Reduced size of Windows executables

src/exe/pe/EXE.go

@@ -24,7 +24,7 @@ type DLL struct {
 
 // Write writes the EXE file to the given writer.
 func Write(writer io.Writer, code []byte, data []byte) {
-	NumSections := 3
+	NumSections := 2
 	HeaderEnd := DOSHeaderSize + NTHeaderSize + OptionalHeader64Size + SectionHeaderSize*NumSections
 	codeStart, codePadding := exe.Align(HeaderEnd, config.Align)
 	dataStart, dataPadding := exe.Align(codeStart+len(code), config.Align)
@@ -61,29 +61,36 @@ func Write(writer io.Writer, code []byte, data []byte) {
 	}
 
 	dllAddresses = append(dllAddresses, 0)
-	importStart, importPadding := exe.Align(dataStart+len(data), config.Align)
-	itblSize := DLLImportSize*len(dlls) + DLLImportSize
-	iatblSize := 8 * len(dllAddresses)
-	itblStart := importStart
-	iatblStart := itblStart + itblSize
+
+	// Add the address table to the data section
+	functionAddressesStart := dataStart + len(data)
+	functionAddressesSize := 8 * len(dllAddresses)
+	data, err := binary.Append(data, binary.LittleEndian, &dllAddresses)
+
+	if err != nil {
+		panic(err)
+	}
 
 	dllImports = append(dllImports, DLLImport{
-		RvaFunctionNameList:    uint32(iatblStart),
+		RvaFunctionNameList:    uint32(functionAddressesStart),
 		TimeDateStamp:          0,
 		ForwarderChain:         0,
 		RvaModuleName:          uint32(dataStart + dllName),
-		RvaFunctionAddressList: uint32(iatblStart),
+		RvaFunctionAddressList: uint32(functionAddressesStart),
 	})
 
-	dllImports = append(dllImports, DLLImport{
-		RvaFunctionNameList:    0,
-		TimeDateStamp:          0,
-		ForwarderChain:         0,
-		RvaModuleName:          0, // must be zero
-		RvaFunctionAddressList: 0,
-	})
+	dllImports = append(dllImports, DLLImport{}) // a zeroed structure marks the end of the list
 
-	imageSize := iatblStart + iatblSize
+	// Add imports to the data section
+	importsStart := dataStart + len(data)
+	importsSize := DLLImportSize * len(dllImports)
+	data, err = binary.Append(data, binary.LittleEndian, &dllImports)
+
+	if err != nil {
+		panic(err)
+	}
+
+	imageSize := functionAddressesStart + functionAddressesSize
 	imageSize, _ = exe.Align(imageSize, config.Align)
 
 	pe := &EXE{
@@ -102,7 +109,7 @@ func Write(writer io.Writer, code []byte, data []byte) {
 			Characteristics:      IMAGE_FILE_EXECUTABLE_IMAGE | IMAGE_FILE_LARGE_ADDRESS_AWARE,
 		},
 		OptionalHeader64: OptionalHeader64{
-			Magic:                       0x020B, // PE32+ executable
+			Magic:                       0x020B, // PE32+ / 64-bit executable
 			MajorLinkerVersion:          0x0E,
 			MinorLinkerVersion:          0x16,
 			SizeOfCode:                  uint32(len(code)),
@@ -133,7 +140,7 @@ func Write(writer io.Writer, code []byte, data []byte) {
 			NumberOfRvaAndSizes:         16,
 			DataDirectory: [16]DataDirectory{
 				{VirtualAddress: 0, Size: 0},
-				{VirtualAddress: uint32(itblStart), Size: uint32(itblSize)}, // RVA of the imported function table
+				{VirtualAddress: uint32(importsStart), Size: uint32(importsSize)}, // RVA of the imported function table
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
@@ -144,7 +151,7 @@ func Write(writer io.Writer, code []byte, data []byte) {
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
-				{VirtualAddress: uint32(iatblStart), Size: uint32(iatblSize)}, // RVA of the import address table
+				{VirtualAddress: uint32(functionAddressesStart), Size: uint32(functionAddressesSize)}, // RVA of the import address table
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
 				{VirtualAddress: 0, Size: 0},
@@ -167,14 +174,6 @@ func Write(writer io.Writer, code []byte, data []byte) {
 				RawAddress:      uint32(dataStart),
 				Characteristics: IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ,
 			},
-			{
-				Name:            [8]byte{'.', 'i', 'd', 'a', 't', 'a'},
-				VirtualSize:     uint32(iatblSize + itblSize),
-				VirtualAddress:  uint32(importStart),
-				RawSize:         uint32(iatblSize + itblSize),
-				RawAddress:      uint32(importStart),
-				Characteristics: IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE,
-			},
 		},
 	}
 
@@ -187,7 +186,4 @@ func Write(writer io.Writer, code []byte, data []byte) {
 	writer.Write(code)
 	writer.Write(bytes.Repeat([]byte{0x00}, int(dataPadding)))
 	writer.Write(data)
-	writer.Write(bytes.Repeat([]byte{0x00}, int(importPadding)))
-	binary.Write(writer, binary.LittleEndian, &dllImports)
-	binary.Write(writer, binary.LittleEndian, &dllAddresses)
 }

src/exe/pe/pe.md

@@ -5,6 +5,13 @@
 Unlike Linux, Windows does not ignore zero-length sections at the end of a file and will
 fail loading them because they don't exist within the file. Adding a single byte to the
 section can fix this problem, but it's easier to just remove the section header entirely.
+The solution used here is to guarantee that the data section is never empty by always
+importing a few core functions from "kernel32.dll".
+
+## DLL function pointers
+
+The section where the DLL function pointers are stored does not need to be marked as writable.
+The Windows executable loader resolves the pointers before they are loaded into memory.
 
 ## Links