linux-kernel - [RFC][PATCH 2/2] pidns: Remove proc flush races when a pid namespaces are exiting.

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Message-ID: <m1mxu15kv4.fsf_-_@fess.ebiederm.org>
Date:	Thu, 08 Jul 2010 21:39:59 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: [RFC][PATCH 2/2] pidns: Remove proc flush races when a pid namespaces are exiting.


Currently it is possible to put proc_mnt before we have flushed the
last process that will use the proc_mnt to flush it's proc entries.

This race is fixed by not flushing proc entries for dead pid
namespaces, and calling pid_ns_release_proc unconditionally from
zap_pid_ns_processes after the pid namespace has been declared dead.

To ensure we don't unnecessarily leak any dcache entries with skipped
flushes pid_ns_release_proc flushes the entire proc_mnt when it is
called.

Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
---
 fs/proc/base.c         |    9 +++++----
 fs/proc/root.c         |    3 +++
 kernel/pid_namespace.c |    1 +
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index acb7ef8..e9d84e1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2742,13 +2742,14 @@ void proc_flush_task(struct task_struct *task)
 
 	for (i = 0; i <= pid->level; i++) {
 		upid = &pid->numbers[i];
+
+		/* Don't bother flushing dead pid namespaces */
+		if (test_bit(PIDNS_DEAD, &upid->ns->flags))
+			continue;
+
 		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,
diff --git a/fs/proc/root.c b/fs/proc/root.c
index cfdf032..2298fdd 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -209,5 +209,8 @@ int pid_ns_prepare_proc(struct pid_namespace *ns)
 
 void pid_ns_release_proc(struct pid_namespace *ns)
 {
+	/* Flush any cached proc dentries for this pid namespace */
+	shrink_dcache_parent(ns->proc_mnt->mnt_root);
+
 	mntput(ns->proc_mnt);
 }
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 92032d1..43dec5d 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -189,6 +189,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 		rc = sys_wait4(-1, NULL, __WALL, NULL);
 	} while (rc != -ECHILD);
 
+	pid_ns_release_proc(pid_ns);
 	acct_exit_ns(pid_ns);
 	return;
 }
-- 
1.6.5.2.143.g8cc62

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