Make MTU configurable for Netlink

Sources/Containerization/Agent/Vminitd.swift

@@ -198,10 +198,11 @@ extension Vminitd: VirtualMachineAgent {
         _ = try await client.deleteProcess(request)
     }
 
-    public func up(name: String) async throws {
+    public func up(name: String, mtu: UInt32? = nil) async throws {
         let request = Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest.with {
             $0.interface = name
             $0.up = true
+            if let mtu { $0.mtu = mtu }
         }
         _ = try await client.ipLinkSet(request)
     }

Sources/Containerization/LinuxContainer.swift

@@ -504,7 +504,7 @@ extension LinuxContainer {
                 for (index, i) in self.interfaces.enumerated() {
                     let name = "eth\(index)"
                     try await agent.addressAdd(name: name, address: i.address)
-                    try await agent.up(name: name)
+                    try await agent.up(name: name, mtu: 1280)
                     if let gateway = i.gateway {
                         try await agent.routeAddDefault(name: name, gateway: gateway)
                     }

Sources/Containerization/SandboxContext/SandboxContext.pb.swift

@@ -658,9 +658,20 @@ public struct Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest: Sendable {
 
   public var up: Bool = false
 
+  public var mtu: UInt32 {
+    get {return _mtu ?? 0}
+    set {_mtu = newValue}
+  }
+  /// Returns true if `mtu` has been explicitly set.
+  public var hasMtu: Bool {return self._mtu != nil}
+  /// Clears the value of `mtu`. Subsequent reads from it will return its default value.
+  public mutating func clearMtu() {self._mtu = nil}
+
   public var unknownFields = SwiftProtobuf.UnknownStorage()
 
   public init() {}
+
+  fileprivate var _mtu: UInt32? = nil
 }
 
 public struct Com_Apple_Containerization_Sandbox_V3_IpLinkSetResponse: Sendable {
@@ -1992,6 +2003,7 @@ extension Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest: SwiftProtobuf.
   public static let _protobuf_nameMap: SwiftProtobuf._NameMap = [
     1: .same(proto: "interface"),
     2: .same(proto: "up"),
+    3: .same(proto: "mtu"),
   ]
 
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
@@ -2002,24 +2014,33 @@ extension Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest: SwiftProtobuf.
       switch fieldNumber {
       case 1: try { try decoder.decodeSingularStringField(value: &self.interface) }()
       case 2: try { try decoder.decodeSingularBoolField(value: &self.up) }()
+      case 3: try { try decoder.decodeSingularUInt32Field(value: &self._mtu) }()
       default: break
       }
     }
   }
 
   public func traverse<V: SwiftProtobuf.Visitor>(visitor: inout V) throws {
+    // The use of inline closures is to circumvent an issue where the compiler
+    // allocates stack space for every if/case branch local when no optimizations
+    // are enabled. https://github.com/apple/swift-protobuf/issues/1034 and
+    // https://github.com/apple/swift-protobuf/issues/1182
     if !self.interface.isEmpty {
       try visitor.visitSingularStringField(value: self.interface, fieldNumber: 1)
     }
     if self.up != false {
       try visitor.visitSingularBoolField(value: self.up, fieldNumber: 2)
     }
+    try { if let v = self._mtu {
+      try visitor.visitSingularUInt32Field(value: v, fieldNumber: 3)
+    } }()
     try unknownFields.traverse(visitor: &visitor)
   }
 
   public static func ==(lhs: Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest, rhs: Com_Apple_Containerization_Sandbox_V3_IpLinkSetRequest) -> Bool {
     if lhs.interface != rhs.interface {return false}
     if lhs.up != rhs.up {return false}
+    if lhs._mtu != rhs._mtu {return false}
     if lhs.unknownFields != rhs.unknownFields {return false}
     return true
   }

Sources/Containerization/SandboxContext/SandboxContext.proto

@@ -193,6 +193,7 @@ message MkdirResponse {}
 message IpLinkSetRequest {
   string interface = 1;
   bool up = 2;
+  optional uint32 mtu = 3;
 }
 
 message IpLinkSetResponse {}

Sources/Containerization/VirtualMachineAgent.swift

@@ -53,7 +53,7 @@ public protocol VirtualMachineAgent: Sendable {
     func deleteProcess(id: String, containerID: String?) async throws
 
     // Networking
-    func up(name: String) async throws
+    func up(name: String, mtu: UInt32?) async throws
     func down(name: String) async throws
     func addressAdd(name: String, address: String) async throws
     func routeAddDefault(name: String, gateway: String) async throws

Sources/ContainerizationNetlink/NetlinkSession.swift

@@ -67,12 +67,17 @@ public struct NetlinkSession {
     /// - Parameters:
     ///   - interface: The name of the interface.
     ///   - up: The value to set the interface state to.
-    public func linkSet(interface: String, up: Bool) throws {
+    public func linkSet(interface: String, up: Bool, mtu: UInt32? = nil) throws {
         // ip link set dev [interface] [up|down]
         let interfaceIndex = try getInterfaceIndex(interface)
-        let mtuAttr = RTAttribute(
-            len: UInt16(RTAttribute.size + MemoryLayout<UInt32>.size), type: LinkAttributeType.IFLA_MTU)
-        let requestSize = NetlinkMessageHeader.size + InterfaceInfo.size + mtuAttr.paddedLen
+        // build the attribute only when mtu is supplied
+        let attr: RTAttribute? =
+            (mtu != nil)
+            ? RTAttribute(
+                len: UInt16(RTAttribute.size + MemoryLayout<UInt32>.size),
+                type: LinkAttributeType.IFLA_MTU)
+            : nil
+        let requestSize = NetlinkMessageHeader.size + InterfaceInfo.size + (attr?.paddedLen ?? 0)
         var requestBuffer = [UInt8](repeating: 0, count: requestSize)
         var requestOffset = 0
 
@@ -91,16 +96,16 @@ public struct NetlinkSession {
             change: InterfaceFlags.DEFAULT_CHANGE)
         requestOffset = try requestInfo.appendBuffer(&requestBuffer, offset: requestOffset)
 
-        requestOffset = try mtuAttr.appendBuffer(&requestBuffer, offset: requestOffset)
-        guard
-            let newRequestOffset = requestBuffer.copyIn(
-                as: UInt32.self,
-                value: Self.mtu,
-                offset: requestOffset)
-        else {
-            throw NetlinkDataError.sendMarshalFailure
+        if let attr = attr, let m = mtu {
+            requestOffset = try attr.appendBuffer(&requestBuffer, offset: requestOffset)
+            guard
+                let newRequestOffset =
+                    requestBuffer.copyIn(as: UInt32.self, value: m, offset: requestOffset)
+            else {
+                throw NetlinkDataError.sendMarshalFailure
+            }
+            requestOffset = newRequestOffset
         }
-        requestOffset = newRequestOffset
 
         guard requestOffset == requestSize else {
             throw Error.unexpectedOffset(offset: requestOffset, size: requestSize)