This helper walks an input path to its parent. Logic are added to handle
walking across mount tree.
This will be used by landlock, and BPF LSM.
Suggested-by: Neil Brown <neil@xxxxxxxxxx>
Signed-off-by: Song Liu <song@xxxxxxxxxx>
---
fs/namei.c | 99 +++++++++++++++++++++++++++++++++++++------
include/linux/namei.h | 2 +
2 files changed, 87 insertions(+), 14 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 4bb889fc980b..bc65361c5d13 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2048,36 +2048,107 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd)
return nd->path.dentry;
}
-static struct dentry *follow_dotdot(struct nameidata *nd)
+/**
+ * __path_walk_parent - Find the parent of the given struct path
+ * @path - The struct path to start from
+ * @root - A struct path which serves as a boundary not to be crosses.
+ * - If @root is zero'ed, walk all the way to global root.
+ * @flags - Some LOOKUP_ flags.
+ *
+ * Find and return the dentry for the parent of the given path
+ * (mount/dentry). If the given path is the root of a mounted tree, it
+ * is first updated to the mount point on which that tree is mounted.
+ *
+ * If %LOOKUP_NO_XDEV is given, then *after* the path is updated to a new
+ * mount, the error EXDEV is returned.
+ *
+ * If no parent can be found, either because the tree is not mounted or
+ * because the @path matches the @root, then @path->dentry is returned
+ * unless @flags contains %LOOKUP_BENEATH, in which case -EXDEV is returned.
+ *
+ * Returns: either an ERR_PTR() or the chosen parent which will have had
+ * the refcount incremented.
+ */
+static struct dentry *__path_walk_parent(struct path *path, const struct path *root, int flags)
{
struct dentry *parent;
- if (path_equal(&nd->path, &nd->root))
+ if (path_equal(path, root))
goto in_root;
- if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
- struct path path;
+ if (unlikely(path->dentry == path->mnt->mnt_root)) {
+ struct path new_path;
- if (!choose_mountpoint(real_mount(nd->path.mnt),
- &nd->root, &path))
+ if (!choose_mountpoint(real_mount(path->mnt),
+ root, &new_path))
goto in_root;
- path_put(&nd->path);
- nd->path = path;
- nd->inode = path.dentry->d_inode;
- if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+ path_put(path);
+ *path = new_path;
+ if (unlikely(flags & LOOKUP_NO_XDEV))
return ERR_PTR(-EXDEV);
}
/* rare case of legitimate dget_parent()... */
- parent = dget_parent(nd->path.dentry);
- if (unlikely(!path_connected(nd->path.mnt, parent))) {
+ parent = dget_parent(path->dentry);
+ if (unlikely(!path_connected(path->mnt, parent))) {
dput(parent);
return ERR_PTR(-ENOENT);
}
return parent;
in_root:
- if (unlikely(nd->flags & LOOKUP_BENEATH))
+ if (unlikely(flags & LOOKUP_BENEATH))
return ERR_PTR(-EXDEV);
- return dget(nd->path.dentry);
+ return dget(path->dentry);
+}
+
+/**
+ * path_walk_parent - Walk to the parent of path
+ * @path: input and output path.
+ * @root: root of the path walk, do not go beyond this root. If @root is
+ * zero'ed, walk all the way to real root.
+ *
+ * Given a path, find the parent path. Replace @path with the parent path.
+ * If we were already at the real root or a disconnected root, @path is
+ * released and zero'ed.
+ *
+ * Returns:
+ * true - if @path is updated to its parent.
+ * false - if @path is already the root (real root or @root).
+ */
+bool path_walk_parent(struct path *path, const struct path *root)
+{
+ struct dentry *parent;
+
+ parent = __path_walk_parent(path, root, LOOKUP_BENEATH);
+
+ if (IS_ERR(parent))
+ goto false_out;
+
+ if (parent == path->dentry) {
+ dput(parent);
+ goto false_out;
+ }
+ dput(path->dentry);
+ path->dentry = parent;
+ return true;
+
+false_out:
+ path_put(path);
+ memset(path, 0, sizeof(*path));
+ return false;
+}
+
+static struct dentry *follow_dotdot(struct nameidata *nd)
+{
+ struct dentry *parent = __path_walk_parent(&nd->path, &nd->root, nd->flags);
+
+ if (IS_ERR(parent))
+ return parent;
+ if (unlikely(!path_connected(nd->path.mnt, parent))) {
+ dput(parent);
+ return ERR_PTR(-ENOENT);
+ }
+ nd->inode = nd->path.dentry->d_inode;
+ return parent;
}
static const char *handle_dots(struct nameidata *nd, int type)
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 5d085428e471..cba5373ecf86 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -85,6 +85,8 @@ extern int follow_down_one(struct path *);
extern int follow_down(struct path *path, unsigned int flags);
extern int follow_up(struct path *);
+bool path_walk_parent(struct path *path, const struct path *root);
+
extern struct dentry *lock_rename(struct dentry *, struct dentry *);
extern struct dentry *lock_rename_child(struct dentry *, struct dentry *);
extern void unlock_rename(struct dentry *, struct dentry *);
--
2.47.1
