Assembly code to dynamically respond to multi-processed HTTP GET requests

multi_processed_dynamic_http_get_requests.asm

.intel_syntax noprefix
.global _start

.section .text

_start:
    # create socket
    mov rdi, 2          # AF_INET
    mov rsi, 1          # SOCK_STREAM
    xor rdx, rdx        # IPPROTO_IP
    mov rax, 41         # sys_socket
    syscall

    mov r8, rax         # save the socket fd in r8

    # bind
    push 0              # padding
    push 0              # INADDR_ANY
    mov word ptr [rsp+2], 0x5000 # port 80 (network byte order)
    mov word ptr [rsp], 2        # AF_INET
    mov rdi, r8
    mov rsi, rsp
    mov rdx, 16
    mov rax, 49         # sys_bind
    syscall
    add rsp, 16

    # listen
    mov rdi, r8
    mov rsi, 0          # backlog
    mov rax, 50         # sys_listen
    syscall

accept_loop:
    # accept new connection
    mov rdi, r8
    xor rsi, rsi
    xor rdx, rdx
    mov rax, 43         # sys_accept
    syscall

    mov r9, rax         # save client fd

    # fork a new process
    mov rax, 57         # sys_fork
    syscall

    cmp rax, 0
    je handle_request   # child process handles the request
    
    # parent process
    mov rdi, r9
    mov rax, 3          # sys_close
    syscall             # close client fd in parent
    
    jmp accept_loop     # parent goes back to accepting

handle_request:
    # child process handles the connection
    mov rdi, r8
    mov rax, 3          # sys_close
    syscall             # close listening socket in child

    # read request
    mov rdi, r9
    lea rsi, [buffer]
    mov rdx, 1024
    mov rax, 0          # sys_read
    syscall

    # parse HTTP request to get file path
    lea rsi, [buffer + 4]  # skip "GET "
    lea rdi, [file_path]
    mov rcx, 256        # max path length

parse_path:
    cmp byte ptr [rsi], 0x20  # check for space
    je done_parsing
    cmp rcx, 0
    je done_parsing
    mov al, byte ptr [rsi]
    mov byte ptr [rdi], al
    inc rsi
    inc rdi
    dec rcx
    jmp parse_path

done_parsing:
    mov byte ptr [rdi], 0    # null terminate path

    # open file
    lea rdi, [file_path]
    xor rsi, rsi        # O_RDONLY
    mov rax, 2          # sys_open
    syscall

    cmp rax, 0
    js file_not_found

    mov r10, rax        # save fd

    # read file
    mov rdi, r10
    lea rsi, [file_content]
    mov rdx, 1024
    mov rax, 0          # sys_read
    syscall

    push rax            # save bytes read

    # close file
    mov rdi, r10
    mov rax, 3          # sys_close
    syscall

    # send header
    mov rdi, r9
    lea rsi, [response_header]
    mov rdx, 19
    mov rax, 1          # sys_write
    syscall

    # send file content
    pop rdx             # restore bytes read as length
    mov rdi, r9
    lea rsi, [file_content]
    mov rax, 1          # sys_write
    syscall

    jmp exit_child

file_not_found:
    # send 404
    mov rdi, r9
    lea rsi, [not_found_response]
    mov rdx, 26
    mov rax, 1          # sys_write
    syscall

exit_child:
    # close client socket
    mov rdi, r9
    mov rax, 3          # sys_close
    syscall

    # exit child process
    xor rdi, rdi
    mov rax, 60         # sys_exit
    syscall

.section .data
    response_header: .ascii "HTTP/1.0 200 OK\r\n\r\n"
    not_found_response: .ascii "HTTP/1.0 404 Not Found\r\n\r\n"

.section .bss
    buffer: .space 1024
    file_path: .space 256
    file_content: .space 1024