btrfs: Add unprivileged ioctl which returns subvolume's ROOT_REF

Add unprivileged ioctl BTRFS_IOC_GET_SUBVOL_ROOTREF which returns
ROOT_REF information of the subvolume containing this inode except the
subvolume name (this is because to prevent potential name leak). The
subvolume name will be gained by user version of ino_lookup ioctl
(BTRFS_IOC_INO_LOOKUP_USER) which also performs permission check.

The min id of root ref's subvolume to be searched is specified by
@min_id in struct btrfs_ioctl_get_subvol_rootref_args. After the search
ends, @min_id is set to the last searched root ref's subvolid + 1. Also,
if there are more root refs than BTRFS_MAX_ROOTREF_BUFFER_NUM,
-EOVERFLOW is returned. Therefore the caller can just call this ioctl
again without changing the argument to continue search.

Reviewed-by: Qu Wenruo <[e-mail]>
Reviewed-by: Gu Jinxiang <[e-mail]>
Tested-by: Gu Jinxiang <[e-mail]>
Signed-off-by: Tomohiro Misono <[e-mail]>
[ style fixes and struct item renames ]
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ioctl.c

···
 	return ret;
 }
 
+/*
+ * Return ROOT_REF information of the subvolume containing this inode
+ * except the subvolume name.
+ */
+static int btrfs_ioctl_get_subvol_rootref(struct file *file, void __user *argp)
+{
+	struct btrfs_ioctl_get_subvol_rootref_args *rootrefs;
+	struct btrfs_root_ref *rref;
+	struct btrfs_root *root;
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	struct extent_buffer *leaf;
+	struct inode *inode;
+	u64 objectid;
+	int slot;
+	int ret;
+	u8 found;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	rootrefs = memdup_user(argp, sizeof(*rootrefs));
+	if (IS_ERR(rootrefs)) {
+		btrfs_free_path(path);
+		return PTR_ERR(rootrefs);
+	}
+
+	inode = file_inode(file);
+	root = BTRFS_I(inode)->root->fs_info->tree_root;
+	objectid = BTRFS_I(inode)->root->root_key.objectid;
+
+	key.objectid = objectid;
+	key.type = BTRFS_ROOT_REF_KEY;
+	key.offset = rootrefs->min_treeid;
+	found = 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;
+		}
+	}
+	while (1) {
+		leaf = path->nodes[0];
+		slot = path->slots[0];
+
+		btrfs_item_key_to_cpu(leaf, &key, slot);
+		if (key.objectid != objectid || key.type != BTRFS_ROOT_REF_KEY) {
+			ret = 0;
+			goto out;
+		}
+
+		if (found == BTRFS_MAX_ROOTREF_BUFFER_NUM) {
+			ret = -EOVERFLOW;
+			goto out;
+		}
+
+		rref = btrfs_item_ptr(leaf, slot, struct btrfs_root_ref);
+		rootrefs->rootref[found].treeid = key.offset;
+		rootrefs->rootref[found].dirid =
+				  btrfs_root_ref_dirid(leaf, rref);
+		found++;
+
+		ret = btrfs_next_item(root, path);
+		if (ret < 0) {
+			goto out;
+		} else if (ret > 0) {
+			ret = -EUCLEAN;
+			goto out;
+		}
+	}
+
+out:
+	if (!ret || ret == -EOVERFLOW) {
+		rootrefs->num_items = found;
+		/* update min_treeid for next search */
+		if (found)
+			rootrefs->min_treeid =
+				rootrefs->rootref[found - 1].treeid + 1;
+		if (copy_to_user(argp, rootrefs, sizeof(*rootrefs)))
+			ret = -EFAULT;
+	}
+
+	kfree(rootrefs);
+	btrfs_free_path(path);
+
+	return ret;
+}
+
 static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 					     void __user *arg)
 {
···
 		return btrfs_ioctl_fssetxattr(file, argp);
 	case BTRFS_IOC_GET_SUBVOL_INFO:
 		return btrfs_ioctl_get_subvol_info(file, argp);
+	case BTRFS_IOC_GET_SUBVOL_ROOTREF:
+		return btrfs_ioctl_get_subvol_rootref(file, argp);
 	}
 
 	return -ENOTTY;

include/uapi/linux/btrfs.h

···
 	__u64 reserved[8];
 };
 
+#define BTRFS_MAX_ROOTREF_BUFFER_NUM 255
+struct btrfs_ioctl_get_subvol_rootref_args {
+		/* in/out, minimum id of rootref's treeid to be searched */
+		__u64 min_treeid;
+
+		/* out */
+		struct {
+			__u64 treeid;
+			__u64 dirid;
+		} rootref[BTRFS_MAX_ROOTREF_BUFFER_NUM];
+
+		/* out, number of found items */
+		__u8 num_items;
+		__u8 align[7];
+};
+
 /* Error codes as returned by the kernel */
 enum btrfs_err_code {
 	BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET = 1,
···
 					struct btrfs_ioctl_logical_ino_args)
 #define BTRFS_IOC_GET_SUBVOL_INFO _IOR(BTRFS_IOCTL_MAGIC, 60, \
 				struct btrfs_ioctl_get_subvol_info_args)
+#define BTRFS_IOC_GET_SUBVOL_ROOTREF _IOWR(BTRFS_IOCTL_MAGIC, 61, \
+				struct btrfs_ioctl_get_subvol_rootref_args)
 
 #endif /* _UAPI_LINUX_BTRFS_H */