Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]

[David Howells]

Okay, this appears to fix the cycle-creation problem.

It could probably be improved by comparing sequence numbers as Alan suggests,
but I need to work out how to get at that.

David
---
commit 069c3376f7849044117c866aeafbb1a525f84926
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Thu Oct 4 23:18:59 2018 +0100

    fixes

diff --git a/fs/internal.h b/fs/internal.h
index 17029b30e196..47a6c80c3c51 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -172,6 +172,7 @@ extern void mnt_pin_kill(struct mount *m);
  * fs/nsfs.c
  */
 extern const struct dentry_operations ns_dentry_operations;
+extern struct file_system_type nsfs;
 
 /*
  * fs/ioctl.c
diff --git a/fs/namespace.c b/fs/namespace.c
index e969ded7d54b..25ecd8b3c76b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2388,6 +2388,27 @@ static inline int tree_contains_unbindable(struct mount *mnt)
 	return 0;
 }
 
+/*
+ * Object if there are any nsfs mounts in the specified subtree.  These can act
+ * as pins for mount namespaces that aren't checked by the mount-cycle checking
+ * code, thereby allowing cycles to be made.
+ */
+static bool check_for_nsfs_mounts(struct mount *subtree)
+{
+	struct mount *p;
+	bool ret = false;
+
+	lock_mount_hash();
+	for (p = subtree; p; p = next_mnt(p, subtree))
+		if (p->mnt.mnt_sb->s_type == &nsfs)
+			goto out;
+
+	ret = true;
+out:
+	unlock_mount_hash();
+	return ret;
+}
+
 static int do_move_mount(struct path *old_path, struct path *new_path)
 {
 	struct path parent_path = {.mnt = NULL, .dentry = NULL};
@@ -2442,6 +2463,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
 		goto out1;
 	err = -ELOOP;
+	if (!check_for_nsfs_mounts(old))
+		goto out1;
 	for (; mnt_has_parent(p); p = p->mnt_parent)
 		if (p == old)
 			goto out1;
diff --git a/fs/nsfs.c b/fs/nsfs.c
index f069eb6495b0..d3abcd5c2a23 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -269,7 +269,7 @@ static struct dentry *nsfs_mount(struct file_system_type *fs_type,
 	return mount_pseudo(fs_type, "nsfs:", &nsfs_ops,
 			&ns_dentry_operations, NSFS_MAGIC);
 }
-static struct file_system_type nsfs = {
+struct file_system_type nsfs = {
 	.name = "nsfs",
 	.mount = nsfs_mount,
 	.kill_sb = kill_anon_super,