main()
{int shift=8;
unsigned long underflow=125656699,a=54781556;
__asm__("shrdl %2,%1,%0;"
:"=r"(underflow),"=r"(a)
:"c"((char)shift),""(underflow));
return a,underflow ;
}
这个程序中执行完shrdl指令后用`gdb`查看underflow=4093640704,a=3086766068;按书上讲的这个指令执行完以后'a'移动1字节到'underflow','a'本身不变吗，为什么实际的结果会不一样呢？请高手指点！
烟消云散 于 2013-06-06 00:30:17发表:
我是这样处理你的这个程序的
:
:
:
:
先用gcc编译成功了这个程序
在就是在终端中运行了这个程序但是没有任何的输出
然后我就把它反编译了反编译的结果如下：
整个执行过程如下：
主程序很短我就不再这对其进行分析了，从mian：开始的，如果你不了解汇编和反编译以及Linux系统汇编AT&T格式的话希望你还是先去了解一下在深入学习你的程序。
maker@maker-virtual-machine:~/桌面$ gcc 123.c -o 123
maker@maker-virtual-machine:~/桌面$ ./123
maker@maker-virtual-machine:~/桌面$ objdump -d 123
123： 文件格式 elf32-i386
Disassembly of section .init:
08048294 <_init>:
8048294: 53 push %ebx
8048295: 83 ec 08 sub $0x8,%esp
8048298: e8 83 00 00 00 call 8048320 <__x86.get_pc_thunk.bx>
804829d: 81 c3 63 1d 00 00 add $0x1d63,%ebx
80482a3: 8b 83 fc ff ff ff mov -0x4(%ebx),%eax
80482a9: 85 c0 test %eax,%eax
80482ab: 74 05 je 80482b2 <_init+0x1e>
80482ad: e8 1e 00 00 00 call 80482d0 <__gmon_start__@plt>
80482b2: 83 c4 08 add $0x8,%esp
80482b5: 5b pop %ebx
80482b6: c3 ret
Disassembly of section .plt:
080482c0 <__gmon_start__@plt-0x10>:
80482c0: ff 35 04 a0 04 08 pushl 0x804a004
80482c6: ff 25 08 a0 04 08 jmp *0x804a008
80482cc: 00 00 add %al,(%eax)
...
080482d0 <__gmon_start__@plt>:
80482d0: ff 25 0c a0 04 08 jmp *0x804a00c
80482d6: 68 00 00 00 00 push $0x0
80482db: e9 e0 ff ff ff jmp 80482c0 <_init+0x2c>
080482e0 <__libc_start_main@plt>:
80482e0: ff 25 10 a0 04 08 jmp *0x804a010
80482e6: 68 08 00 00 00 push $0x8
80482eb: e9 d0 ff ff ff jmp 80482c0 <_init+0x2c>
Disassembly of section .text:
080482f0 <_start>:
80482f0: 31 ed xor %ebp,%ebp
80482f2: 5e pop %esi
80482f3: 89 e1 mov %esp,%ecx
80482f5: 83 e4 f0 and $0xfffffff0,%esp
80482f8: 50 push %eax
80482f9: 54 push %esp
80482fa: 52 push %edx
80482fb: 68 90 84 04 08 push $0x8048490
8048300: 68 20 84 04 08 push $0x8048420
8048305: 51 push %ecx
8048306: 56 push %esi
8048307: 68 f0 83 04 08 push $0x80483f0
804830c: e8 cf ff ff ff call 80482e0 <__libc_start_main@plt>
8048311: f4 hlt
8048312: 66 90 xchg %ax,%ax
8048314: 66 90 xchg %ax,%ax
8048316: 66 90 xchg %ax,%ax
8048318: 66 90 xchg %ax,%ax
804831a: 66 90 xchg %ax,%ax
804831c: 66 90 xchg %ax,%ax
804831e: 66 90 xchg %ax,%ax
08048320 <__x86.get_pc_thunk.bx>:
8048320: 8b 1c 24 mov (%esp),%ebx
8048323: c3 ret
