Pass Softnet policy control FD through Tart (#1287)

* Pass Softnet policy control FD through Tart

* Update Softnet control test for policy set

Sources/tart/Commands/Run.swift

@@ -224,6 +224,13 @@ struct Run: AsyncParsableCommand {
   """, valueName: "comma-separated CIDRs"))
   var netSoftnetBlock: String?
 
+  @Option(help: ArgumentHelp("Connected Unix stream socket file descriptor to use for the Softnet control channel (e.g. --net-softnet-control-fd=3)", discussion: """
+  This option enables the Softnet control channel on an inherited Unix stream socket. It can be used to dynamically replace Softnet allow and block lists while the VM is running.
+
+  The file descriptor must be greater than 2. Implies --net-softnet.
+  """, valueName: "file descriptor"))
+  var netSoftnetControlFd: Int32?
+
   @Option(help: ArgumentHelp("Comma-separated list of TCP ports to expose (e.g. --net-softnet-expose 2222:22,8080:80)", discussion: """
   Options are comma-separated and are as follows:
 
@@ -313,7 +320,7 @@ struct Run: AsyncParsableCommand {
     }
 
     // Automatically enable --net-softnet when any of its related options are specified
-    if netSoftnetAllow != nil || netSoftnetBlock != nil || netSoftnetExpose != nil {
+    if netSoftnetAllow != nil || netSoftnetBlock != nil || netSoftnetExpose != nil || netSoftnetControlFd != nil {
       netSoftnet = true
     }
 
@@ -681,13 +688,13 @@ struct Run: AsyncParsableCommand {
     if netSoftnet {
       let config = try VMConfig.init(fromURL: vmDir.configURL)
 
-      return try Softnet(vmMACAddress: config.macAddress.string, extraArguments: softnetExtraArguments)
+      return try Softnet(vmMACAddress: config.macAddress.string, extraArguments: softnetExtraArguments, controlFD: netSoftnetControlFd)
     }
 
     if netHost {
       let config = try VMConfig.init(fromURL: vmDir.configURL)
 
-      return try Softnet(vmMACAddress: config.macAddress.string, extraArguments: ["--vm-net-type", "host"] + softnetExtraArguments)
+      return try Softnet(vmMACAddress: config.macAddress.string, extraArguments: ["--vm-net-type", "host"] + softnetExtraArguments, controlFD: netSoftnetControlFd)
     }
 
     if netBridged.count > 0 {

Sources/tart/Network/Softnet.swift

@@ -16,7 +16,18 @@ class Softnet: Network {
 
   let vmFD: Int32
 
-  init(vmMACAddress: String, extraArguments: [String] = []) throws {
+  init(vmMACAddress: String, extraArguments: [String] = [], controlFD: Int32? = nil) throws {
+    var controlFileHandle: FileHandle?
+
+    if let controlFD = controlFD {
+      guard controlFD > STDERR_FILENO else {
+        throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor must be greater than 2")
+      }
+
+      controlFileHandle = FileHandle(fileDescriptor: controlFD, closeOnDealloc: true)
+      try Self.validateControlFD(controlFD)
+    }
+
     let fds = UnsafeMutablePointer<Int32>.allocate(capacity: MemoryLayout<Int>.stride * 2)
 
     let ret = socketpair(AF_UNIX, SOCK_DGRAM, 0, fds)
@@ -33,6 +44,44 @@ class Softnet: Network {
     process.executableURL = try Self.softnetExecutableURL()
     process.arguments = ["--vm-fd", String(STDIN_FILENO), "--vm-mac-address", vmMACAddress] + extraArguments
     process.standardInput = FileHandle(fileDescriptor: softnetFD, closeOnDealloc: false)
+
+    if let controlFileHandle = controlFileHandle {
+      process.arguments! += ["--control-fd", String(STDOUT_FILENO)]
+      process.standardOutput = controlFileHandle
+    }
+  }
+
+  static func validateControlFD(_ fd: Int32) throws {
+    guard fd > STDERR_FILENO else {
+      throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor must be greater than 2")
+    }
+
+    var socketType: Int32 = 0
+    var socketTypeLength = socklen_t(MemoryLayout<Int32>.size)
+    guard getsockopt(fd, SOL_SOCKET, SO_TYPE, &socketType, &socketTypeLength) == 0 else {
+      let details = Errno(rawValue: CInt(errno))
+      throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor is not a socket: \(details)")
+    }
+
+    guard socketType == SOCK_STREAM else {
+      throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor must be a Unix stream socket")
+    }
+
+    var peerAddress = sockaddr_storage()
+    var peerAddressLength = socklen_t(MemoryLayout<sockaddr_storage>.size)
+    let result = withUnsafeMutablePointer(to: &peerAddress) { pointer in
+      pointer.withMemoryRebound(to: sockaddr.self, capacity: 1) {
+        getpeername(fd, $0, &peerAddressLength)
+      }
+    }
+    guard result == 0 else {
+      let details = Errno(rawValue: CInt(errno))
+      throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor is not connected: \(details)")
+    }
+
+    guard peerAddress.ss_family == sa_family_t(AF_UNIX) else {
+      throw SoftnetError.InitializationFailed(why: "Softnet control file descriptor must be a Unix stream socket")
+    }
   }
 
   static func softnetExecutableURL() throws -> URL {
@@ -46,6 +95,8 @@ class Softnet: Network {
   }
 
   func run(_ sema: AsyncSemaphore) throws {
+    defer { try? (process.standardOutput as? FileHandle)?.close() }
+
     try process.run()
 
     monitorTask = Task {