What is the smallest possible Windows (PE) executa

Question

As a precursor to writing a compiler I'm trying to understand the Windows (32-bit) Portable Executable format. In particular I'd like to see an example of a bare-bones executable which does nothing except load correctly, run and exit.

I've tried writing and compiling a simple C main function which does nothing but the resulting .exe is ~22KB and contains many imports from KERNEL32.DLL (presumably used by LIBC to set up environment, heaps etc.). Even the DOS Header could probably be smaller (it currently prints the default 'This program cannot be run in DOS mode').

What is the structure of the smallest possible Windows 32-bit executable?

Answer 1

On Windows XP (x32) the smallest PE executable is 97 bytes. On 32bit versions of Vista and 7 the smallest PE executable is 252 bytes. On 64bit versions of Windows the smallest 32bit executable is 268 bytes. On this forum you find a bit-map of such executable.

The smallest x64 PE executable is 268 bytes. It is even possible to execute every byte in an executable of this size. You can find a link on this forum as well.

The code below is a x64 PE (aka PE32+) executable file of size 268 bytes.

; PE64smallest.asm   Aug 19, 2018 (c) DrakoPensulo
; A smallest PE32+ executable (x64)
; 
; Features:
;  - Windows Vista/7/8/10 compatible
;  - Size: 268 bytes (an executable file on x64 Windows cannot be smaller)
;  - No sections
;  - No Data Directories (in particular no imports and no TLS callbacks)
;  - Exits with code 0x2a (this executable does nothing else than that)
;
;
; Compile using FASM (https://flatassembler.net)  command line: fasm.exe PE64smallest.asm

format binary as 'exe' 
use64 


EntryPoint:
db 'MZ'     ; DOS signature
dw 0faceh

dd 00004550h    ; Signature PE\0\0
dw 8664h    ; Machine
dw 0000h    ; NumberOfSections

dd 0facefaceh   ; TimeDateStamp

dd 0facefaceh   ; PointerToSymbolTable

dd 0facefaceh   ; NumberOfSymbols

dw 0        ; SizeOfOptionalHeader      ; must be multiple of 8 not too large 
dw 002fh    ; Characteristics       ; must be bit 1=1 bit 13=0

dw 020Bh    ; PE32+ Magic
db 0fah     ; MajorLinkerVersion
db 0fah     ; MinorLinkerVersion

dd 0facefaceh   ; SizeOfCode

dd 0facefaceh   ; SizeOfInitializedData

dd 0facefaceh   ; SizeOfUninitializedData

dd start    ; AddressOfEntryPoint       ; cannot be smaller than SizeOfHeaders

dd 0facefaceh   ; BaseOfCode

dq 0000000100000000h    ; ImageBase     ; must be multiple of 64k

dd 4        ; SectionAlignment and e_lfanew ; PE header offset in file

dd 4        ; FileAlignment

dw 0faceh   ; MajorOperatingSystemVersiom

dw 0faceh   ; MinorOperatingSystemVersion

dw 0faceh   ; MajorImageVersion

dw 0faceh   ; MinorImageVersion

dw 5        ; MajorSubsystemVersion     ; >3.1 or 4  
dw 0h       ; MinorSubsystemVersion

dd 0facefaceh   ; Win32VersionValue     

dd 0400h    ; SizeOfImage           ; MSB has to be small, must be >0200h

dd start    ; SizeOfHeaders         ; SizeOfHeaders has to be < SizeOfImage

dd 0facefaceh   ; CheckSum

dw 0002h    ; Subsystem 2-GUI 3-CUI
dw 0        ; DllCharacteristics

dd 000cefaceh
dd 0        ; SizeOfStackReserve  upper dword has to be 0, MSB of lower dword has to be small

dd 000cefaceh
dd 0        ; SizeOfStackCommit  upper dword has to be 0, MSB of lower dword has to be small

dd 000cefaceh
dd 0        ; SizeOfHeapReserve  upper dword has to be 0, MSB of lower dword has to be small

dd 000cefaceh
dd 0        ; SizeOfHeapCommit  upper dword has to be 0, MSB of lower dword has to be small

dd 0facefaceh   ; LoaderFlags

dd 0        ; NumberofRvaAndSizes   

dd 0facefaceh
dd 0facefaceh   ; Export Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Import Directory Address and Size

dd 0facefaceh   
dd 0facefaceh   ; Resource Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Exception Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Security Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Base Relocation Table Address and Size

    dd 0facefaceh
dd 0facefaceh   ; Debug Directory Address and Size

dd 0facefaceh   
dd 0facefaceh   ; Architecture Specific Data Address and Size

dd 0facefaceh
dd 0facefaceh   ; RVA of GlobalPtr Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; TLS Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Load Configuration Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Bound Import Directory Address and Size

dd 0facefaceh
dd 0facefaceh   ; Import Address Table Address and Size

dd 0facefaceh
dd 0facefaceh   ; Delay Load Import Descriptors Address and Size

dd 0facefaceh
dd 0facefaceh   ; COM runtime Descriptors Address and Size

dd 0facefaceh

start:
push 2ah
pop rax
ret     ; Reserved Descriptor

BTW On this blog entry you find a small (316 bytes) x32 executable with assembler source code and many technical details.

