用GDB跟踪观察共享库函数的地址翻译过程

用GDB观察共享库函数的翻译过程

研究了一下共享库函数是怎样加载到当前进程中的.开始共享库函数地址放在GOT中,
第一次调用时,ld将其翻译成函数在程序空间的真实地址.用GDB跟踪了一下整个过程,
记录在下面.  

PLT (Procedure Linkage Table) 和 GOT (Global Offset Table)背景,google.  
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### 准备 ### ### 环境Ubuntu 11.04 amd64, ### 安装libc debug symbol.

 
sudo apt-get install libc-dbg

### 安装libc6 source，假设目录是~/codes/debsrc/eglibc-2.13

 
sudo apt-get install build-essential
sudo apt-get source libc6

### 使用实验源文件https://files.cnblogs.com/dyno/plt.zip,

 
mkdir whatever; cd whatever; make
main.c  <---- 主程序
test.c  <---- 共享库
test.h
Makefile

foo & foo2是两个共享库中的函数，

 
[dyno@ubuntu:plt]$ objdump --syms main.exe | grep -E "(foo|xyz)"
0000000000000000       F *UND*	0000000000000000              foo2 <---- 1
0000000000000000       F *UND*	0000000000000000              foo  <---- 2
0000000000601028 g     O .bss	0000000000000004              xyz

[dyno@ubuntu:plt]$ readelf --sections --wide main.exe | grep got 
  [22] .got              PROGBITS        0000000000600fe0 000fe0 000008 08  WA  0   0  8
  [23] .got.plt          PROGBITS        0000000000600fe8 000fe8 000030 08  WA  0   0  8

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### 实验 ###

 
export LD_LIBRARY_PATH=$PWD
gdb main.exe

(gdb) break main
(gdb) run
Breakpoint 1, main () at main.c:4
4       xyz = 100;

### 加载ld的符号表，(/usr/lib/debug/lib/*是libc6-dbg安装的debug symbol。)
### 注意 add-symbol-file的第三个参数，地址是如何得到的。

 
(gdb) info sharedlibrary 
From                To                  Syms Read   Shared Object Library
0x00007ffff7ddcaf0  0x00007ffff7df5a66  Yes (*)     /lib64/ld-linux-x86-64.so.2
0x00007ffff7bda500  0x00007ffff7bda628  Yes         /home/dyno/codes/plt/libtest.so
0x00007ffff7864c00  0x00007ffff79817ec  Yes         /lib/x86_64-linux-gnu/libc.so.6

(gdb) add-symbol-file /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so 0x00007ffff7ddcaf0
(gdb) directory ~/codes/debsrc/eglibc-2.13/elf
(gdb) set disassemble-next-line on

### foo() 现在是 <foo@plt>

 
(gdb) disassemble main
Dump of assembler code for function main:
   0x0000000000400674 <+0>: push   %rbp
   0x0000000000400675 <+1>: mov    %rsp,%rbp
   0x0000000000400678 <+4>: movl   $0x64,0x2009a6(%rip)        # 0x601028 
   0x0000000000400682 <+14>:	mov    $0x0,%eax
   0x0000000000400687 <+19>:	callq  0x400578  <----
   0x000000000040068c <+24>:	mov    $0x0,%eax
   0x0000000000400691 <+29>:	callq  0x400578 
...

(gdb) disassemble 0x400578
Dump of assembler code for function foo@plt:
   0x0000000000400578 <+0>: jmpq   *0x200a92(%rip)        # 0x601010 <_GLOBAL_OFFSET_TABLE_+40>
   0x000000000040057e <+6>: pushq  $0x2	         <----
   0x0000000000400583 <+11>:	jmpq   0x400548  <----
End of assembler dump.

### pushq是什么？翻译函数所需要的参数，这个是第一个参数reloc_index，是函数foo在GOT中的偏移量。
### $rip里存了下一条指令,所以实际上将要执行顺序下一条指令

 
(gdb) p/x 0x40057e + 0x200a92
$3 = 0x601010

### 这就是PLT的精妙之处，第一次执行，转到哪里去了呢？

 
(gdb) disassemble 0x400548
No function contains specified address.
(gdb) x/5i 0x400548
   0x400548:	pushq  0x200aa2(%rip)        # 0x600ff0 <_GLOBAL_OFFSET_TABLE_+8>  <----
   0x40054e:	jmpq   *0x200aa4(%rip)       # 0x600ff8 <_GLOBAL_OFFSET_TABLE_+16> <----
   0x400554:	nopl   0x0(%rax)
   0x400558 <__libc_start_main@plt>:	jmpq   *0x200aa2(%rip)        # 0x601000 <_GLOBAL_OFFSET_TABLE_+24>
   0x40055e <__libc_start_main@plt+6>:	pushq  $0x0

