proc: Allow creating permanently empty directories that serve as mount points

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.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

4 files changed, 35 insertions, 2 deletions

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index df6327a2b865..e5dee5c3188e 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -373,6 +373,10 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
                 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 char *name,
 }
 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,
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 8272aaba1bb0..e3eb5524639f 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -423,6 +423,10 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
                 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;
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index c835b94c0cd3..aa2781095bd1 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -191,6 +191,12 @@ static inline struct proc_dir_entry *pde_get(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
  */
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 64e1ab64bde6..68feb0f70e63 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -179,10 +179,10 @@ 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);