Answer 2

As quoted from source (Creating the smallest possible PE executable): ¹

• Smallest possible PE file: 97 bytes
• Smallest possible PE file on Windows 2000: 133 bytes
• Smallest PE file that downloads a file over WebDAV and executes it: 133 bytes

The files above are the smallest possible PE files due to requirements of the PE file format and cannot be improved further.

This result was achieved with some clever NASM tricks, such as removing the step that links to C stdlib and removing a number of header fields and data directories.

The full source code is below. It is effectively the same as the article with these modification:

• Removal of blank lines
• sectalign label renamed to sect_align. Since the time this assembly code was written sectalign became a NASM keyword. Rename it to avoid warnings and errors.

The code is as follows:

; tiny97.asm, copyright Alexander Sotirov

BITS 32
;
; MZ header
; The only two fields that matter are e_magic and e_lfanew

mzhdr:
    dw "MZ"       ; e_magic
    dw 0          ; e_cblp UNUSED

; PE signature
pesig:
    dd "PE"       ; e_cp, e_crlc UNUSED       ; PE signature

; PE header
pehdr:
    dw 0x014C     ; e_cparhdr UNUSED          ; Machine (Intel 386)
    dw 1          ; e_minalloc UNUSED         ; NumberOfSections

;   dd 0xC3582A6A ; e_maxalloc, e_ss UNUSED   ; TimeDateStamp UNUSED

; Entry point
start:
    push byte 42
    pop eax
    ret

codesize equ $ - start

    dd 0          ; e_sp, e_csum UNUSED       ; PointerToSymbolTable UNUSED
    dd 0          ; e_ip, e_cs UNUSED         ; NumberOfSymbols UNUSED
    dw sections-opthdr ; e_lsarlc UNUSED      ; SizeOfOptionalHeader
    dw 0x103      ; e_ovno UNUSED             ; Characteristics

; PE optional header
; The debug directory size at offset 0x94 from here must be 0

filealign equ 4
sect_align equ 4  ; must be 4 because of e_lfanew

%define round(n, r) (((n+(r-1))/r)*r)

opthdr:
    dw 0x10B      ; e_res UNUSED              ; Magic (PE32)
    db 8                                      ; MajorLinkerVersion UNUSED
    db 0                                      ; MinorLinkerVersion UNUSED

; PE code section
sections:
    dd round(codesize, filealign)  ; SizeOfCode UNUSED  ; Name UNUSED
    dd 0  ; e_oemid, e_oeminfo UNUSED ; SizeOfInitializedData UNUSED
    dd codesize  ; e_res2 UNUSED  ; SizeOfUninitializedData UNUSED  ; VirtualSize
    dd start  ; AddressOfEntryPoint  ; VirtualAddress
    dd codesize  ; BaseOfCode UNUSED  ; SizeOfRawData
    dd start  ; BaseOfData UNUSED  ; PointerToRawData
    dd 0x400000  ; ImageBase  ; PointerToRelocations UNUSED
    dd sect_align ; e_lfanew  ; SectionAlignment  ; PointerToLinenumbers UNUSED
    dd filealign  ; FileAlignment  ; NumberOfRelocations, NumberOfLinenumbers UNUSED
    dw 4  ; MajorOperatingSystemVersion UNUSED ; Characteristics UNUSED
    dw 0  ; MinorOperatingSystemVersion UNUSED
    dw 0  ; MajorImageVersion UNUSED
    dw 0  ; MinorImageVersion UNUSED
    dw 4  ; MajorSubsystemVersion
    dw 0  ; MinorSubsystemVersion UNUSED
    dd 0  ; Win32VersionValue UNUSED
    dd round(hdrsize, sect_align)+round(codesize,sect_align) ; SizeOfImage
    dd round(hdrsize, filealign)  ; SizeOfHeaders
    dd 0  ; CheckSum UNUSED
    db 2  ; Subsystem (Win32 GUI)

hdrsize equ $ - $$
filesize equ $ - $$

To build into an executable use:

nasm -f bin tiny97.asm -o tiny97.exe

For GNU/Linux ELF executables, See the article "Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux". TL;DR: 1340 bytes, using NASM

Note: This answer is an expansion of J...'s comment on Dec 3 '16 at 17:31, in order to preserve the information found in the link (in case that too goes dead).

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1. Tiny PE; Alexander Sotirov; viewed 15/11/2017 @ 17:50 SAST