btrfs: add a btrfs_get_dev_args_from_path helper

We are going to want to populate our device lookup args outside of any
locks and then do the actual device lookup later, so add a helper to do
this work and make btrfs_find_device_by_devspec() use this helper for
now.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2325,45 +2325,81 @@ void btrfs_destroy_dev_replace_tgtdev(struct btrfs_device *tgtdev)
 	btrfs_free_device(tgtdev);
 }
 
-static struct btrfs_device *btrfs_find_device_by_path(
-		struct btrfs_fs_info *fs_info, const char *device_path)
+/**
+ * Populate args from device at path
+ *
+ * @fs_info:	the filesystem
+ * @args:	the args to populate
+ * @path:	the path to the device
+ *
+ * This will read the super block of the device at @path and populate @args with
+ * the devid, fsid, and uuid.  This is meant to be used for ioctls that need to
+ * lookup a device to operate on, but need to do it before we take any locks.
+ * This properly handles the special case of "missing" that a user may pass in,
+ * and does some basic sanity checks.  The caller must make sure that @path is
+ * properly NUL terminated before calling in, and must call
+ * btrfs_put_dev_args_from_path() in order to free up the temporary fsid and
+ * uuid buffers.
+ *
+ * Return: 0 for success, -errno for failure
+ */
+int btrfs_get_dev_args_from_path(struct btrfs_fs_info *fs_info,
+				 struct btrfs_dev_lookup_args *args,
+				 const char *path)
 {
-	BTRFS_DEV_LOOKUP_ARGS(args);
-	int ret = 0;
 	struct btrfs_super_block *disk_super;
 	struct block_device *bdev;
-	struct btrfs_device *device;
+	int ret;
 
-	ret = btrfs_get_bdev_and_sb(device_path, FMODE_READ,
-				    fs_info->bdev_holder, 0, &bdev, &disk_super);
-	if (ret)
-		return ERR_PTR(ret);
+	if (!path || !path[0])
+		return -EINVAL;
+	if (!strcmp(path, "missing")) {
+		args->missing = true;
+		return 0;
+	}
+
+	args->uuid = kzalloc(BTRFS_UUID_SIZE, GFP_KERNEL);
+	args->fsid = kzalloc(BTRFS_FSID_SIZE, GFP_KERNEL);
+	if (!args->uuid || !args->fsid) {
+		btrfs_put_dev_args_from_path(args);
+		return -ENOMEM;
+	}
 
-	args.devid = btrfs_stack_device_id(&disk_super->dev_item);
-	args.uuid = disk_super->dev_item.uuid;
+	ret = btrfs_get_bdev_and_sb(path, FMODE_READ, fs_info->bdev_holder, 0,
+				    &bdev, &disk_super);
+	if (ret)
+		return ret;
+	args->devid = btrfs_stack_device_id(&disk_super->dev_item);
+	memcpy(args->uuid, disk_super->dev_item.uuid, BTRFS_UUID_SIZE);
 	if (btrfs_fs_incompat(fs_info, METADATA_UUID))
-		args.fsid = disk_super->metadata_uuid;
+		memcpy(args->fsid, disk_super->metadata_uuid, BTRFS_FSID_SIZE);
 	else
-		args.fsid = disk_super->fsid;
-
-	device = btrfs_find_device(fs_info->fs_devices, &args);
-
+		memcpy(args->fsid, disk_super->fsid, BTRFS_FSID_SIZE);
 	btrfs_release_disk_super(disk_super);
-	if (!device)
-		device = ERR_PTR(-ENOENT);
 	blkdev_put(bdev, FMODE_READ);
-	return device;
+	return 0;
 }
 
 /*
- * Lookup a device given by device id, or the path if the id is 0.
+ * Only use this jointly with btrfs_get_dev_args_from_path() because we will
+ * allocate our ->uuid and ->fsid pointers, everybody else uses local variables
+ * that don't need to be freed.
  */
+void btrfs_put_dev_args_from_path(struct btrfs_dev_lookup_args *args)
+{
+	kfree(args->uuid);
+	kfree(args->fsid);
+	args->uuid = NULL;
+	args->fsid = NULL;
+}
+
 struct btrfs_device *btrfs_find_device_by_devspec(
 		struct btrfs_fs_info *fs_info, u64 devid,
 		const char *device_path)
 {
 	BTRFS_DEV_LOOKUP_ARGS(args);
 	struct btrfs_device *device;
+	int ret;
 
 	if (devid) {
 		args.devid = devid;
@@ -2373,18 +2409,14 @@ struct btrfs_device *btrfs_find_device_by_devspec(
 		return device;
 	}
 
-	if (!device_path || !device_path[0])
-		return ERR_PTR(-EINVAL);
-
-	if (strcmp(device_path, "missing") == 0) {
-		args.missing = true;
-		device = btrfs_find_device(fs_info->fs_devices, &args);
-		if (!device)
-			return ERR_PTR(-ENOENT);
-		return device;
-	}
-
-	return btrfs_find_device_by_path(fs_info, device_path);
+	ret = btrfs_get_dev_args_from_path(fs_info, &args, device_path);
+	if (ret)
+		return ERR_PTR(ret);
+	device = btrfs_find_device(fs_info->fs_devices, &args);
+	btrfs_put_dev_args_from_path(&args);
+	if (!device)
+		return ERR_PTR(-ENOENT);
+	return device;
 }
 
 /*

fs/btrfs/volumes.h

@@ -520,9 +520,13 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
 struct btrfs_device *btrfs_find_device_by_devspec(struct btrfs_fs_info *fs_info,
 						  u64 devid,
 						  const char *devpath);
+int btrfs_get_dev_args_from_path(struct btrfs_fs_info *fs_info,
+				 struct btrfs_dev_lookup_args *args,
+				 const char *path);
 struct btrfs_device *btrfs_alloc_device(struct btrfs_fs_info *fs_info,
 					const u64 *devid,
 					const u8 *uuid);
+void btrfs_put_dev_args_from_path(struct btrfs_dev_lookup_args *args);
 void btrfs_free_device(struct btrfs_device *device);
 int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 		    const char *device_path, u64 devid,