fs: add filp_clone_open API

I need an API that allows me to obtain a clone of the current file
pointer to pass in to an exec handler.  I've labelled this as an
internal API because I can't see how it would be useful outside of the
fs subsystem.  The use case will be a persistent binfmt_misc handler.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Jan Kara <jack@suse.cz>

fs/internal.h

@@ -108,6 +108,7 @@ extern long do_handle_open(int mountdirfd,
 			   struct file_handle __user *ufh, int open_flag);
 extern int open_check_o_direct(struct file *f);
 extern int vfs_open(const struct path *, struct file *, const struct cred *);
+extern struct file *filp_clone_open(struct file *);
 
 /*
  * inode.c

fs/open.c

@@ -1002,6 +1002,26 @@ struct file *file_open_root(struct dentry *dentry, struct vfsmount *mnt,
 }
 EXPORT_SYMBOL(file_open_root);
 
+struct file *filp_clone_open(struct file *oldfile)
+{
+	struct file *file;
+	int retval;
+
+	file = get_empty_filp();
+	if (IS_ERR(file))
+		return file;
+
+	file->f_flags = oldfile->f_flags;
+	retval = vfs_open(&oldfile->f_path, file, oldfile->f_cred);
+	if (retval) {
+		put_filp(file);
+		return ERR_PTR(retval);
+	}
+
+	return file;
+}
+EXPORT_SYMBOL(filp_clone_open);
+
 long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
 {
 	struct open_flags op;