8048324: 66 90 xchg %ax,%ax
8048326: 66 90 xchg %ax,%ax
8048328: 66 90 xchg %ax,%ax
804832a: 66 90 xchg %ax,%ax
804832c: 66 90 xchg %ax,%ax
804832e: 66 90 xchg %ax,%ax
08048330
8048330: b8 1f a0 04 08 mov $0x804a01f,%eax
8048335: 2d 1c a0 04 08 sub $0x804a01c,%eax
804833a: 83 f8 06 cmp $0x6,%eax
804833d: 77 01 ja 8048340
804833f: c3 ret
8048340: b8 00 00 00 00 mov $0x0,%eax
8048345: 85 c0 test %eax,%eax
8048347: 74 f6 je 804833f
8048349: 55 push %ebp
804834a: 89 e5 mov %esp,%ebp
804834c: 83 ec 18 sub $0x18,%esp
804834f: c7 04 24 1c a0 04 08 movl $0x804a01c,(%esp)
8048356: ff d0 call *%eax
8048358: c9 leave
8048359: c3 ret
804835a: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
08048360
8048360: b8 1c a0 04 08 mov $0x804a01c,%eax
8048365: 2d 1c a0 04 08 sub $0x804a01c,%eax
804836a: c1 f8 02 sar $0x2,%eax
804836d: 89 c2 mov %eax,%edx
804836f: c1 ea 1f shr $0x1f,%edx
8048372: 01 d0 add %edx,%eax
8048374: d1 f8 sar %eax
8048376: 75 01 jne 8048379
8048378: c3 ret
8048379: ba 00 00 00 00 mov $0x0,%edx
804837e: 85 d2 test %edx,%edx
8048380: 74 f6 je 8048378
8048382: 55 push %ebp
8048383: 89 e5 mov %esp,%ebp
8048385: 83 ec 18 sub $0x18,%esp
8048388: 89 44 24 04 mov %eax,0x4(%esp)
804838c: c7 04 24 1c a0 04 08 movl $0x804a01c,(%esp)
8048393: ff d2 call *%edx
8048395: c9 leave
8048396: c3 ret
8048397: 89 f6 mov %esi,%esi
8048399: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi
080483a0 <__do_global_dtors_aux>:
80483a0: 80 3d 1c a0 04 08 00 cmpb $0x0,0x804a01c
80483a7: 75 13 jne 80483bc <__do_global_dtors_aux+0x1c>
80483a9: 55 push %ebp
80483aa: 89 e5 mov %esp,%ebp
80483ac: 83 ec 08 sub $0x8,%esp
80483af: e8 7c ff ff ff call 8048330
80483b4: c6 05 1c a0 04 08 01 movb $0x1,0x804a01c
80483bb: c9 leave
80483bc: f3 c3 repz ret
80483be: 66 90 xchg %ax,%ax
080483c0
80483c0: a1 10 9f 04 08 mov 0x8049f10,%eax
80483c5: 85 c0 test %eax,%eax
80483c7: 74 1f je 80483e8
80483c9: b8 00 00 00 00 mov $0x0,%eax
80483ce: 85 c0 test %eax,%eax
80483d0: 74 16 je 80483e8
80483d2: 55 push %ebp
80483d3: 89 e5 mov %esp,%ebp
80483d5: 83 ec 18 sub $0x18,%esp
80483d8: c7 04 24 10 9f 04 08 movl $0x8049f10,(%esp)
80483df: ff d0 call *%eax
80483e1: c9 leave
80483e2: e9 79 ff ff ff jmp 8048360
80483e7: 90 nop
80483e8: e9 73 ff ff ff jmp 8048360
80483ed: 66 90 xchg %ax,%ax
80483ef: 90 nop
080483f0
80483f0: 55 push %ebp
80483f1: 89 e5 mov %esp,%ebp
80483f3: 83 ec 10 sub $0x10,%esp
