【android逆向】 ARM for 逆向

C源码

#include <stdio.h>
int nums[5] = {1, 2, 3, 4, 5};
int for1(int n){	//普通for循环
	int i = 0;
	int s = 0;
	for (i = 0; i < n; i++){
		s += i * 2;
	}
	return s;
}
int for2(int n){	//访问全局数组
	int i = 0;
	int s = 0;
	for (i = 0; i < n; i++){
		s += i * i + nums[n-1];
	}
	return s;
}
int main(int argc, char* argv[]){
	printf("for1:%d\n", for1(5));
	printf("for2:%d\n", for2(5));
	return 0;
}

android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_ARM_MODE := arm
LOCAL_MODULE    := for
LOCAL_SRC_FILES := for.c
include $(BUILD_EXECUTABLE)

逆向代码+注释

main

.text:000085FC ; int __cdecl main(int argc, const char **argv, const char **envp)
.text:000085FC main                                    ; CODE XREF: j_main↑j
.text:000085FC ; __unwind {
.text:000085FC                 PUSH    {R4,LR}
.text:00008600                 MOV     R0, #5
.text:00008604                 BL      for1
.text:00008608                 MOV     R1, R0
.text:0000860C                 LDR     R0, =(aFor1D - 0x8618) ; "for1:%d\n"
.text:00008610                 ADD     R0, PC, R0      ; "for1:%d\n"
.text:00008614                 BL      printf
.text:00008618                 MOV     R0, #5
.text:0000861C                 BL      for2
.text:00008620                 MOV     R1, R0
.text:00008624                 LDR     R0, =(aFor2D - 0x8630) ; "for2:%d\n"
.text:00008628                 ADD     R0, PC, R0      ; "for2:%d\n"
.text:0000862C                 BL      printf
.text:00008630                 MOV     R0, #0
.text:00008634                 POP     {R4,PC}
.text:00008634 ; End of function main
.text:00008634
.text:00008634 ; ---------------------------------------------------------------------------
.text:00008638 off_8638        DCD aFor1D - 0x8618     ; DATA XREF: main+10↑r
.text:00008638                                         ; "for1:%d\n"
.text:0000863C off_863C        DCD aFor2D - 0x8630     ; DATA XREF: main+28↑r
.text:0000863C ; } // starts at 85FC

for1

.text:00008590 ; =============== S U B R O U T I N E =======================================
.text:00008590
.text:00008590
.text:00008590 for1                                    ; CODE XREF: main+8↓p
.text:00008590 ; __unwind {
.text:00008590                 CMP     R0, #0
.text:00008594                 MOVLE   R0, #0          ; if arg0 <=0 s = 0
.text:00008598                 BXLE    LR              ; if arg0 <= 0 return s = 0
.text:0000859C                 MOV     R3, #0          ; i = 0;
.text:000085A0                 MOV     R2, R0,LSL#1    ; n = arg0*2 = 10
.text:000085A4                 MOV     R0, R3          ; s = 0
.text:000085A8
.text:000085A8 loc_85A8                                ; CODE XREF: for1+24↓j
.text:000085A8                 ADD     R0, R0, R3      ; s += i
.text:000085AC                 ADD     R3, R3, #2      ; i += 2
.text:000085B0                 CMP     R3, R2
.text:000085B4                 BNE     loc_85A8        ; if (i != n) continue
.text:000085B8                 BX      LR
.text:000085B8 ; } // starts at 8590
.text:000085B8 ; End of function for1

for2

.text:000085BC ; =============== S U B R O U T I N E =======================================
.text:000085BC
.text:000085BC
.text:000085BC for2                                    ; CODE XREF: main+20↓p
.text:000085BC ; __unwind {
.text:000085BC                 SUBS    R1, R0, #0
.text:000085C0                 MOVLE   R0, #0          ; s = 0
.text:000085C4                 BXLE    LR
.text:000085C8                 LDR     R3, =(__data_start_ptr - 0x85DC) ; nums 首地址在GOT中的偏移
.text:000085CC                 SUB     R2, R1, #1      ; n = arg0 - 1 = 4
.text:000085D0                 MOV     R0, #0          ; i = 0
.text:000085D4                 LDR     R3, [PC,R3]     ; __data_start ; num 首地址
.text:000085D8                 LDR     R12, [R3,R2,LSL#2] ; num + 4*4 = num[4]
.text:000085DC                 MOV     R3, R0          ; i = 0
.text:000085E0
.text:000085E0 loc_85E0                                ; CODE XREF: for2+34↓j
.text:000085E0                 MLA     R2, R3, R3, R12 ; R2 = i*i + num[4]
.text:000085E4                 ADD     R3, R3, #1      ; i++
.text:000085E8                 CMP     R3, R1          ; i < n
.text:000085EC                 ADD     R0, R0, R2      ; s += R2
.text:000085F0                 BNE     loc_85E0        ; i < n ;继续循环
.text:000085F4                 BX      LR
.text:000085F4 ; End of function for2