = 0; i < PIDMAP_ENTRIES; i++)
 		kfree(ns->pidmap[i].page);
 	kmem_cache_free(pid_ns_cachep, ns);
--- 35-rc3/fs/proc/base.c~PNS_6_BREAK_CIRCLE	2010-06-24 15:16:21.000000000 +0200
+++ 35-rc3/fs/proc/base.c	2010-06-24 20:48:18.000000000 +0200
@@ -2349,11 +2349,17 @@ static const struct file_operations proc
 /*
  * /proc/self:
  */
+
+static inline pid_t self_tgid(struct dentry *dentry)
+{
+	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
+	return ns ? task_tgid_nr_ns(current, ns) : 0;
+}
+
 static int proc_self_readlink(struct dentry *dentry, char __user *buffer,
 			      int buflen)
 {
-	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
-	pid_t tgid = task_tgid_nr_ns(current, ns);
+	pid_t tgid = self_tgid(dentry);
 	char tmp[PROC_NUMBUF];
 	if (!tgid)
 		return -ENOENT;
@@ -2363,8 +2369,7 @@ static int proc_self_readlink(struct den
 
 static void *proc_self_follow_link(struct dentry *dentry, struct nameidata *nd)
 {
-	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
-	pid_t tgid = task_tgid_nr_ns(current, ns);
+	pid_t tgid = self_tgid(dentry);
 	char *name = ERR_PTR(-ENOENT);
 	if (tgid) {
 		name = __getname();
@@ -2745,10 +2750,6 @@ void proc_flush_task(struct task_struct 
 		proc_flush_task_mnt(upid->ns->proc_mnt, upid->nr,
 					tgid->numbers[i].nr);
 	}
-
-	upid = &pid->numbers[pid->level];
-	if (upid->nr == 1)
-		pid_ns_release_proc(upid->ns);
 }
 
 static struct dentry *proc_pid_instantiate(struct inode *dir,
@@ -2796,6 +2797,9 @@ struct dentry *proc_pid_lookup(struct in
 		goto out;
 
 	ns = dentry->d_sb->s_fs_info;
+	if (!ns)
+		goto out;
+
 	rcu_read_lock();
 	task = find_task_by_pid_ns(tgid, ns);
 	if (task)
--- 35-rc3/fs/proc/root.c~PNS_6_BREAK_CIRCLE	2010-06-23 22:06:01.000000000 +0200
+++ 35-rc3/fs/proc/root.c	2010-06-24 20:48:18.000000000 +0200
@@ -31,7 +31,7 @@ static int proc_set_super(struct super_b
 	struct pid_namespace *ns;
 
 	ns = (struct pid_namespace *)data;
-	sb->s_fs_info = get_pid_ns(ns);
+	sb->s_fs_info = ns;
 	return set_anon_super(sb, NULL);
 }
 
@@ -68,7 +68,7 @@ static int proc_get_sb(struct file_syste
 		struct proc_inode *ei = PROC_I(sb->s_root->d_inode);
 		if (!ei->pid) {
 			rcu_read_lock();
-			ei->pid = get_pid(find_pid_ns(1, ns));
+			ei->pid = find_pid_ns(1, ns);
 			rcu_read_unlock();
 		}
 	}
@@ -83,7 +83,6 @@ static void proc_kill_sb(struct super_bl
 
 	ns = (struct pid_namespace *)sb->s_fs_info;
 	kill_anon_super(sb);
-	put_pid_ns(ns);
 }
 
 static struct file_system_type proc_fs_type = {
@@ -209,5 +208,9 @@ int pid_ns_prepare_proc(struct pid_names
 
 void pid_ns_release_proc(struct pid_namespace *ns)
 {
-	mntput(ns->proc_mnt);
+	if (ns->proc_mnt) {
+		ns->proc_mnt->mnt_sb->s_fs_info = NULL;
+		PROC_I(ns->proc_mnt->mnt_sb->s_root->d_inode)->pid = NULL;
+		mntput(ns->proc_mnt);
+	}
 }