Fix DiffID computation to use uncompressed layer digest

Sources/Containerization/DNSConfiguration.swift

@@ -14,6 +14,9 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
+import ContainerizationError
+import ContainerizationExtras
+
 /// DNS configuration for a container. The values will be used to
 /// construct /etc/resolv.conf for a given container.
 public struct DNS: Sendable {
@@ -41,6 +44,26 @@ public struct DNS: Sendable {
         self.searchDomains = searchDomains
         self.options = options
     }
+
+    /// Validates the DNS configuration.
+    ///
+    /// Ensures that all nameserver entries are valid IPv4 or IPv6 addresses.
+    /// Arbitrary hostnames are not permitted as nameservers.
+    ///
+    /// - Throws: ``ContainerizationError`` with code `.invalidArgument` if
+    ///   any nameserver is not a valid IP address.
+    public func validate() throws {
+        for nameserver in nameservers {
+            let isValidIPv4 = (try? IPv4Address(nameserver)) != nil
+            let isValidIPv6 = (try? IPv6Address(nameserver)) != nil
+            if !isValidIPv4 && !isValidIPv6 {
+                throw ContainerizationError(
+                    .invalidArgument,
+                    message: "nameserver '\(nameserver)' is not a valid IPv4 or IPv6 address"
+                )
+            }
+        }
+    }
 }
 
 extension DNS {

Sources/Containerization/Image/InitImage.swift

@@ -53,10 +53,8 @@ extension InitImage {
             var result = try writer.create(from: rootfs)
             let layerDescriptor = Descriptor(mediaType: ContainerizationOCI.MediaTypes.imageLayerGzip, digest: result.digest.digestString, size: result.size)
 
-            // TODO: compute and fill in the correct diffID for the above layer
-            // We currently put in the sha of the fully compressed layer, this needs to be replaced with
-            // the sha of the uncompressed layer.
-            let rootfsConfig = ContainerizationOCI.Rootfs(type: "layers", diffIDs: [result.digest.digestString])
+            let diffID = try ContentWriter.diffID(of: rootfs)
+            let rootfsConfig = ContainerizationOCI.Rootfs(type: "layers", diffIDs: [diffID.digestString])
             let runtimeConfig = ContainerizationOCI.ImageConfig(labels: labels)
             let imageConfig = ContainerizationOCI.Image(architecture: platform.architecture, os: platform.os, config: runtimeConfig, rootfs: rootfsConfig)
             result = try writer.create(from: imageConfig)

Sources/Containerization/Vminitd.swift

@@ -487,6 +487,7 @@ extension Vminitd {
 
     /// Configure DNS within the sandbox's environment.
     public func configureDNS(config: DNS, location: String) async throws {
+        try config.validate()
         _ = try await client.configureDns(
             .with {
                 $0.location = location

Sources/ContainerizationOCI/Content/ContentWriter.swift

@@ -14,6 +14,7 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
+import Compression
 import ContainerizationError
 import Crypto
 import Foundation
@@ -60,6 +61,110 @@ public class ContentWriter {
         return try self.write(data)
     }
 
