What is the smallest possible Windows (PE) executable?
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?
Solution 1:
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
-
sectalignlabel renamed tosect_align. Since the time this assembly code was writtensectalignbecame 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).
- Tiny PE; Alexander Sotirov; viewed 15/11/2017 @ 17:50 SAST
Solution 2:
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.