首先看看asm代码：
(gdb) disass main
Dump of assembler code for function main:
0x00008d3c <+0>: push {r4, r11, lr}
0x00008d40 <+4>: add r11, sp, #8
0x00008d44 <+8>: sub sp, sp, #12
0x00008d48 <+12>: mov r3, #0
0x00008d4c <+16>: str r3, [r11, #-16]
0x00008d50 <+20>: ldr r0, [pc, #104] ; 0x8dc0 <main+132>
0x00008d54 <+24>: bl 0xfb6c <printf>
0x00008d58 <+28>: sub r3, r11, #16
0x00008d5c <+32>: ldr r0, [pc, #96] ; 0x8dc4 <main+136>
0x00008d60 <+36>: mov r1, r3
0x00008d64 <+40>: bl 0xfbd8 <__isoc99_scanf>
0x00008d68 <+44>: bl 0x8cd4 <key1>
0x00008d6c <+48>: mov r4, r0
0x00008d70 <+52>: bl 0x8cf0 <key2>
0x00008d74 <+56>: mov r3, r0
0x00008d78 <+60>: add r4, r4, r3
0x00008d7c <+64>: bl 0x8d20 <key3>
0x00008d80 <+68>: mov r3, r0
0x00008d84 <+72>: add r2, r4, r3
0x00008d88 <+76>: ldr r3, [r11, #-16]
0x00008d8c <+80>: cmp r2, r3
0x00008d90 <+84>: bne 0x8da8 <main+108>
0x00008d94 <+88>: ldr r0, [pc, #44] ; 0x8dc8 <main+140>
0x00008d98 <+92>: bl 0x1050c <puts>
0x00008d9c <+96>: ldr r0, [pc, #40] ; 0x8dcc <main+144>
0x00008da0 <+100>: bl 0xf89c <system>
0x00008da4 <+104>: b 0x8db0 <main+116>
0x00008da8 <+108>: ldr r0, [pc, #32] ; 0x8dd0 <main+148>
0x00008dac <+112>: bl 0x1050c <puts>
0x00008db0 <+116>: mov r3, #0
0x00008db4 <+120>: mov r0, r3
0x00008db8 <+124>: sub sp, r11, #8
0x00008dbc <+128>: pop {r4, r11, pc}
0x00008dc0 <+132>: andeq r10, r6, r12, lsl #9
0x00008dc4 <+136>: andeq r10, r6, r12, lsr #9
0x00008dc8 <+140>: ; <UNDEFINED> instruction: 0x0006a4b0
0x00008dcc <+144>: ; <UNDEFINED> instruction: 0x0006a4bc
0x00008dd0 <+148>: andeq r10, r6, r4, asr #9
End of assembler dump.
(gdb) disass key1
Dump of assembler code for function key1:
0x00008cd4 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008cd8 <+4>: add r11, sp, #0
0x00008cdc <+8>: mov r3, pc
0x00008ce0 <+12>: mov r0, r3
0x00008ce4 <+16>: sub sp, r11, #0
0x00008ce8 <+20>: pop {r11} ; (ldr r11, [sp], #4)
0x00008cec <+24>: bx lr
End of assembler dump.
(gdb) disass key2
Dump of assembler code for function key2:
0x00008cf0 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008cf4 <+4>: add r11, sp, #0
0x00008cf8 <+8>: push {r6} ; (str r6, [sp, #-4]!)
0x00008cfc <+12>: add r6, pc, #1
0x00008d00 <+16>: bx r6
0x00008d04 <+20>: mov r3, pc
0x00008d06 <+22>: adds r3, #4
0x00008d08 <+24>: push {r3}
0x00008d0a <+26>: pop {pc}
0x00008d0c <+28>: pop {r6} ; (ldr r6, [sp], #4)
0x00008d10 <+32>: mov r0, r3
0x00008d14 <+36>: sub sp, r11, #0
0x00008d18 <+40>: pop {r11} ; (ldr r11, [sp], #4)
0x00008d1c <+44>: bx lr
End of assembler dump.
(gdb) disass key3
Dump of assembler code for function key3:
0x00008d20 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008d24 <+4>: add r11, sp, #0
0x00008d28 <+8>: mov r3, lr
0x00008d2c <+12>: mov r0, r3
0x00008d30 <+16>: sub sp, r11, #0
0x00008d34 <+20>: pop {r11} ; (ldr r11, [sp], #4)
0x00008d38 <+24>: bx lr
End of assembler dump.
