[RFC PATCH 8/9] namespace: add sb_revalidate_bindmounts helper

[Alexander Mikhalitsyn]

Useful if for some reason bindmounts root dentries get invalidated
but it's needed to revalidate existing bindmounts without remounting.

Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: Stéphane Graber <stgraber@xxxxxxxxxx>
Cc: Seth Forshee <sforshee@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Andrei Vagin <avagin@xxxxxxxxx>
Cc: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: criu@xxxxxxxxxx
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
---
 fs/namespace.c                | 90 +++++++++++++++++++++++++++++++++++
 include/linux/mnt_namespace.h |  3 ++
 2 files changed, 93 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index ab467ee58341..88491f9c8bbd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -682,6 +682,96 @@ static int mnt_make_readonly(struct mount *mnt)
 	return ret;
 }
 
+struct bind_mount_list_item {
+	struct list_head list;
+	struct vfsmount *mnt;
+};
+
+/*
+ * sb_revalidate_bindmounts - Relookup/reset bindmounts root dentries
+ *
+ * Useful if for some reason bindmount root dentries get invalidated
+ * but it's needed to revalidate existing bindmounts without remounting.
+ */
+int sb_revalidate_bindmounts(struct super_block *sb)
+{
+	struct mount *mnt;
+	struct bind_mount_list_item *bmnt, *next;
+	int err = 0;
+	struct vfsmount *root_mnt = NULL;
+	LIST_HEAD(mnt_to_update);
+	char *buf;
+
+	buf = (char *) __get_free_page(GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	lock_mount_hash();
+	list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+		/* we only want to touch bindmounts */
+		if (mnt->mnt.mnt_root == sb->s_root) {
+			if (!root_mnt)
+				root_mnt = mntget(&mnt->mnt);
+
+			continue;
+		}
+
+		bmnt = kzalloc(sizeof(struct bind_mount_list_item), GFP_NOWAIT | __GFP_NOWARN);
+		if (!bmnt) {
+			err = -ENOMEM;
+			goto exit;
+		}
+
+		bmnt->mnt = mntget(&mnt->mnt);
+		list_add_tail(&bmnt->list, &mnt_to_update);
+	}
+	unlock_mount_hash();
+
+	/* TODO: get rid of this limitation */
+	if (!root_mnt) {
+		err = -ENOENT;
+		goto exit;
+	}
+
+	list_for_each_entry_safe(bmnt, next, &mnt_to_update, list) {
+		struct vfsmount *cur_mnt = bmnt->mnt;
+		struct path path;
+		struct dentry *old_root;
+		char *p;
+
+		p = dentry_path(cur_mnt->mnt_root, buf, PAGE_SIZE);
+		if (IS_ERR(p))
+			goto exit;
+
+		/* TODO: are these lookup flags fully safe and correct? */
+		err = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
+				p, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT|LOOKUP_REVAL, &path);
+		if (err)
+			goto exit;
+
+		/* replace bindmount root dentry */
+		lock_mount_hash();
+		old_root = cur_mnt->mnt_root;
+		cur_mnt->mnt_root = dget(path.dentry);
+		dput(old_root);
+		unlock_mount_hash();
+
+		path_put(&path);
+	}
+
+exit:
+	free_page((unsigned long) buf);
+	mntput(root_mnt);
+	list_for_each_entry_safe(bmnt, next, &mnt_to_update, list) {
+		list_del(&bmnt->list);
+		mntput(bmnt->mnt);
+		kfree(bmnt);
+	}
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(sb_revalidate_bindmounts);
+
 int sb_prepare_remount_readonly(struct super_block *sb)
 {
 	struct mount *mnt;
diff --git a/include/linux/mnt_namespace.h b/include/linux/mnt_namespace.h
index 8f882f5881e8..20ac29e702f5 100644
--- a/include/linux/mnt_namespace.h
+++ b/include/linux/mnt_namespace.h
@@ -3,6 +3,7 @@
 #define _NAMESPACE_H_
 #ifdef __KERNEL__
 
+struct super_block;
 struct mnt_namespace;
 struct fs_struct;
 struct user_namespace;
@@ -13,6 +14,8 @@ extern struct mnt_namespace *copy_mnt_ns(unsigned long, struct mnt_namespace *,
 extern void put_mnt_ns(struct mnt_namespace *ns);
 extern struct ns_common *from_mnt_ns(struct mnt_namespace *);
 
+extern int sb_revalidate_bindmounts(struct super_block *sb);
+
 extern const struct file_operations proc_mounts_operations;
 extern const struct file_operations proc_mountinfo_operations;
 extern const struct file_operations proc_mountstats_operations;
-- 
2.34.1