Tag: tech

;Chastity's Source for ELF 64-bit executable creation
;
;All data as defined in this file is based off of the specification of the ELF file format.
;I first looked at the type of file created by FASM's "format ELF executable" directive.
;It is great that FASM can create an executable file automatically. (Thanks Tomasz Grysztar, you are a true warrior!)
;However, I wanted to understand the format for theoretical use in other assemblers like NASM.

;The Github repository with the spec I used is here.
;<https://github.com/xinuos/gabi>
;And this is the wikipedia article which linked me to the specification document
;<https://en.wikipedia.org/wiki/Executable_and_Linkable_Format>

;This rest of this file contains a raw binary ELF64 header created using db,dw,dd,dq commands.
;After that, it proceeds to assemble a real "Hello World!" program

;Header for 64 bit ELF executable (with comments based on specification)

db 0x7F,"ELF" ;ELFMAGIC: 4 bytes that identify this as an ELF file. The magic numbers you could say.
db 2          ;EI_CLASS: 1=32-bit 2=64-bit
db 1          ;EI_DATA: The endianness of the data. 1=ELFDATA2LSB 2=ELFDATA2MSB For Intel x86 this is always 1 as far as I know.
db 1          ;EI_VERSION: 1=EV_CURRENT (ELF identity version 1) (which is current at time of specification Version 4.2 I was using)
db 9 dup 0    ;padding zeros to bring us to address 0x10
dw 2          ;e_type: 2=ET_EXEC (executable instead of object file)
dw 0x3E       ;e_machine : 3=EM_386 (Intel 80386) 0x3E (AMD x86-64 architecture)
dd 1          ;e_version: 1=EV_CURRENT (ELF object file version.)

p_vaddr=0x400000
e_entry=0x400078 ;we will be reusing this constant later 

dq e_entry    ;e_entry: the virtual address at which the program starts
dq 0x40       ;e_phoff: where in the file the program header offset is
db 12 dup 0   ;e_shoff and e_flags are unused in this example,therefore all zeros
dw 0x40       ;e_ehsize: size of the ELF header
dw 0x38       ;e_phentsize: size of program header which happens after ELF header
dw 1          ;e_phnum: How many program headers. Only 1 in this case
dw 0x40       ;e_shentsize: Size of a section header
dw 0          ;e_shnum number of section headers
dw 0          ;e_shstrndx: section header string index (not used here)

;That is the end of the 0x40 byte (64 bytes decimal) ELF header. Sadly, this is not the end and a program header is also required (what drunk person made this format?)

dd 1           ;p_type: 1=PT_LOAD
dd 7           ;p_flags: permission flags: 7=4(Read)+2(Write)+1(Execute)
dq 0           ;p_offset: Base address from file (zero)
dq p_vaddr     ;p_vaddr: Virtual address in memory where the file will be.
dq p_vaddr     ;p_paddr: Physical address. Same as previous

image_size=0x1000 ;Chosen size for file and memory size. At minimum this must be as big as the actual binary file (code after header included)
                  ;By choosing a default size of 0x1000, I am assuming all assembly programs I write will be less than 4 kilobytes

dq image_size  ;p_filesz: Size of file image of the segment. Must be equal to the file size or greater
dq image_size  ;p_memsz: Size of memory image of the segment, which may be equal to or greater than file image.

dq 0           ;p_align; Alignment (none)

;important FASM directives
use64          ;tell assembler that 64 bit code is being used
org e_entry    ;origin of new code begins at the entry point

;now, the actual hello world program
mov rax,1   ; invoke SYS_WRITE (kernel opcode 1 on 64 bit systems)
mov rdi,1   ; write to the STDOUT file
mov rsi,msg ; pointer/address of string to write
mov rdx,13  ; number of bytes to write
syscall

mov rax,0x3C ; invoke SYS_EXIT (kernel opcode 0x3C on 64 bit systems)
mov rdi,0    ; return 0 status on exit - 'No Errors'
syscall

msg db 'Hello World!',0Ah

;This is the makefile I use when assembling and running this program

;main-fasm:
;	fasm ELF-64-hello.asm
;	chmod +x ELF-64-hello.bin
;	./ELF-64-hello.bin