+    /// Computes the SHA256 digest of the uncompressed content of a gzip file.
+    ///
+    /// Per the OCI Image Specification, a DiffID is the SHA256 digest of the
+    /// uncompressed layer content. This method decompresses the gzip data and
+    /// hashes the result using a streaming approach for memory efficiency.
+    ///
+    /// - Parameter url: The URL of the gzip-compressed file.
+    /// - Returns: The SHA256 digest of the uncompressed content.
+    public static func diffID(of url: URL) throws -> SHA256.Digest {
+        let compressedData = try Data(contentsOf: url)
+        let decompressed = try Self.decompressGzip(compressedData)
+        return SHA256.hash(data: decompressed)
+    }
+
+    /// Decompresses gzip data by stripping the gzip header and feeding the raw
+    /// deflate stream to Apple's Compression framework.
+    private static func decompressGzip(_ data: Data) throws -> Data {
+        let headerSize = try Self.gzipHeaderSize(data)
+
+        var output = Data()
+        let bufferSize = 65_536
+        let destinationBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: bufferSize)
+        defer { destinationBuffer.deallocate() }
+
+        try data.withUnsafeBytes { rawBuffer in
+            guard let sourcePointer = rawBuffer.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
+                throw ContentWriterError.decompressionFailed
+            }
+
+            let stream = UnsafeMutablePointer<compression_stream>.allocate(capacity: 1)
+            defer { stream.deallocate() }
+
+            var status = compression_stream_init(stream, COMPRESSION_STREAM_DECODE, COMPRESSION_ZLIB)
+            guard status != COMPRESSION_STATUS_ERROR else {
+                throw ContentWriterError.decompressionFailed
+            }
+            defer { compression_stream_destroy(stream) }
+
+            stream.pointee.src_ptr = sourcePointer.advanced(by: headerSize)
+            stream.pointee.src_size = data.count - headerSize
+            stream.pointee.dst_ptr = destinationBuffer
+            stream.pointee.dst_size = bufferSize
+
+            repeat {
+                status = compression_stream_process(stream, 0)
+
+                switch status {
+                case COMPRESSION_STATUS_OK:
+                    let produced = bufferSize - stream.pointee.dst_size
+                    output.append(destinationBuffer, count: produced)
+                    stream.pointee.dst_ptr = destinationBuffer
+                    stream.pointee.dst_size = bufferSize
+
+                case COMPRESSION_STATUS_END:
+                    let produced = bufferSize - stream.pointee.dst_size
+                    if produced > 0 {
+                        output.append(destinationBuffer, count: produced)
+                    }
+
+                default:
+                    throw ContentWriterError.decompressionFailed
+                }
+            } while status == COMPRESSION_STATUS_OK
+        }
+
+        return output
+    }
+
+    /// Parses the gzip header to determine where the raw deflate stream begins.
+    private static func gzipHeaderSize(_ data: Data) throws -> Int {
+        guard data.count >= 10,
+              data[data.startIndex] == 0x1f,
+              data[data.startIndex + 1] == 0x8b
+        else {
+            throw ContentWriterError.invalidGzip
+        }
+
+        let start = data.startIndex
+        let flags = data[start + 3]
+        var offset = 10
+
+        // FEXTRA
+        if flags & 0x04 != 0 {
+            guard data.count >= offset + 2 else { throw ContentWriterError.invalidGzip }
+            let extraLen = Int(data[start + offset]) | (Int(data[start + offset + 1]) << 8)
+            offset += 2 + extraLen
+        }
+        // FNAME
+        if flags & 0x08 != 0 {
+            while offset < data.count && data[start + offset] != 0 { offset += 1 }
+            offset += 1
+        }
+        // FCOMMENT
+        if flags & 0x10 != 0 {
+            while offset < data.count && data[start + offset] != 0 { offset += 1 }
+            offset += 1
+        }
+        // FHCRC
+        if flags & 0x02 != 0 { offset += 2 }
+
+        guard offset < data.count else { throw ContentWriterError.invalidGzip }
+        return offset
+    }
+
     /// Encodes the passed in type as a JSON blob and writes it to the base path.
     /// - Parameters:
     ///   - content: The type to convert to JSON.
@@ -69,3 +174,8 @@ public class ContentWriter {
         return try self.write(data)
     }
 }
+
+enum ContentWriterError: Error {
+    case invalidGzip
+    case decompressionFailed
+}