(gdb) 再看看leg.c的源码：
#include <stdio.h>
#include <fcntl.h>
int key1(){
asm("mov r3, pc\n");
}
int key2(){
asm(
"push {r6}\n"
"add r6, pc, $1\n"
"bx r6\n"
".code 16\n"
"mov r3, pc\n"
"add r3, $0x4\n"
"push {r3}\n"
"pop {pc}\n"
".code 32\n"
"pop {r6}\n"
);
}
int key3(){
asm("mov r3, lr\n");
}
int main(){
int key=0;
printf("Daddy has very strong arm! : ");
scanf("%d", &key);
if( (key1()+key2()+key3()) == key ){
printf("Congratz!\n");
int fd = open("flag", O_RDONLY);
char buf[100];
int r = read(fd, buf, 100);
write(0, buf, r);
}
else{
printf("I have strong leg :P\n");
}
return 0;
}通过阅读这段c代码可以知道这段代码的意思便是判断key1、key2、key3是否都满足条件，满足，就输出flag，接下来对代码进行分析。
key1：
0x00008d68 <+44>: bl 0x8cd4 <key1>
0x00008d6c <+48>: mov r4, r0(gdb) disass key1
Dump of assembler code for function key1:
0x00008cd4 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008cd8 <+4>: add r11, sp, #0
0x00008cdc <+8>: mov r3, pc
0x00008ce0 <+12>: mov r0, r3
0x00008ce4 <+16>: sub sp, r11, #0
0x00008ce8 <+20>: pop {r11} ; (ldr r11, [sp], #4)
0x00008cec <+24>: bx lr
End of assembler dump.int key1(){
asm("mov r3, pc\n");
}这个key最简单，只要知道r0的返回值即可，r0=r3=pc，由于是pc，所以指向该语句的地址，所以key1是0x00008cdc+0x8；
key2：
0x00008d70 <+52>: bl 0x8cf0 <key2>
0x00008d74 <+56>: mov r3, r0
0x00008d78 <+60>: add r4, r4, r3(gdb) disass key2
Dump of assembler code for function key2:
0x00008cf0 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008cf4 <+4>: add r11, sp, #0
0x00008cf8 <+8>: push {r6} ; (str r6, [sp, #-4]!)
0x00008cfc <+12>: add r6, pc, #1
0x00008d00 <+16>: bx r6
0x00008d04 <+20>: mov r3, pc
0x00008d06 <+22>: adds r3, #4
0x00008d08 <+24>: push {r3}
0x00008d0a <+26>: pop {pc}
0x00008d0c <+28>: pop {r6} ; (ldr r6, [sp], #4)
0x00008d10 <+32>: mov r0, r3
0x00008d14 <+36>: sub sp, r11, #0
0x00008d18 <+40>: pop {r11} ; (ldr r11, [sp], #4)
0x00008d1c <+44>: bx lr
End of assembler dump.int key2(){
asm(
"push {r6}\n"
"add r6, pc, $1\n"
"bx r6\n"
".code 16\n"
"mov r3, pc\n"
"add r3, $0x4\n"
"push {r3}\n"
"pop {pc}\n"
".code 32\n"
"pop {r6}\n"
);
}第一次看这段代码感觉很繁琐，但是进一步对这段代码进行分析，可以一步步分析出key2的值，首先是pc指向的是8cfc+12，之后的0x00008d04 <+20>: mov r3, pc指令，所以综合分析得到key2的值为8d04+8;
key3：
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0x00008d7c <+64>: bl 0x8d20 <key3>
0x00008d80 <+68>: mov r3, r0
0x00008d84 <+72>: add r2, r4, r3(gdb) disass key3
Dump of assembler code for function key3:
0x00008d20 <+0>: push {r11} ; (str r11, [sp, #-4]!)
0x00008d24 <+4>: add r11, sp, #0
0x00008d28 <+8>: mov r3, lr
0x00008d2c <+12>: mov r0, r3
0x00008d30 <+16>: sub sp, r11, #0
0x00008d34 <+20>: pop {r11} ; (ldr r11, [sp], #4)
0x00008d38 <+24>: bx lr
End of assembler dump.
(gdb) int key3(){
asm("mov r3, lr\n");
}易得key3为8d80；
所以最终的key是三者之和，即为key1+key2+key3=10840(dec);
大功告成！！！！hhhh跨度了半天的一篇博客。