mnt: Add filesystem private data to mount points

This starts to add private data associated directly
to mount points. The intent is to give filesystems
a sense of where they have come from, as a means of
letting a filesystem take different actions based on
this information.

Change-Id: Ie769d7b3bb2f5972afe05c1bf16cf88c91647ab2
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>

fs/namespace.c

@@ -482,6 +482,7 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 
 static void free_vfsmnt(struct mount *mnt)
 {
+	kfree(mnt->mnt.data);
 	kfree(mnt->mnt_devname);
 	mnt_free_id(mnt);
 #ifdef CONFIG_SMP
@@ -698,11 +699,21 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
 	if (!mnt)
 		return ERR_PTR(-ENOMEM);
 
+	mnt->mnt.data = NULL;
+	if (type->alloc_mnt_data) {
+		mnt->mnt.data = type->alloc_mnt_data();
+		if (!mnt->mnt.data) {
+			mnt_free_id(mnt);
+			free_vfsmnt(mnt);
+			return ERR_PTR(-ENOMEM);
+		}
+	}
 	if (flags & MS_KERNMOUNT)
 		mnt->mnt.mnt_flags = MNT_INTERNAL;
 
 	root = mount_fs(type, flags, name, data);
 	if (IS_ERR(root)) {
+		kfree(mnt->mnt.data);
 		free_vfsmnt(mnt);
 		return ERR_CAST(root);
 	}
@@ -729,6 +740,13 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	if (!mnt)
 		return ERR_PTR(-ENOMEM);
 
+	if (sb->s_op->clone_mnt_data) {
+		mnt->mnt.data = sb->s_op->clone_mnt_data(old->mnt.data);
+		if (!mnt->mnt.data) {
+			goto out_free;
+		}
+	}
+
 	if (flag & (CL_SLAVE | CL_PRIVATE))
 		mnt->mnt_group_id = 0; /* not a peer of original */
 	else
@@ -774,6 +792,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	return mnt;
 
  out_free:
+	kfree(mnt->mnt.data);
 	free_vfsmnt(mnt);
 	return ERR_PTR(err);
 }
@@ -1711,8 +1730,12 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 	down_write(&sb->s_umount);
 	if (flags & MS_BIND)
 		err = change_mount_flags(path->mnt, flags);
-	else
+	else {
 		err = do_remount_sb(sb, flags, data, 0);
+		br_write_lock(&vfsmount_lock);
+		propagate_remount(mnt);
+		br_write_unlock(&vfsmount_lock);
+	}
 	if (!err) {
 		br_write_lock(&vfsmount_lock);
 		mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;

fs/pnode.c

@@ -356,3 +356,17 @@ int propagate_umount(struct list_head *list)
 		__propagate_umount(mnt);
 	return 0;
 }
+
+int propagate_remount(struct mount *mnt) {
+	struct mount *m;
+	struct super_block *sb = mnt->mnt.mnt_sb;
+	int ret = 0;
+
+	if (sb->s_op->copy_mnt_data) {
+		for (m = first_slave(mnt); m->mnt_slave.next != &mnt->mnt_slave_list; m = next_slave(m)) {
+			sb->s_op->copy_mnt_data(m->mnt.data, mnt->mnt.data);
+		}
+	}
+
+	return ret;
+}

fs/pnode.h

@@ -35,6 +35,7 @@ int propagate_mnt(struct mount *, struct dentry *, struct mount *,
 		struct list_head *);
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct mount *, int);
+int propagate_remount(struct mount *);
 void mnt_release_group_id(struct mount *);
 int get_dominating_id(struct mount *mnt, const struct path *root);
 unsigned int mnt_get_count(struct mount *mnt);

include/linux/fs.h

@@ -1725,6 +1725,8 @@ struct super_operations {
 	int (*unfreeze_fs) (struct super_block *);
 	int (*statfs) (struct dentry *, struct kstatfs *);
 	int (*remount_fs) (struct super_block *, int *, char *);
+	void *(*clone_mnt_data) (void *);
+	void (*copy_mnt_data) (void *, void *);
 	void (*umount_begin) (struct super_block *);
 
 	int (*show_options)(struct seq_file *, struct dentry *);
@@ -1881,6 +1883,7 @@ struct file_system_type {
 #define FS_RENAME_DOES_D_MOVE  32768   /* FS will handle d_move() during rename() internally. */
 	struct dentry *(*mount) (struct file_system_type *, int,
 		       const char *, void *);
+	void *(*alloc_mnt_data) (void);
 	void (*kill_sb) (struct super_block *);
 	struct module *owner;
 	struct file_system_type * next;

include/linux/mount.h

@@ -53,6 +53,7 @@ struct vfsmount {
 	struct dentry *mnt_root;	/* root of the mounted tree */
 	struct super_block *mnt_sb;	/* pointer to superblock */
 	int mnt_flags;
+	void *data;
 };
 
 struct file; /* forward dec */