diff --git a/compositediskconfig/src/lib.rs b/compositediskconfig/src/lib.rs
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+++ b/compositediskconfig/src/lib.rs
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+// Copyright 2021, The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! JSON configuration for composite disks, as used for running `mk_cdisk` and by the `vm` tool.
+
+use serde::{Deserialize, Serialize};
+use std::io::Write;
+use std::path::PathBuf;
+
+/// Configuration for running `mk_cdisk`.
+#[derive(Clone, Debug, Default, Deserialize, Eq, PartialEq, Serialize)]
+pub struct Config {
+    /// The set of partitions to be assembled into a composite image.
+    pub partitions: Vec<Partition>,
+}
+
+/// A partition to be assembled into a composite image.
+#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+pub struct Partition {
+    /// A label for the partition.
+    pub label: String,
+    /// The filename of the partition image.
+    pub path: PathBuf,
+    /// Whether the partition should be writable.
+    #[serde(default)]
+    pub writable: bool,
+}
+
+impl Config {
+    /// Write the configuration as JSON, in the format used by `mk_cdisk`.
+    pub fn write_json(&self, writer: impl Write) -> serde_json::Result<()> {
+        serde_json::to_writer(writer, self)
+    }
+}