### 又一个pushq， link_map .got.plt,是翻译需要的第二个参数。
### 再次jumpq，where？where？

 
(gdb) x/a 0x600ff8
0x600ff8 <_GLOBAL_OFFSET_TABLE_+16>:	0x7ffff7df0760
(gdb) info symbol 0x7ffff7df0760
_dl_runtime_resolve in section .text of /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so

### 看看_dl_runtime_resolve是怎么工作的...

 
(gdb) break _dl_runtime_resolve
(gdb) info breakpoints 
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000000000400678 in main at main.c:4
    breakpoint already hit 1 time
2       breakpoint     keep y   0x00007ffff7df0760 ../sysdeps/x86_64/dl-trampoline.S:30

(gdb) si
0x0000000000400548 in ?? ()
=> 0x0000000000400548:	 ff 35 a2 0a 20 00  pushq  0x200aa2(%rip)        # 0x600ff0 <_GLOBAL_OFFSET_TABLE_+8>

### 上面提到的第二个参数

 
(gdb) x/x 0x600ff0
0x600ff0 <_GLOBAL_OFFSET_TABLE_+8>: 0x00007ffff7ffe2e8

(gdb) list _dl_runtime_resolve
...
29  _dl_runtime_resolve:
30  subq $56,%rsp
31  cfi_adjust_cfa_offset(72) # Incorporate PLT
32  movq %rax,(%rsp)    # Preserve registers otherwise clobbered.
...
(gdb) list +
...
39  movq 64(%rsp), %rsi # Copy args pushed by PLT in register.
40  movq 56(%rsp), %rdi # %rdi: link_map, %rsi: reloc_index  <----前面提到的两个参数
41  call _dl_fixup      # Call resolver.
42  movq %rax, %r11     # Save return value	     <----真正的共享库里函数地址
43  movq 48(%rsp), %r9  # Get register content back.
...

### 设置断点，看地址在GOT表中的变化

 
(gdb) info line _dl_runtime_resolve 
Line 30 of "../sysdeps/x86_64/dl-trampoline.S" starts at address 0x7ffff7df0760 <_dl_runtime_resolve>
   and ends at 0x7ffff7df0764 <_dl_runtime_resolve+4>.
(gdb) break ../sysdeps/x86_64/dl-trampoline.S:40
Breakpoint 3 at 0x7ffff7df078b: file ../sysdeps/x86_64/dl-trampoline.S, line 40.

(gdb) c
(gdb) x/a 0x601010
0x601010 <_GLOBAL_OFFSET_TABLE_+40>:	0x40057e   <---- _dl_fixup 之前
(gdb) ni
42  movq %rax, %r11     # Save return value
=> 0x00007ffff7df0795 <_dl_runtime_resolve+53>:	 49 89 c3   mov    %rax,%r11
(gdb) x/a 0x601010
0x601010 <_GLOBAL_OFFSET_TABLE_+40>:	0x7ffff7bda5cc   <---- _dl_fixup 之后

### 以后再次调用foo就直接到这里了。

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### 延伸阅读 ###

[1] Reversing the ELF Stepping with GDB during PLT uses and .GOT fixup
    http://packetstormsecurity.org/files/view/25642/elf-runtime-fixup.txt
[2] AMD64 Application Binary Interface (v 0.99)
    http://www.x86-64.org/documentation/abi.pdf
[3] PLT and GOT - the key to code sharing and dynamic libraries
    http://www.technovelty.org/linux/pltgot.html
[4] examining PLT/GOT structures
    http://althing.cs.dartmouth.edu/secref/resources/plt-got.txt
[5] Debugging with GDB
    http://sourceware.org/gdb/current/onlinedocs/gdb/
[6] 共享库函数调用原理
    http://blog.csdn.net/absurd/article/details/3169860
[7] How main() is executed on Linux 
    http://linuxgazette.net/issue84/hawk.html
[8] Gentle Introduction to x86-64 Assembly
    http://www.x86-64.org/documentation/assembly.html