80483f6: c7 45 f4 08 00 00 00 movl $0x8,-0xc(%ebp)
80483fd: c7 45 f8 7b 5e 7d 07 movl $0x77d5e7b,-0x8(%ebp)
8048404: c7 45 fc 74 e6 43 03 movl $0x343e674,-0x4(%ebp)
804840b: 8b 45 f4 mov -0xc(%ebp),%eax
804840e: 89 c1 mov %eax,%ecx
8048410: 0f ad c2 shrd %cl,%eax,%edx
8048413: 89 55 f8 mov %edx,-0x8(%ebp)
8048416: 89 45 fc mov %eax,-0x4(%ebp)
8048419: 8b 45 f8 mov -0x8(%ebp),%eax
804841c: c9 leave
804841d: c3 ret
804841e: 66 90 xchg %ax,%ax
08048420 <__libc_csu_init>:
8048420: 55 push %ebp
8048421: 57 push %edi
8048422: 31 ff xor %edi,%edi
8048424: 56 push %esi
8048425: 53 push %ebx
8048426: e8 f5 fe ff ff call 8048320 <__x86.get_pc_thunk.bx>
804842b: 81 c3 d5 1b 00 00 add $0x1bd5,%ebx
8048431: 83 ec 1c sub $0x1c,%esp
8048434: 8b 6c 24 30 mov 0x30(%esp),%ebp
8048438: 8d b3 0c ff ff ff lea -0xf4(%ebx),%esi
804843e: e8 51 fe ff ff call 8048294 <_init>
8048443: 8d 83 08 ff ff ff lea -0xf8(%ebx),%eax
8048449: 29 c6 sub %eax,%esi
804844b: c1 fe 02 sar $0x2,%esi
804844e: 85 f6 test %esi,%esi
8048450: 74 27 je 8048479 <__libc_csu_init+0x59>
8048452: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
8048458: 8b 44 24 38 mov 0x38(%esp),%eax
804845c: 89 2c 24 mov %ebp,(%esp)
804845f: 89 44 24 08 mov %eax,0x8(%esp)
8048463: 8b 44 24 34 mov 0x34(%esp),%eax
8048467: 89 44 24 04 mov %eax,0x4(%esp)
804846b: ff 94 bb 08 ff ff ff call *-0xf8(%ebx,%edi,4)
8048472: 83 c7 01 add $0x1,%edi
8048475: 39 f7 cmp %esi,%edi
8048477: 75 df jne 8048458 <__libc_csu_init+0x38>
8048479: 83 c4 1c add $0x1c,%esp
804847c: 5b pop %ebx
804847d: 5e pop %esi
804847e: 5f pop %edi
804847f: 5d pop %ebp
8048480: c3 ret
8048481: eb 0d jmp 8048490 <__libc_csu_fini>
8048483: 90 nop
8048484: 90 nop
8048485: 90 nop
8048486: 90 nop
8048487: 90 nop
8048488: 90 nop
8048489: 90 nop
804848a: 90 nop
804848b: 90 nop
804848c: 90 nop
804848d: 90 nop
804848e: 90 nop
804848f: 90 nop
08048490 <__libc_csu_fini>:
8048490: f3 c3 repz ret
8048492: 66 90 xchg %ax,%ax
Disassembly of section .fini:
08048494 <_fini>:
8048494: 53 push %ebx
8048495: 83 ec 08 sub $0x8,%esp
8048498: e8 83 fe ff ff call 8048320 <__x86.get_pc_thunk.bx>
804849d: 81 c3 63 1b 00 00 add $0x1b63,%ebx
80484a3: 83 c4 08 add $0x8,%esp
80484a6: 5b pop %ebx
80484a7: c3 ret
maker@maker-virtual-machine:~/桌面$
deem_passion 于 2013-05-13 15:39:06发表:
不错，谢谢
zhaochangwei 于 2013-05-13 00:29:54发表:
飞过
haohu137 于 2013-05-09 19:05:48发表:
路过，看看。