kernfs: define kernfs_node_dentry

Add a new kernfs api is added to lookup the dentry for a particular
kernfs path.

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 71 insertions, 0 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 8eaf417187f1..b67dbccdaf88 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -14,6 +14,7 @@
 #include <linux/magic.h>
 #include <linux/slab.h>
 #include <linux/pagemap.h>
+#include <linux/namei.h>
 
 #include "kernfs-internal.h"
 
@@ -62,6 +63,74 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
         return NULL;
 }
 
+/*
+ * find the next ancestor in the path down to @child, where @parent was the
+ * ancestor whose descendant we want to find.
+ *
+ * Say the path is /a/b/c/d.  @child is d, @parent is NULL.  We return the root
+ * node.  If @parent is b, then we return the node for c.
+ * Passing in d as @parent is not ok.
+ */
+static struct kernfs_node *find_next_ancestor(struct kernfs_node *child,
+                                              struct kernfs_node *parent)
+{
+        if (child == parent) {
+                pr_crit_once("BUG in find_next_ancestor: called with parent == child");
+                return NULL;
+        }
+
+        while (child->parent != parent) {
+                if (!child->parent)
+                        return NULL;
+                child = child->parent;
+        }
+
+        return child;
+}
+
+/**
+ * kernfs_node_dentry - get a dentry for the given kernfs_node
+ * @kn: kernfs_node for which a dentry is needed
+ * @sb: the kernfs super_block
+ */
+struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
+                                  struct super_block *sb)
+{
+        struct dentry *dentry;
+        struct kernfs_node *knparent = NULL;
+
+        BUG_ON(sb->s_op != &kernfs_sops);
+
+        dentry = dget(sb->s_root);
+
+        /* Check if this is the root kernfs_node */
+        if (!kn->parent)
+                return dentry;
+
+        knparent = find_next_ancestor(kn, NULL);
+        if (WARN_ON(!knparent))
+                return ERR_PTR(-EINVAL);
+
+        do {
+                struct dentry *dtmp;
+                struct kernfs_node *kntmp;
+
+                if (kn == knparent)
+                        return dentry;
+                kntmp = find_next_ancestor(kn, knparent);
+                if (WARN_ON(!kntmp))
+                        return ERR_PTR(-EINVAL);
+                mutex_lock(&d_inode(dentry)->i_mutex);
+                dtmp = lookup_one_len(kntmp->name, dentry, strlen(kntmp->name));
+                mutex_unlock(&d_inode(dentry)->i_mutex);
+                dput(dentry);
+                if (IS_ERR(dtmp))
+                        return dtmp;
+                knparent = kntmp;
+                dentry = dtmp;
+        } while (true);
+}
+
 static int kernfs_fill_super(struct super_block *sb, unsigned long magic)
 {
         struct kernfs_super_info *info = kernfs_info(sb);
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 716bfdede5f5..c06c44242f39 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -284,6 +284,8 @@ struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry);
 struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
 struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn);
 
+struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
+                                  struct super_block *sb);
 struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
                                        unsigned int flags, void *priv);
 void kernfs_destroy_root(struct kernfs_root *root);