[PATCH 4.0 03/58] proc: Allow creating permanently empty directories that serve as mount points

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebiederm@xmission.com>

commit eb6d38d5427b3ad42f5268da0f1dd31bb0af1264 upstream.

Add a new function proc_create_mount_point that when used to creates a
directory that can not be added to.

Add a new function is_empty_pde to test if a function is a mount
point.

Update the code to use make_empty_dir_inode when reporting
a permanently empty directory to the vfs.

Update the code to not allow adding to permanently empty directories.

Update /proc/openprom and /proc/fs/nfsd to be permanently empty directories.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/proc/generic.c  |   23 +++++++++++++++++++++++
 fs/proc/inode.c    |    4 ++++
 fs/proc/internal.h |    6 ++++++
 fs/proc/root.c     |    4 ++--
 4 files changed, 35 insertions(+), 2 deletions(-)

--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -373,6 +373,10 @@ static struct proc_dir_entry *__proc_cre
 		WARN(1, "create '/proc/%s' by hand\n", qstr.name);
 		return NULL;
 	}
+	if (is_empty_pde(*parent)) {
+		WARN(1, "attempt to add to permanently empty directory");
+		return NULL;
+	}
 
 	ent = kzalloc(sizeof(struct proc_dir_entry) + qstr.len + 1, GFP_KERNEL);
 	if (!ent)
@@ -455,6 +459,25 @@ struct proc_dir_entry *proc_mkdir(const
 }
 EXPORT_SYMBOL(proc_mkdir);
 
+struct proc_dir_entry *proc_create_mount_point(const char *name)
+{
+	umode_t mode = S_IFDIR | S_IRUGO | S_IXUGO;
+	struct proc_dir_entry *ent, *parent = NULL;
+
+	ent = __proc_create(&parent, name, mode, 2);
+	if (ent) {
+		ent->data = NULL;
+		ent->proc_fops = NULL;
+		ent->proc_iops = NULL;
+		if (proc_register(parent, ent) < 0) {
+			kfree(ent);
+			parent->nlink--;
+			ent = NULL;
+		}
+	}
+	return ent;
+}
+
 struct proc_dir_entry *proc_create_data(const char *name, umode_t mode,
 					struct proc_dir_entry *parent,
 					const struct file_operations *proc_fops,
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -423,6 +423,10 @@ struct inode *proc_get_inode(struct supe
 		inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 		PROC_I(inode)->pde = de;
 
+		if (is_empty_pde(de)) {
+			make_empty_dir_inode(inode);
+			return inode;
+		}
 		if (de->mode) {
 			inode->i_mode = de->mode;
 			inode->i_uid = de->uid;
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -191,6 +191,12 @@ static inline struct proc_dir_entry *pde
 }
 extern void pde_put(struct proc_dir_entry *);
 
+static inline bool is_empty_pde(const struct proc_dir_entry *pde)
+{
+	return S_ISDIR(pde->mode) && !pde->proc_iops;
+}
+struct proc_dir_entry *proc_create_mount_point(const char *name);
+
 /*
  * inode.c
  */
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -182,10 +182,10 @@ void __init proc_root_init(void)
 #endif
 	proc_mkdir("fs", NULL);
 	proc_mkdir("driver", NULL);
-	proc_mkdir("fs/nfsd", NULL); /* somewhere for the nfsd filesystem to be mounted */
+	proc_create_mount_point("fs/nfsd"); /* somewhere for the nfsd filesystem to be mounted */
 #if defined(CONFIG_SUN_OPENPROMFS) || defined(CONFIG_SUN_OPENPROMFS_MODULE)
 	/* just give it a mountpoint */
-	proc_mkdir("openprom", NULL);
+	proc_create_mount_point("openprom");
 #endif
 	proc_tty_init();
 	proc_mkdir("bus", NULL);