btrfs: Add unprivileged ioctl which returns subvolume information

Add new unprivileged ioctl BTRFS_IOC_GET_SUBVOL_INFO which returns
the information of subvolume containing this inode.
(i.e. returns the information in ROOT_ITEM and ROOT_BACKREF.)

Reviewed-by: Gu Jinxiang <[e-mail]>
Tested-by: Gu Jinxiang <[e-mail]>
Signed-off-by: Tomohiro Misono <[e-mail]>
[ minor style fixes, update struct comments ]
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ioctl.c

···
 	return ret;
 }
 
+/* Get the subvolume information in BTRFS_ROOT_ITEM and BTRFS_ROOT_BACKREF */
+static int btrfs_ioctl_get_subvol_info(struct file *file, void __user *argp)
+{
+	struct btrfs_ioctl_get_subvol_info_args *subvol_info;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_root *root;
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	struct btrfs_root_item *root_item;
+	struct btrfs_root_ref *rref;
+	struct extent_buffer *leaf;
+	unsigned long item_off;
+	unsigned long item_len;
+	struct inode *inode;
+	int slot;
+	int ret = 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	subvol_info = kzalloc(sizeof(*subvol_info), GFP_KERNEL);
+	if (!subvol_info) {
+		btrfs_free_path(path);
+		return -ENOMEM;
+	}
+
+	inode = file_inode(file);
+	fs_info = BTRFS_I(inode)->root->fs_info;
+
+	/* Get root_item of inode's subvolume */
+	key.objectid = BTRFS_I(inode)->root->root_key.objectid;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = (u64)-1;
+	root = btrfs_read_fs_root_no_name(fs_info, &key);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+		goto out;
+	}
+	root_item = &root->root_item;
+
+	subvol_info->treeid = key.objectid;
+
+	subvol_info->generation = btrfs_root_generation(root_item);
+	subvol_info->flags = btrfs_root_flags(root_item);
+
+	memcpy(subvol_info->uuid, root_item->uuid, BTRFS_UUID_SIZE);
+	memcpy(subvol_info->parent_uuid, root_item->parent_uuid,
+						    BTRFS_UUID_SIZE);
+	memcpy(subvol_info->received_uuid, root_item->received_uuid,
+						    BTRFS_UUID_SIZE);
+
+	subvol_info->ctransid = btrfs_root_ctransid(root_item);
+	subvol_info->ctime.sec = btrfs_stack_timespec_sec(&root_item->ctime);
+	subvol_info->ctime.nsec = btrfs_stack_timespec_nsec(&root_item->ctime);
+
+	subvol_info->otransid = btrfs_root_otransid(root_item);
+	subvol_info->otime.sec = btrfs_stack_timespec_sec(&root_item->otime);
+	subvol_info->otime.nsec = btrfs_stack_timespec_nsec(&root_item->otime);
+
+	subvol_info->stransid = btrfs_root_stransid(root_item);
+	subvol_info->stime.sec = btrfs_stack_timespec_sec(&root_item->stime);
+	subvol_info->stime.nsec = btrfs_stack_timespec_nsec(&root_item->stime);
+
+	subvol_info->rtransid = btrfs_root_rtransid(root_item);
+	subvol_info->rtime.sec = btrfs_stack_timespec_sec(&root_item->rtime);
+	subvol_info->rtime.nsec = btrfs_stack_timespec_nsec(&root_item->rtime);
+
+	if (key.objectid != BTRFS_FS_TREE_OBJECTID) {
+		/* Search root tree for ROOT_BACKREF of this subvolume */
+		root = fs_info->tree_root;
+
+		key.type = BTRFS_ROOT_BACKREF_KEY;
+		key.offset = 0;
+		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		if (ret < 0) {
+			goto out;
+		} else if (path->slots[0] >=
+			   btrfs_header_nritems(path->nodes[0])) {
+			ret = btrfs_next_leaf(root, path);
+			if (ret < 0) {
+				goto out;
+			} else if (ret > 0) {
+				ret = -EUCLEAN;
+				goto out;
+			}
+		}
+
+		leaf = path->nodes[0];
+		slot = path->slots[0];
+		btrfs_item_key_to_cpu(leaf, &key, slot);
+		if (key.objectid == subvol_info->treeid &&
+		    key.type == BTRFS_ROOT_BACKREF_KEY) {
+			subvol_info->parent_id = key.offset;
+
+			rref = btrfs_item_ptr(leaf, slot, struct btrfs_root_ref);
+			subvol_info->dirid = btrfs_root_ref_dirid(leaf, rref);
+
+			item_off = btrfs_item_ptr_offset(leaf, slot)
+					+ sizeof(struct btrfs_root_ref);
+			item_len = btrfs_item_size_nr(leaf, slot)
+					- sizeof(struct btrfs_root_ref);
+			read_extent_buffer(leaf, subvol_info->name,
+					   item_off, item_len);
+		} else {
+			ret = -ENOENT;
+			goto out;
+		}
+	}
+
+	if (copy_to_user(argp, subvol_info, sizeof(*subvol_info)))
+		ret = -EFAULT;
+
+out:
+	btrfs_free_path(path);
+	kzfree(subvol_info);
+	return ret;
+}
+
 static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 					     void __user *arg)
 {
···
 		return btrfs_ioctl_fsgetxattr(file, argp);
 	case FS_IOC_FSSETXATTR:
 		return btrfs_ioctl_fssetxattr(file, argp);
+	case BTRFS_IOC_GET_SUBVOL_INFO:
+		return btrfs_ioctl_get_subvol_info(file, argp);
 	}
 
