bcachefs: support get fs label

Implement support for FS_IOC_GETFSLABEL ioctl to read filesystem
label.
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-ioctl.c

@@ -277,6 +277,30 @@ static int bch2_ioc_getversion(struct bch_inode_info *inode, u32 __user *arg)
 	return put_user(inode->v.i_generation, arg);
 }
 
+static int bch2_ioc_getlabel(struct bch_fs *c, char __user *user_label)
+{
+	int ret;
+	size_t len;
+	char label[BCH_SB_LABEL_SIZE];
+
+	BUILD_BUG_ON(BCH_SB_LABEL_SIZE >= FSLABEL_MAX);
+
+	mutex_lock(&c->sb_lock);
+	memcpy(label, c->disk_sb.sb->label, BCH_SB_LABEL_SIZE);
+	mutex_unlock(&c->sb_lock);
+
+	len = strnlen(label, BCH_SB_LABEL_SIZE);
+	if (len == BCH_SB_LABEL_SIZE) {
+		bch_warn(c,
+			"label is too long, return the first %zu bytes",
+			--len);
+	}
+
+	ret = copy_to_user(user_label, label, len);
+
+	return ret ? -EFAULT : 0;
+}
+
 static int bch2_ioc_goingdown(struct bch_fs *c, u32 __user *arg)
 {
 	u32 flags;
@@ -511,6 +535,10 @@ long bch2_fs_file_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 		ret = -ENOTTY;
 		break;
 
+	case FS_IOC_GETFSLABEL:
+		ret = bch2_ioc_getlabel(c, (void __user *) arg);
+		break;
+
 	case FS_IOC_GOINGDOWN:
 		ret = bch2_ioc_goingdown(c, (u32 __user *) arg);
 		break;
@@ -555,6 +583,8 @@ long bch2_compat_fs_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 	case FS_IOC32_GETVERSION:
 		cmd = FS_IOC_GETVERSION;
 		break;
+	case FS_IOC_GETFSLABEL:
+		break;
 	default:
 		return -ENOIOCTLCMD;
 	}