linux-kernel - [PATCH 11/13] vfs: Implement mount_super

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Date:	Mon,  4 Apr 2016 20:29:27 -0500
From:	"Eric W. Biederman" <ebiederm@...ssion.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>, security@...ntu.com,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 11/13] vfs: Implement mount_super_once

The devpts filesystem has a notion of a system or primary instance of
devpts.  To retain the notion of a primary system instance of devpts
the code needs a way to allow userspace to mount the internally
mounted instance of devpts when it is not currently mounted by
userspace.  The new helper mount_super_once allows that.

Ideally mount_super_once would ignore still referenced lazy unmounts,
but in testing I was not able to find an existing distribution that
cared.  Since no one actually cares I did not try and solve the
formidable challenges of adding a test to ignore still referenced lazy
mounts in a race free way.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/super.c         | 12 ++++++++++++
 include/linux/fs.h |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 74914b1bae70..98a569412036 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1101,6 +1101,18 @@ struct dentry *mount_single(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(mount_single);
 
+struct dentry *mount_super_once(struct super_block *sb, int flags, void *data)
+{
+	/* Allow mounting the specified superblock by userspace if there
+	 * are not any existing userspace mounts of it.
+	 */
+	if (atomic_cmpxchg(&sb->s_active, 1, 2) != 1)
+		return ERR_PTR(-EBUSY);
+	down_write(&sb->s_umount);
+	do_remount_sb(sb, flags, data, 0);
+	return dget(sb->s_root);
+}
+
 struct dentry *
 mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
 {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 02a980bfad5c..515f874a6907 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2002,6 +2002,8 @@ extern struct dentry *mount_single(struct file_system_type *fs_type,
 extern struct dentry *mount_nodev(struct file_system_type *fs_type,
 	int flags, void *data,
 	int (*fill_super)(struct super_block *, void *, int));
+extern struct dentry *mount_super_once(struct super_block *sb,
+	int flags, void *data);
 extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path);
 void generic_shutdown_super(struct super_block *sb);
 void kill_block_super(struct super_block *sb);
-- 
2.6.3