memmod: register exception handler tables

Otherwise recent WDK binaries fail on ARM64, where an exception handler
is used for trapping an illegal instruction when ARMv8.1 atomics are
being tested for functionality.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/wintun/memmod/memmod_windows.go

@@ -159,6 +159,16 @@ func (module *Module) finalizeSection(sectionData *sectionFinalizeData) error {
 	return nil
 }
 
+var rtlAddFunctionTable = windows.NewLazySystemDLL("ntdll.dll").NewProc("RtlAddFunctionTable")
+
+func (module *Module) registerExceptionHandlers() {
+	directory := module.headerDirectory(IMAGE_DIRECTORY_ENTRY_EXCEPTION)
+	if directory.Size == 0 || directory.VirtualAddress == 0 {
+		return
+	}
+	rtlAddFunctionTable.Call(module.codeBase+uintptr(directory.VirtualAddress), uintptr(directory.Size)/unsafe.Sizeof(IMAGE_RUNTIME_FUNCTION_ENTRY{}), module.codeBase)
+}
+
 func (module *Module) finalizeSections() error {
 	sections := module.headers.Sections()
 	imageOffset := module.headers.OptionalHeader.imageOffset()
@@ -500,6 +510,9 @@ func LoadLibrary(data []byte) (module *Module, err error) {
 		return
 	}
 
+	// Register exception tables, if they exist.
+	module.registerExceptionHandlers()
+
 	// TLS callbacks are executed BEFORE the main loading.
 	module.executeTLS()
 

tun/wintun/memmod/syscall_windows.go

@@ -370,6 +370,12 @@ const (
 	IMAGE_GUARD_CF_FUNCTION_TABLE_SIZE_SHIFT       = 28
 )
 
+type IMAGE_RUNTIME_FUNCTION_ENTRY struct {
+	BeginAddress      uint32
+	EndAddress        uint32
+	UnwindInfoAddress uint32
+}
+
 const (
 	DLL_PROCESS_ATTACH = 1
 	DLL_THREAD_ATTACH  = 2