PoC: Box64 hybrid NaCl emulation for unsupported platforms

Run x86_64 NaCl loader and amd64 .nexe game modules under Box64
emulation on platforms without native NaCl support. The native engine
communicates with the emulated NaCl sandbox processes via
architecture-independent Unix domain socket IPC, requiring no changes
to game modules.

- Add opt-in DAEMON_NACL_BOX64_EMULATION CMake option (OFF by default,
  Linux only) that remaps NACL_ARCH to amd64
- Replace architecture-specific NaCl dummy defines with a generic
  fallback for any unsupported architecture
- Prepend box64 to NaCl loader args with PATH resolution
- Skip bootstrap helper (incompatible with Box64 double-exec)
- Disable platform qualification for emulated loader
- Add cvars: vm.box64.path, workaround.box64.disableBootstrap,
  workaround.box64.disableQualification

Author: runlevel5

cmake/DaemonArchitecture.cmake

@@ -81,6 +81,17 @@ if (LINUX OR FREEBSD)
 		# The nexe is system agnostic so there should be no difference with armel.
 		set(NACL_ARCH "armhf")
 	endif()
+
+	set(BOX64_USAGE ppc64el riscv64)
+	if (ARCH IN_LIST BOX64_USAGE)
+		option(DAEMON_NACL_BOX64_EMULATION "Use Box64 to emulate x86_64 NaCl loader on unsupported platforms" ON)
+		if (DAEMON_NACL_BOX64_EMULATION)
+			# Use Box64 to run x86_64 NaCl loader and amd64 nexe.
+			# Box64 must be installed and available in PATH at runtime.
+			set(NACL_ARCH "amd64")
+			add_definitions(-DDAEMON_NACL_BOX64_EMULATION)
+		endif()
+	endif()
 elseif(APPLE)
 	if ("${ARCH}" STREQUAL arm64)
 		# You can get emulated NaCl going like this:

cmake/DaemonNacl.cmake

@@ -71,13 +71,12 @@ else()
     add_definitions( -DNACL_BUILD_SUBARCH=32 )
   elseif( NACL_ARCH STREQUAL "armhf" )
     add_definitions( -DNACL_BUILD_ARCH=arm )
-  elseif( NACL_ARCH STREQUAL "ppc64el" OR NACL_ARCH STREQUAL "ppc64" )
-    # NaCl does not support PPC, but these defines must be set for native
-    # builds. Use dummy x86 values as PNaCl does for arch-independent builds.
+  else()
+    # NaCl does not support this architecture natively, but these defines must
+    # be set because nacl_config.h is included unconditionally. Use dummy x86
+    # values as PNaCl does for architecture-independent builds.
     add_definitions( -DNACL_BUILD_ARCH=x86 )
     add_definitions( -DNACL_BUILD_SUBARCH=64 )
-  else()
-    message(WARNING "Unknown architecture ${NACL_ARCH}")
   endif()
 endif()
 

src/engine/framework/VirtualMachine.cpp

@@ -73,6 +73,54 @@ static Cvar::Cvar<bool> workaround_naclSystem_freebsd_disableQualification(
 	"Disable platform qualification when running Linux NaCl loader on FreeBSD through Linuxulator",
 	Cvar::NONE, true);
 
