用GDB跟蹤觀察共享庫函數的地址翻譯過程
用GDB觀察共享庫函數的翻譯過程
研究了一下共享庫函數是怎樣加載到當前進程中的.開始共享庫函數地址放在GOT中, 第一次調用時,ld將其翻譯成函數在程序空間的真實地址.用GDB跟蹤了一下整個過程, 記錄在下面.
PLT (Procedure Linkage Table) 和 GOT (Global Offset Table)背景
準備環境Ubuntu 11.04 amd64,安裝libc debug symbol.
sudo apt-get install libc-dbg
安裝libc6 source
sudo apt-get install build-essential
sudo apt-get source libc6
- main.c
#include "test.h"
int main() {
xyz = 100;
foo();
foo();
foo2();
foo2();
return 0;
}
- test.c
#include "test.h"
int xyz = 4;
int foo() {
return xyz;
}
int foo2() {
return xyz * 2;
}
- test.h
#ifndef TEST_H
#define TEST_H
extern int xyz;
int foo();
#endif
all:
gcc -g -fPIC -c -o test.o test.c
gcc -g -shared -Wl,-soname,libtest.so -o libtest.so test.o -lc
gcc -g -I. -L. main.c -o main.exe -ltest
tar:
tar zcvf main.tar.gz main.c test.c test.h Makefile
clean:
rm -f *.so *.o *.exe
.PHONY: all tar clean
mkdir whatever; cd whatever; make
main.c <---- 主程序
test.c <---- 共享庫
test.h
Makefile
foo & foo2是兩個共享庫中的函數，
[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
實驗
export LD_LIBRARY_PATH=$PWD
gdb main.exe
(gdb) break main
(gdb) run
Breakpoint 1, main () at main.c:4
4 xyz = 100;
### 加載ld的符號表，(/usr/lib/debug/lib/*是libc6-dbg安裝的debug symbol。)
### 注意 add-symbol-file的第三個參數，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() 現在是 <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 <xyz>
0x0000000000400682 <+14>: mov $0x0,%eax
0x0000000000400687 <+19>: callq 0x400578 <foo@plt> <----
0x000000000040068c <+24>: mov $0x0,%eax
0x0000000000400691 <+29>: callq 0x400578 <foo@plt>
(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.
</foo@plt></foo@plt></xyz>
### pushq是什麼？翻譯函數所需要的參數，這個是第一個參數reloc_index，是函數foo在GOT中的偏移量。
### $rip裡存了下一條指令,所以實際上將要執行順序下一條指令
(gdb) p/x 0x40057e + 0x200a92
$3 = 0x601010
### 這就是PLT的精妙之處，第一次執行，轉到哪裡去了呢？
?
(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
### 又一個pushq， link_map .got.plt,是翻譯需要的第二個參數。
### 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是怎麼工作的...
(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>
### 上面提到的第二個參數
(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 <----前面提到的兩個參數
41 call _dl_fixup # Call resolver.
42 movq %rax, %r11 # Save return value <----真正的共享庫裡函數地址
43 movq 48(%rsp), %r9 # Get register content back.
...
### 設置斷點，看地址在GOT表中的變化
(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 <foo@plt+6> <---- _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 <foo> <---- _dl_fixup 之後
</foo></foo@plt+6>
### 以後再次調用foo就直接到這裡了。
延伸閱讀
[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] 共享庫函數調用原理 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