pidns: Make the pidns proc mount/umount logic obvious.

Track the number of pids in the proc hash table.  When the number of
pids goes to 0 schedule work to unmount the kernel mount of proc.

Move the mount of proc into alloc_pid when we allocate the pid for
init.

Remove the surprising calls of pid_ns_release proc in fork and
proc_flush_task.  Those code paths really shouldn't know about proc
namespace implementation details and people have demonstrated several
times that finding and understanding those code paths is difficult and
non-obvious.

Because of the call path detach pid is alwasy called with the
rtnl_lock held free_pid is not allowed to sleep, so the work to
unmounting proc is moved to a work queue.  This has the side benefit
of not blocking the entire world waiting for the unnecessary
rcu_barrier in deactivate_locked_super.

In the process of making the code clear and obvious this fixes a bug
reported by Gao feng <gaofeng@cn.fujitsu.com> where we would leak a
mount of proc during clone(CLONE_NEWPID|CLONE_NEWNET) if copy_pid_ns
succeeded and copy_net_ns failed.

Acked-by: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 17 insertions, 4 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index 3a5f238c1ca0..e957f8b09136 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -36,6 +36,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/syscalls.h>
+#include <linux/proc_fs.h>
 
 #define pid_hashfn(nr, ns)      \
         hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
@@ -270,8 +271,12 @@ void free_pid(struct pid *pid)
         unsigned long flags;
 
         spin_lock_irqsave(&pidmap_lock, flags);
-        for (i = 0; i <= pid->level; i++)
-                hlist_del_rcu(&pid->numbers[i].pid_chain);
+        for (i = 0; i <= pid->level; i++) {
+                struct upid *upid = pid->numbers + i;
+                hlist_del_rcu(&upid->pid_chain);
+                if (--upid->ns->nr_hashed == 0)
+                        schedule_work(&upid->ns->proc_work);
+        }
         spin_unlock_irqrestore(&pidmap_lock, flags);
 
         for (i = 0; i <= pid->level; i++)
@@ -293,6 +298,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
                 goto out;
 
         tmp = ns;
+        pid->level = ns->level;
         for (i = ns->level; i >= 0; i--) {
                 nr = alloc_pidmap(tmp);
                 if (nr < 0)
@@ -303,17 +309,23 @@ struct pid *alloc_pid(struct pid_namespace *ns)
                 tmp = tmp->parent;
         }
 
+        if (unlikely(is_child_reaper(pid))) {
+                if (pid_ns_prepare_proc(ns))
+                        goto out_free;
+        }
+
         get_pid_ns(ns);
-        pid->level = ns->level;
         atomic_set(&pid->count, 1);
         for (type = 0; type < PIDTYPE_MAX; ++type)
                 INIT_HLIST_HEAD(&pid->tasks[type]);
 
         upid = pid->numbers + ns->level;
         spin_lock_irq(&pidmap_lock);
-        for ( ; upid >= pid->numbers; --upid)
+        for ( ; upid >= pid->numbers; --upid) {
                 hlist_add_head_rcu(&upid->pid_chain,
                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
+                upid->ns->nr_hashed++;
+        }
         spin_unlock_irq(&pidmap_lock);
 
 out:
@@ -570,6 +582,7 @@ void __init pidmap_init(void)
         /* Reserve PID 0. We never call free_pidmap(0) */
         set_bit(0, init_pid_ns.pidmap[0].page);
         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
+        init_pid_ns.nr_hashed = 1;
 
         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);