Just a code in assembly which open his own executable when he is mapped and executed in nasm (in order to do for example self mofifying code)

open_self_linux_nasm.asm

BITS 64

section .text
    global _start

_start:
    call _main__

    mov rax, 60
    mov rdi, 0x0
    syscall ; exit(0);

_main__:
    push rbp
    mov rbp, rsp
    sub rsp, 144 ; stat_file
    mov rdi, [rbp+0x18]
    lea rsi, [rsp]
    call _open_self ; open self
    push r12 ; len file
    push rax ; addr
    mov r14, rsi

    

    mov rdi, [rbp+0x18] ; pathname
    pop rsi ; addr
    pop rdx ; len
    push rdx
    push rsi
    call __create
    mov r13, rax ; second fd

    mov rdi, r14 ; fd
    pop rsi ; addr -> mmap
    pop rdx ; len_file
    call __close_unmap

    mov rax, 87
    mov rdi, [rbp+0x18]
    syscall

    mov rax, 0x3 ; close(scnd_fd);
    mov rdi, r13
    syscall

    mov rax, 86
    push 'nasm'
    lea rdi, [rsp]
    mov rsi, [rbp+0x18]
    syscall ; link tmp name to original name

    mov rax, 87
    lea rdi, [rsp]
    syscall ; delete old tmp file

    leave
    ret


; ===============================

; Open himself
_open_self:
    push rbp
    mov rbp, rsp

    mov r15, rsi ; &stat_file
    mov r12, rdi ; *pathname

    mov rax, 0x2
    mov rsi, 0x0 ; 0_RD
    mov rdx, 509
    syscall

    push rax ; fd

    mov rdi, rax ; fd
    mov rsi, r15 ; struct stat
    mov rax, 5 ; fstat
    syscall

    xor rdi, rdi
    mov rsi, qword [r15+48]
    mov rdx, 0x4
    mov r10, 0x2
    pop r8
    push r8
    mov r9, 0x0
    mov rax, 9
    syscall ; mmap

    ; rax -> byte of the executable that we gonna dump

    mov r12, qword [r15+48]

    pop rsi ; fd
    leave
    ret

; ===============================

; int __create(const char *pathname, void *addr, ssize_t len_bytes_mapped);

__create:
    push rbp
    mov rbp, rsp
    push rsi ; addr
    push rcx ; len

    push 'nasm'
    lea rdi, [rsp]
    mov rax, 0x2
    mov rsi, 0x42 ; 0_CREAT | O_RDWR
    mov rdx, 509
    syscall ; sys_open

    add rsp, 0x8 ; 'nasm'
    mov r9, rax ; fd
    mov rdi, rax ; fd

    mov rax, 0x1
    pop rdx
    pop rsi
    syscall ; sys_write

    mov rax, r9 ; fd final

    leave
    ret

; int __close_unmap(int fd, unsigned lon addr, ssize_t len_file);

__close_unmap:
    push rbp
    mov rbp, rsp

    push rdi

    mov rdi, rsi
    mov rsi, rdx
    mov rax, 11
    syscall ; munmap(addr, len_file)

    pop rdi
    mov rax, 3
    syscall ; close(fd);

    leave
    ret

;=========================================================

exit:
    mov rax, 60
    mov rdi, 0
    syscall

_ret:
    leave ; mov rsp, rbp ; pop rbp
    ret ; pop rip && jmp rip

;============================================