+#if defined(DAEMON_NACL_BOX64_EMULATION)
+static Cvar::Cvar<bool> workaround_box64_disableQualification(
+	"workaround.box64.disableQualification",
+	"Disable platform qualification when running amd64 NaCl loader under Box64 emulation",
+	Cvar::NONE, true);
+
+static Cvar::Cvar<bool> workaround_box64_disableBootstrap(
+	"workaround.box64.disableBootstrap",
+	"Disable NaCl bootstrap helper when using Box64 emulation",
+	Cvar::NONE, true);
+
+static Cvar::Cvar<std::string> vm_box64_path(
+	"vm.box64.path",
+	"Path to the box64 binary for NaCl emulation (empty = search PATH)",
+	Cvar::NONE, "");
+
+// Resolve box64 binary path by searching PATH if not explicitly set.
+static std::string ResolveBox64Path() {
+	std::string path = vm_box64_path.Get();
+	if (!path.empty()) {
+		return path;
+	}
+
+	const char* envPath = getenv("PATH");
+	if (!envPath) {
+		Sys::Error("Box64 emulation is enabled but PATH is not set and vm.box64.path is empty.");
+	}
+
+	std::string pathStr(envPath);
+	size_t start = 0;
+	while (start < pathStr.size()) {
+		size_t end = pathStr.find(':', start);
+		if (end == std::string::npos) {
+			end = pathStr.size();
+		}
+		std::string candidate = pathStr.substr(start, end - start) + "/box64";
+		if (access(candidate.c_str(), X_OK) == 0) {
+			return candidate;
+		}
+		start = end + 1;
+	}
+
+	Sys::Error("Box64 emulation is enabled but 'box64' was not found in PATH. "
+	           "Install Box64 or set vm.box64.path to the full path of the box64 binary.");
+	return ""; // unreachable
+}
+#endif
+
 static Cvar::Cvar<bool> vm_nacl_qualification(
 	"vm.nacl.qualification",
 	"Enable NaCl loader platform qualification",
@@ -255,6 +303,9 @@ static std::pair<Sys::OSHandle, IPC::Socket> CreateNaClVM(std::pair<IPC::Socket,
 	char rootSocketRedir[32];
 	std::string module, nacl_loader, irt, bootstrap, modulePath, verbosity;
 	FS::File stderrRedirect;
+#if defined(DAEMON_NACL_BOX64_EMULATION)
+	std::string box64Path;
+#endif
 #if !defined(_WIN32) || defined(_WIN64)
 	constexpr bool win32Force64Bit = false;
 #else
@@ -304,6 +355,33 @@ static std::pair<Sys::OSHandle, IPC::Socket> CreateNaClVM(std::pair<IPC::Socket,
 	}
 
 #if defined(__linux__) || defined(__FreeBSD__)
+#if defined(DAEMON_NACL_BOX64_EMULATION)
+	/* Use Box64 to run the x86_64 NaCl loader on non-x86 architectures.
+	The bootstrap helper uses a double-exec pattern that Box64 cannot handle,
+	so we skip it and prepend "box64" to the nacl_loader command instead. */
+	if (!workaround_box64_disableBootstrap.Get() && vm_nacl_bootstrap.Get()) {
+		bootstrap = FS::Path::Build(naclPath, "nacl_helper_bootstrap");
+
+		if (!FS::RawPath::FileExists(bootstrap)) {
+			Sys::Error("NaCl bootstrap helper not found: %s", bootstrap);
+		}
+
+		args.push_back(bootstrap.c_str());
+		args.push_back(nacl_loader.c_str());
+		args.push_back("--r_debug=0xXXXXXXXXXXXXXXXX");
+		args.push_back("--reserved_at_zero=0xXXXXXXXXXXXXXXXX");
+	} else {
+		if (workaround_box64_disableBootstrap.Get()) {
+			Log::Notice("Skipping NaCl bootstrap helper for Box64 emulation.");
+		} else {
+			Log::Warn("Not using NaCl bootstrap helper.");
+		}
+		box64Path = ResolveBox64Path();
+		Log::Notice("Using Box64 emulator: %s", box64Path);
+		args.push_back(box64Path.c_str());
+		args.push_back(nacl_loader.c_str());
+	}
+#else
 	if (vm_nacl_bootstrap.Get()) {
 #if defined(DAEMON_ARCH_arm64)
 		bootstrap = FS::Path::Build(naclPath, "nacl_helper_bootstrap-armhf");
@@ -323,6 +401,7 @@ static std::pair<Sys::OSHandle, IPC::Socket> CreateNaClVM(std::pair<IPC::Socket,
 		Log::Warn("Not using NaCl bootstrap helper.");
 		args.push_back(nacl_loader.c_str());
 	}
+#endif
 #else
 	Q_UNUSED(bootstrap);
 	args.push_back(nacl_loader.c_str());
@@ -381,6 +460,17 @@ static std::pair<Sys::OSHandle, IPC::Socket> CreateNaClVM(std::pair<IPC::Socket,
 			enableQualification = false;
 		}
 #endif
+
+#if defined(DAEMON_NACL_BOX64_EMULATION)
+		/* When running the amd64 NaCl loader under Box64, the loader's
+		platform qualification will fail because the CPU is not actually x86_64.
+		Disabling qualification allows the emulated loader to proceed. */
+
+		if (workaround_box64_disableQualification.Get()) {
+			Log::Warn("Disabling NaCL platform qualification for Box64 emulation.");
+			enableQualification = false;
+		}
+#endif
 	}
 	else {
 		Log::Warn("Not using NaCl platform qualification.");