Add a helper to allow the few users that need it to open a file in
init's fs_struct from a kernel thread.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/open.c          | 25 +++++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 26 insertions(+)

diff --git a/fs/open.c b/fs/open.c
index 91f1139591ab..bc97d66b6348 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1342,6 +1342,31 @@ struct file *filp_open(const char *filename, int flags, umode_t mode)
 }
 EXPORT_SYMBOL(filp_open);
 
+/**
+ * filp_open_init - open file resolving paths against init's root
+ *
+ * @filename:	path to open
+ * @flags:	open flags as per the open(2) second argument
+ * @mode:	mode for the new file if O_CREAT is set, else ignored
+ *
+ * Same as filp_open() but path resolution is done relative to init's
+ * root (using pid1_fs) instead of current->fs. Intended for kernel
+ * threads that need to open files by absolute path after being rooted
+ * in nullfs.
+ */
+struct file *filp_open_init(const char *filename, int flags, umode_t mode)
+{
+	struct open_flags op;
+	struct open_how how = build_open_how(flags, mode);
+	int err = build_open_flags(&how, &op);
+	if (err)
+		return ERR_PTR(err);
+	op.lookup_flags |= LOOKUP_IN_INIT;
+	CLASS(filename_kernel, name)(filename);
+	return do_file_open(AT_FDCWD, name, &op);
+}
+EXPORT_SYMBOL(filp_open_init);
+
 struct file *file_open_root(const struct path *root,
 			    const char *filename, int flags, umode_t mode)
 {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8b3dd145b25e..bc0430e72c74 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2459,6 +2459,7 @@ int do_sys_open(int dfd, const char __user *filename, int flags,
 		umode_t mode);
 extern struct file *file_open_name(struct filename *, int, umode_t);
 extern struct file *filp_open(const char *, int, umode_t);
+extern struct file *filp_open_init(const char *, int, umode_t);
 extern struct file *file_open_root(const struct path *,
 				   const char *, int, umode_t);
 static inline struct file *file_open_root_mnt(struct vfsmount *mnt,

-- 
2.47.3