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>

1 files changed, 23 insertions, 0 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,