ANDROID: mnt: Add filesystem private data to mount points (v2)

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.

(v2)
Changes due to commit:
68847c941700 ("namespace: take lock_mount_hash() directly when changing flags")

Bug: 62094374
Bug: 120446149
Bug: 122428178
Change-Id: Ie769d7b3bb2f5972afe05c1bf16cf88c91647ab2
Signed-off-by: Daniel Rosenberg <drosen@google.com>
[adelva: Folded 89a54ed3bf68 ("ANDROID: mnt: Fix next_descendent")
            into this patch]
[drosen: Folded 138993e ("Android: mnt: Propagate remount
         correctly") into this patch, integrated fs_context things
	 Now has update_mnt_data instead of needing remount2
[maennich: Folded cbfbd9e932de ("ANDROID: mnt: Fix null pointer
           dereference") into this patch]
Signed-off-by: Alistair Delva <adelva@google.com>
Signed-off-by: Matthias Maennich <maennich@google.com>

fs/namespace.c

@@ -201,6 +201,7 @@ static struct mount *alloc_vfsmnt(const char *name)
 		mnt->mnt_count = 1;
 		mnt->mnt_writers = 0;
 #endif
+		mnt->mnt.data = NULL;
 
 		INIT_HLIST_NODE(&mnt->mnt_hash);
 		INIT_LIST_HEAD(&mnt->mnt_child);
@@ -540,6 +541,7 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 
 static void free_vfsmnt(struct mount *mnt)
 {
+	kfree(mnt->mnt.data);
 	kfree_const(mnt->mnt_devname);
 #ifdef CONFIG_SMP
 	free_percpu(mnt->mnt_pcp);
@@ -942,14 +944,26 @@ static struct mount *skip_mnt_tree(struct mount *p)
 struct vfsmount *vfs_create_mount(struct fs_context *fc)
 {
 	struct mount *mnt;
+	struct super_block *sb;
 
 	if (!fc->root)
 		return ERR_PTR(-EINVAL);
+	sb = fc->root->d_sb;
 
 	mnt = alloc_vfsmnt(fc->source ?: "none");
 	if (!mnt)
 		return ERR_PTR(-ENOMEM);
 
+	if (fc->fs_type->alloc_mnt_data) {
+		mnt->mnt.data = fc->fs_type->alloc_mnt_data();
+		if (!mnt->mnt.data) {
+			mnt_free_id(mnt);
+			free_vfsmnt(mnt);
+			return ERR_PTR(-ENOMEM);
+		}
+		if (sb->s_op->update_mnt_data)
+			sb->s_op->update_mnt_data(mnt->mnt.data, fc);
+	}
 	if (fc->sb_flags & SB_KERNMOUNT)
 		mnt->mnt.mnt_flags = MNT_INTERNAL;
 
@@ -1033,6 +1047,14 @@ 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) {
+			err = -ENOMEM;
+			goto out_free;
+		}
+	}
+
 	if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
 		mnt->mnt_group_id = 0; /* not a peer of original */
 	else
@@ -2409,7 +2431,17 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 		err = -EPERM;
 		if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
 			err = reconfigure_super(fc);
-			if (!err) {
+			if (!err && sb->s_op->update_mnt_data) {
+				sb->s_op->update_mnt_data(mnt->mnt.data, fc);
+				lock_mount_hash();
+				set_mount_attributes(mnt, mnt_flags);
+				unlock_mount_hash();
+				namespace_lock();
+				lock_mount_hash();
+				propagate_remount(mnt);
+				unlock_mount_hash();
+				namespace_unlock();
+			} else if (!err) {
 				lock_mount_hash();
 				set_mount_attributes(mnt, mnt_flags);
 				unlock_mount_hash();

fs/pnode.c

@@ -607,3 +607,18 @@ int propagate_umount(struct list_head *list)
 	return 0;
 }
 
+void propagate_remount(struct mount *mnt)
+{
+	struct mount *parent = mnt->mnt_parent;
+	struct mount *p = mnt, *m;
+	struct super_block *sb = mnt->mnt.mnt_sb;
+
+	if (!sb->s_op->copy_mnt_data)
+		return;
+	for (p = propagation_next(parent, parent); p;
+				p = propagation_next(p, parent)) {
+		m = __lookup_mnt(&p->mnt, mnt->mnt_mountpoint);
+		if (m)
+			sb->s_op->copy_mnt_data(m->mnt.data, mnt->mnt.data);
+	}
+}

fs/pnode.h

@@ -44,6 +44,7 @@ int propagate_mnt(struct mount *, struct mountpoint *, struct mount *,
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct mount *, int);
 void propagate_mount_unlock(struct mount *);
+void 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

@@ -1974,6 +1974,9 @@ 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 (*update_mnt_data) (void *, struct fs_context *);
 	void (*umount_begin) (struct super_block *);
 	void (*umount_end)(struct super_block *sb, int flags);
 
@@ -2254,6 +2257,7 @@ struct file_system_type {
 	const struct fs_parameter_description *parameters;
 	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

@@ -70,6 +70,7 @@ struct vfsmount {
 	struct dentry *mnt_root;	/* root of the mounted tree */
 	struct super_block *mnt_sb;	/* pointer to superblock */
 	int mnt_flags;
+	void *data;
 	ANDROID_KABI_RESERVE(1);
 	ANDROID_KABI_RESERVE(2);
 	ANDROID_KABI_RESERVE(3);