[PATCH 15/16] devpts: Always return a distinct instance when mounting

[Eric W. Biederman]

When devpts is mounted and the newinstance flag is not passed the code
first checks to see if the system devpts instance has been exported to
userspace.  If it has not the system devpts instance is returned
otherwise a fresh instance of devpts is allocated and returned.

If newinstance is passed a fresh devpts instance is always returned.

Combined with the earlier work to cause mounts of devpts to fail
if devpts is mounted over itself, this ensures that the system devpts
is mounted on /dev/pts on all of the systems I have tested.

This has been verified to work properly on openwrt-15.05, centos5,
centos6, centos7, debian-6.0.2, debian-7.9, debian-8.2, ubuntu-14.04.3,
ubuntu-15.10, fedora23, magia-5, mint-17.3, opensuse-42.1, slackware-14.1,
gentoo-20151225 (13.0?), archlinux-2015-12-01

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/devpts/inode.c | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index c1c346190f35..e5e533fe824a 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -430,13 +430,6 @@ fail:
 }
 
 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
-static int compare_init_pts_sb(struct super_block *s, void *p)
-{
-	if (devpts_mnt)
-		return devpts_mnt->mnt_sb == s;
-	return 0;
-}
-
 /*
  * devpts_mount()
  *
@@ -467,28 +460,26 @@ static int compare_init_pts_sb(struct super_block *s, void *p)
 static struct dentry *devpts_mount(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data)
 {
-	struct super_block *s;
+	struct dentry *root;
 	bool newinstance;
 
 	newinstance = parse_newinstance(data);
+	if (flags & MS_KERNMOUNT)
+		newinstance = true;
 
-	/* Require newinstance for all user namespace mounts to ensure
+	/* Force newinstance for all user namespace mounts to ensure
 	 * the mount options are not changed.
 	 */
-	if ((current_user_ns() != &init_user_ns) && !newinstance)
-		return ERR_PTR(-EINVAL);
-
-	if (newinstance)
-		return mount_nodev(fs_type, flags, data, devpts_fill_super);
-
-	s = sget(fs_type, compare_init_pts_sb, set_anon_super, flags, NULL);
-	if (IS_ERR(s))
-		return ERR_CAST(s);
+	if (current_user_ns() != &init_user_ns)
+		newinstance = true;
 
-	/* Match mount_single ignore errors on remount */
-	devpts_remount(s, &flags, data);
+	root = NULL;
+	if (!newinstance)
+		root = mount_super_once(devpts_mnt->mnt_sb, flags, data);
+	if (IS_ERR_OR_NULL(root))
+		root = mount_nodev(fs_type, flags, data, devpts_fill_super);
 
-	return dget(s->s_root);
+	return root;
 }
 
 #else
-- 
2.8.1