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Subject[PATCH 5/8] kernfs: define kernfs_node_dentry
Date
From: Aditya Kali <adityakali@google.com>

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>
---
Changelog:
20151116 - Don't allow user namespaces to bind new subsystems
20151118 - postpone the FS_USERNS_MOUNT flag until the
last patch, until we can convince ourselves it
is safe.
20151207 - Switch to walking up the kernfs path from kn root.
20151208 - Split out the kernfs change
- Style changes
- Switch from pr_crit to WARN_ON
- Reorder arguments to kernfs_obtain_root
- rename kernfs_obtain_root to kernfs_node_dentry
---
fs/kernfs/mount.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/kernfs.h | 2 ++
2 files changed, 69 insertions(+)

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 8eaf417..7224296 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 +63,72 @@ 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);
+ dtmp = lookup_one_len(kntmp->name, dentry, strlen(kntmp->name));
+ dput(dentry);
+ if (IS_ERR(dtmp))
+ return dtmp;
+ knparent = kntmp;
+ dentry = dtmp;
+ } while (1);
+}
+
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 d025ebd..6eba888 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);
--
1.7.9.5


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