 	return -ENOTTY;

include/uapi/linux/btrfs.h

···
 	__u64 reserved[4];		/* in */
 };
 
+/*
+ * Information about a fs tree root.
+ *
+ * All items are filled by the ioctl
+ */
+struct btrfs_ioctl_get_subvol_info_args {
+	/* Id of this subvolume */
+	__u64 treeid;
+
+	/* Name of this subvolume, used to get the real name at mount point */
+	char name[BTRFS_VOL_NAME_MAX + 1];
+
+	/*
+	 * Id of the subvolume which contains this subvolume.
+	 * Zero for top-level subvolume or a deleted subvolume.
+	 */
+	__u64 parent_id;
+
+	/*
+	 * Inode number of the directory which contains this subvolume.
+	 * Zero for top-level subvolume or a deleted subvolume
+	 */
+	__u64 dirid;
+
+	/* Latest transaction id of this subvolume */
+	__u64 generation;
+
+	/* Flags of this subvolume */
+	__u64 flags;
+
+	/* UUID of this subvolume */
+	__u8 uuid[BTRFS_UUID_SIZE];
+
+	/*
+	 * UUID of the subvolume of which this subvolume is a snapshot.
+	 * All zero for a non-snapshot subvolume.
+	 */
+	__u8 parent_uuid[BTRFS_UUID_SIZE];
+
+	/*
+	 * UUID of the subvolume from which this subvolume was received.
+	 * All zero for non-received subvolume.
+	 */
+	__u8 received_uuid[BTRFS_UUID_SIZE];
+
+	/* Transaction id indicating when change/create/send/receive happened */
+	__u64 ctransid;
+	__u64 otransid;
+	__u64 stransid;
+	__u64 rtransid;
+	/* Time corresponding to c/o/s/rtransid */
+	struct btrfs_ioctl_timespec ctime;
+	struct btrfs_ioctl_timespec otime;
+	struct btrfs_ioctl_timespec stime;
+	struct btrfs_ioctl_timespec rtime;
+
+	/* Must be zero */
+	__u64 reserved[8];
+};
+
 /* Error codes as returned by the kernel */
 enum btrfs_err_code {
 	BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET = 1,
···
 				   struct btrfs_ioctl_vol_args_v2)
 #define BTRFS_IOC_LOGICAL_INO_V2 _IOWR(BTRFS_IOCTL_MAGIC, 59, \
 					struct btrfs_ioctl_logical_ino_args)
+#define BTRFS_IOC_GET_SUBVOL_INFO _IOR(BTRFS_IOCTL_MAGIC, 60, \
+				struct btrfs_ioctl_get_subvol_info_args)
 
 #endif /* _UAPI_LINUX_BTRFS_H */