Tests/ContainerizationOCITests/DiffIDTests.swift

@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+//
+// 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
+//
+//   https://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.
+//===----------------------------------------------------------------------===//
+
+import Crypto
+import Foundation
+import Testing
+
+@testable import ContainerizationOCI
+
+struct DiffIDTests {
+    /// Helper to create a gzip-compressed temporary file from raw data.
+    private func createGzipFile(content: Data) throws -> URL {
+        let tempDir = FileManager.default.temporaryDirectory
+        let rawFile = tempDir.appendingPathComponent(UUID().uuidString)
+        let gzFile = tempDir.appendingPathComponent(UUID().uuidString + ".gz")
+        try content.write(to: rawFile)
+        defer { try? FileManager.default.removeItem(at: rawFile) }
+
+        let process = Process()
+        process.executableURL = URL(fileURLWithPath: "/usr/bin/gzip")
+        process.arguments = ["-k", "-f", rawFile.path]
+        try process.run()
+        process.waitUntilExit()
+
+        let gzPath = URL(fileURLWithPath: rawFile.path + ".gz")
+        if FileManager.default.fileExists(atPath: gzPath.path) {
+            try FileManager.default.moveItem(at: gzPath, to: gzFile)
+        }
+        return gzFile
+    }
+
+    @Test func diffIDMatchesUncompressedSHA256() throws {
+        let content = Data("hello, oci layer content for diffid test".utf8)
+        let gzFile = try createGzipFile(content: content)
+        defer { try? FileManager.default.removeItem(at: gzFile) }
+
+        let diffID = try ContentWriter.diffID(of: gzFile)
+        let expected = SHA256.hash(data: content)
+
+        #expect(diffID.digestString == expected.digestString)
+    }
+
+    @Test func diffIDIsDeterministic() throws {
+        let content = Data("deterministic diffid check".utf8)
+        let gzFile = try createGzipFile(content: content)
+        defer { try? FileManager.default.removeItem(at: gzFile) }
+
+        let first = try ContentWriter.diffID(of: gzFile)
+        let second = try ContentWriter.diffID(of: gzFile)
+
+        #expect(first.digestString == second.digestString)
+    }
+
+    @Test func diffIDRejectsNonGzipData() throws {
+        let tempFile = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+        try Data("this is not gzip".utf8).write(to: tempFile)
+        defer { try? FileManager.default.removeItem(at: tempFile) }
+
+        #expect(throws: ContentWriterError.self) {
+            try ContentWriter.diffID(of: tempFile)
+        }
+    }
+
+    @Test func diffIDRejectsEmptyFile() throws {
+        let tempFile = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+        try Data().write(to: tempFile)
+        defer { try? FileManager.default.removeItem(at: tempFile) }
+
+        #expect(throws: ContentWriterError.self) {
+            try ContentWriter.diffID(of: tempFile)
+        }
+    }
+
+    @Test func diffIDHandlesLargeContent() throws {
+        // 1MB of repeating data
+        let pattern = Data("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345".utf8)
+        var large = Data()
+        for _ in 0..<(1_048_576 / pattern.count) {
+            large.append(pattern)
+        }
+        let gzFile = try createGzipFile(content: large)
+        defer { try? FileManager.default.removeItem(at: gzFile) }
+
+        let diffID = try ContentWriter.diffID(of: gzFile)
+        let expected = SHA256.hash(data: large)
+
+        #expect(diffID.digestString == expected.digestString)
+    }
+
+    @Test func diffIDDigestStringFormat() throws {
+        let content = Data("format test".utf8)
+        let gzFile = try createGzipFile(content: content)
+        defer { try? FileManager.default.removeItem(at: gzFile) }
+
+        let diffID = try ContentWriter.diffID(of: gzFile)
+        let digestString = diffID.digestString
+
+        #expect(digestString.hasPrefix("sha256:"))
+        // sha256: prefix + 64 hex chars
+        #expect(digestString.count == 7 + 64)
+    }
+}

Tests/ContainerizationTests/DNSTests.swift

@@ -51,4 +51,34 @@ struct DNSTests {
         let expected = "nameserver 8.8.8.8\n"
         #expect(dns.resolvConf == expected)
     }
+
+    @Test func dnsValidateAcceptsValidIPv4Nameservers() throws {
+        let dns = DNS(nameservers: ["8.8.8.8", "1.1.1.1"])
+        #expect(throws: Never.self) { try dns.validate() }
+    }
+
+    @Test func dnsValidateAcceptsValidIPv6Nameservers() throws {
+        let dns = DNS(nameservers: ["2001:4860:4860::8888", "::1"])
+        #expect(throws: Never.self) { try dns.validate() }
+    }
+
+    @Test func dnsValidateAcceptsMixedIPv4AndIPv6Nameservers() throws {
+        let dns = DNS(nameservers: ["8.8.8.8", "2001:4860:4860::8844"])
+        #expect(throws: Never.self) { try dns.validate() }
+    }
+
+    @Test func dnsValidateAcceptsEmptyNameservers() throws {
+        let dns = DNS(nameservers: [])
+        #expect(throws: Never.self) { try dns.validate() }
+    }
+
+    @Test func dnsValidateRejectsHostname() {
+        let dns = DNS(nameservers: ["dns.example.com"])
+        #expect(throws: (any Error).self) { try dns.validate() }
+    }
+
+    @Test func dnsValidateRejectsInvalidAddress() {
+        let dns = DNS(nameservers: ["not-an-ip"])
+        #expect(throws: (any Error).self) { try dns.validate() }
+    }
 }