Subject: [PATCH] Rewrite proc_root_follow_link so it is transparent to the user
From: Eric W. Biederman <ebiederman@lnxi.com>
Date: 1129570923 -0600

By using a vfsmnt structure I completely make name of the intermediate
dentry disappear.  And allows me to use whatever name works best for
d_lookup and debugging.

---

 fs/proc/root.c |   60 ++++++++++++++++++++++++++++++++++----------------------
 1 files changed, 36 insertions(+), 24 deletions(-)

e8cbde5d8941a2352702980cb2a9cb11f2378f49
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 1e51a7c..f725980 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -83,43 +83,55 @@ void __init proc_root_init(void)
 	proc_bus = proc_mkdir("bus", NULL);
 }
 
-static void *proc_root_follow_link(struct vfsmount *mnt, struct dentry *dentry, struct nameidata *nd)
+static void *proc_root_follow_link(struct vfsmount *mnt, struct dentry *parent, struct nameidata *nd)
 {
-	/* Follow the pseudo link to the per pspace root of the /proc filesystem */
-	struct inode *root = dentry->d_inode;
+	/* Follow the pseudo link to the per pspace root of the /proc
+	 * filesystem 
+	 */
+	struct inode *root = parent->d_inode;
+	struct dentry *dentry;
 	struct qstr str;
 
-	/* Install the mount and drop the dentry */
-	mntget(mnt);
-	dput(nd->dentry);
-	mntput(nd->mnt);
-	nd->mnt = mnt;
-
-	/* Use a name of length 0 so it is not visible to user space.
-	 * This relies on all of the information needed to be mached
-	 * being placed in str.hash.  
+	/* Each pspace root is stored in the main /proc directory
+	 * it's name is the pointer to it's pspace.
 	 *
-	 * FIXME:  We are safe with 32bit pointers there is a remote
-	 * possibility of a hash collision with 64bit pointers.
+	 * To keep user space from detecting the redirection an
+	 * additional vfsmount is added, which allows me to skip 
+	 * the dentry name when building the d_path name.
 	 */
-	str.name = "\0";
-	str.len = 0;
-	str.hash = ((unsigned long)(current->pspace))/sizeof(struct pspace);
+	str.name = (const unsigned char *)&current->pspace;
+	str.len  = sizeof(current->pspace);
+	str.hash = full_name_hash(str.name, str.len);
 
 	/* See if I already have the pspace dentry and if not create it */
 	down(&root->i_sem);
-	nd->dentry = d_lookup(dentry, &str);
-	if (!nd->dentry) {
-		nd->dentry = d_alloc(dentry, &str);
-		if (nd->dentry) {
-			nd->dentry->d_fsdata = current->pspace;
-			d_add(nd->dentry, root);
+	dentry = d_lookup(parent, &str);
+	if (!dentry) {
+		dentry = d_alloc(parent, &str);
+		if (dentry) {
+			dentry->d_fsdata = current->pspace;
+			d_add(dentry, root);
 			__iget(root);
 		} else {
-			nd->dentry = ERR_PTR(-ENOMEM);
+			dentry = ERR_PTR(-ENOMEM);
 		}
 	}
 	up(&root->i_sem);
+	if (!IS_ERR(dentry)) {
+		struct vfsmount *cover;
+		cover = alloc_vfsmnt(NULL);
+		cover->mnt_sb         = dentry->d_sb;
+		cover->mnt_root       = dget(dentry);
+		cover->mnt_mountpoint = parent;
+		cover->mnt_parent     = mnt;
+		mntget(mnt);
+		mntput(nd->mnt);
+		nd->mnt = cover;
+		atomic_inc(&dentry->d_sb->s_active);
+	}
+	/* Forget nd->dentry */
+	dput(nd->dentry);
+	nd->dentry = dentry;
 
 	return NULL;
 }
-- 
1.0.GIT