fs: add init_pivot_root()

We will soon be able to pivot_root() with the introduction of the
immutable rootfs. Add a wrapper for kernel internal usage.

Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/init.c

···
 #include <linux/security.h>
 #include "internal.h"
 
+int __init init_pivot_root(const char *new_root, const char *put_old)
+{
+	struct path new_path __free(path_put) = {};
+	struct path old_path __free(path_put) = {};
+	int ret;
+
+	ret = kern_path(new_root, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &new_path);
+	if (ret)
+		return ret;
+
+	ret = kern_path(put_old, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &old_path);
+	if (ret)
+		return ret;
+
+	return path_pivot_root(&new_path, &old_path);
+}
+
 int __init init_mount(const char *dev_name, const char *dir_name,
 		const char *type_page, unsigned long flags, void *data_page)
 {

fs/internal.h

···
 int path_mount(const char *dev_name, const struct path *path,
 		const char *type_page, unsigned long flags, void *data_page);
 int path_umount(const struct path *path, int flags);
+int path_pivot_root(struct path *new, struct path *old);
 
 int show_path(struct seq_file *m, struct dentry *root);
 

fs/namespace.c

···
 }
 EXPORT_SYMBOL(path_is_under);
 
+int path_pivot_root(struct path *new, struct path *old)
+{
+	struct path root __free(path_put) = {};
+	struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent, *ex_parent;
+	int error;
+
+	if (!may_mount())
+		return -EPERM;
+
+	error = security_sb_pivotroot(old, new);
+	if (error)
+		return error;
+
+	get_fs_root(current->fs, &root);
+
+	LOCK_MOUNT(old_mp, old);
+	old_mnt = old_mp.parent;
+	if (IS_ERR(old_mnt))
+		return PTR_ERR(old_mnt);
+
+	new_mnt = real_mount(new->mnt);
+	root_mnt = real_mount(root.mnt);
+	ex_parent = new_mnt->mnt_parent;
+	root_parent = root_mnt->mnt_parent;
+	if (IS_MNT_SHARED(old_mnt) ||
+		IS_MNT_SHARED(ex_parent) ||
+		IS_MNT_SHARED(root_parent))
+		return -EINVAL;
+	if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
+		return -EINVAL;
+	if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
+		return -EINVAL;
+	if (d_unlinked(new->dentry))
+		return -ENOENT;
+	if (new_mnt == root_mnt || old_mnt == root_mnt)
+		return -EBUSY; /* loop, on the same file system  */
+	if (!path_mounted(&root))
+		return -EINVAL; /* not a mountpoint */
+	if (!mnt_has_parent(root_mnt))
+		return -EINVAL; /* absolute root */
+	if (!path_mounted(new))
+		return -EINVAL; /* not a mountpoint */
+	if (!mnt_has_parent(new_mnt))
+		return -EINVAL; /* absolute root */
+	/* make sure we can reach put_old from new_root */
+	if (!is_path_reachable(old_mnt, old_mp.mp->m_dentry, new))
+		return -EINVAL;
+	/* make certain new is below the root */
+	if (!is_path_reachable(new_mnt, new->dentry, &root))
+		return -EINVAL;
+	lock_mount_hash();
+	umount_mnt(new_mnt);
+	if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
+		new_mnt->mnt.mnt_flags |= MNT_LOCKED;
+		root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
+	}
+	/* mount new_root on / */
+	attach_mnt(new_mnt, root_parent, root_mnt->mnt_mp);
+	umount_mnt(root_mnt);
+	/* mount old root on put_old */
+	attach_mnt(root_mnt, old_mnt, old_mp.mp);
+	touch_mnt_namespace(current->nsproxy->mnt_ns);
+	/* A moved mount should not expire automatically */
+	list_del_init(&new_mnt->mnt_expire);
+	unlock_mount_hash();
+	mnt_notify_add(root_mnt);
+	mnt_notify_add(new_mnt);
+	chroot_fs_refs(&root, new);
+	return 0;
+}
+
 /*
  * pivot_root Semantics:
  * Moves the root file system of the current process to the directory put_old,
···
 {
 	struct path new __free(path_put) = {};
 	struct path old __free(path_put) = {};
-	struct path root __free(path_put) = {};
-	struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent, *ex_parent;
 	int error;
-
-	if (!may_mount())
-		return -EPERM;
 
 	error = user_path_at(AT_FDCWD, new_root,
 			     LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &new);
···
 	if (error)
 		return error;
 
-	error = security_sb_pivotroot(&old, &new);
-	if (error)
-		return error;
-
-	get_fs_root(current->fs, &root);
-
-	LOCK_MOUNT(old_mp, &old);
-	old_mnt = old_mp.parent;
-	if (IS_ERR(old_mnt))
-		return PTR_ERR(old_mnt);
-
-	new_mnt = real_mount(new.mnt);
-	root_mnt = real_mount(root.mnt);
-	ex_parent = new_mnt->mnt_parent;
-	root_parent = root_mnt->mnt_parent;
-	if (IS_MNT_SHARED(old_mnt) ||
-		IS_MNT_SHARED(ex_parent) ||
-		IS_MNT_SHARED(root_parent))
-		return -EINVAL;
-	if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
-		return -EINVAL;
-	if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
-		return -EINVAL;
-	if (d_unlinked(new.dentry))
-		return -ENOENT;
-	if (new_mnt == root_mnt || old_mnt == root_mnt)
-		return -EBUSY; /* loop, on the same file system  */
-	if (!path_mounted(&root))
-		return -EINVAL; /* not a mountpoint */
-	if (!mnt_has_parent(root_mnt))
-		return -EINVAL; /* absolute root */
-	if (!path_mounted(&new))
-		return -EINVAL; /* not a mountpoint */
-	if (!mnt_has_parent(new_mnt))
-		return -EINVAL; /* absolute root */
-	/* make sure we can reach put_old from new_root */
-	if (!is_path_reachable(old_mnt, old_mp.mp->m_dentry, &new))
-		return -EINVAL;
-	/* make certain new is below the root */
-	if (!is_path_reachable(new_mnt, new.dentry, &root))
-		return -EINVAL;
-	lock_mount_hash();
-	umount_mnt(new_mnt);
-	if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
-		new_mnt->mnt.mnt_flags |= MNT_LOCKED;
-		root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
-	}
-	/* mount new_root on / */
-	attach_mnt(new_mnt, root_parent, root_mnt->mnt_mp);
-	umount_mnt(root_mnt);
-	/* mount old root on put_old */
-	attach_mnt(root_mnt, old_mnt, old_mp.mp);
-	touch_mnt_namespace(current->nsproxy->mnt_ns);
-	/* A moved mount should not expire automatically */
-	list_del_init(&new_mnt->mnt_expire);
-	unlock_mount_hash();
-	mnt_notify_add(root_mnt);
-	mnt_notify_add(new_mnt);
-	chroot_fs_refs(&root, &new);
-	return 0;
+	return path_pivot_root(&new, &old);
 }
 
 static unsigned int recalc_flags(struct mount_kattr *kattr, struct mount *mnt)

include/linux/init_syscalls.h

···
 int __init init_rmdir(const char *pathname);
 int __init init_utimes(char *filename, struct timespec64 *ts);
 int __init init_dup(struct file *file);
+int __init init_pivot_root(const char *new_root, const char *put_old);