在动态链接的应用程序或共享库中，ELF的程序头描述表具有一个PT_DYNAMIC类型的描述符，它指?br>隽?dynamic段的位置，dynamic段用来描述动态链接过程。当应用程序调用的共享库函数时，要通过.plt段进行跳转。plt段又称为过程连接表，它是连接器ld所生成的一组静态的trampline，是只读的可执行的段，包含在.text段一起映射到内存。plt每16个字节为一个槽位，plt的第1个槽位保留给动态解析器使用，其余的槽位表示对不同共享库函数的调用。plt依赖于全局偏移量表(.got段)，GOT表是一可写的数据段，包含在.data段中一起映射到内存，用来存放共享符号的绝对地址。?br>τ贸绦虻饔霉蚕砜夂??褪峭ü齪lt槽位上的一条jmp指令跳转到GOT表所指的一个共享函数指针。这样，共享库的重定位就化为对GOT表项的重定位。GOT表的第1个指针指向.dynamic段，第2、3个?br>刚胗雙lt段的第1个槽位对应，用来安装动态解析器。为了少做无用功，Linux采用了动态解析技术，就是说在加载共享库时，并不进行函数的解析，而是安装动态解析器，让共享库调用指向解析器，只有当函数调用发生时才进行解析。为此，在ld在生成可执行程序时，让其GOT共享函数指针指?br>蚋髯詐lt槽位上的两条指令，一条是pushl指令，将该函数所对应GOT重定位表的索引作为参数压入堆栈，然后通过另一条jmp指令跳转到plt槽位1，它再跳转到GOT表第3个指针所表示的动态解析器?br>肟凇Ｕ庋?狈⑸?⒊晒馕瞿勘旰??诠蚕砜庵械牡刂肥保?煤??诔绦騁OT表中的指针就被实际的地址刷新。


下面是基于动态解释器ld.so-1.9.9版本的简单分析

简单的测试文件testso.c:

int x = 0;

int test()
{
return x;
}

用gcc -S -fPIC testso.c编绎成的汇编代码:

.globl x
.data
.align 4
.type x,@object
.size x,4
x:
.long 0
.text
.align 4
.globl test
.type test,@function
test:
pushl %ebp
movl %esp,%ebp
pushl %ebx
call .L2
.L2:
popl %ebx # 取标号.L2所在的地址
addl $_GLOBAL_OFFSET_TABLE_+[.-.L2],%ebx #_GLOBAL_OFFSET_TABLE_为当前地址到GOT表的偏移
movl x@GOT(%ebx),%eax # ebx在-fPIC编绎的函数中用于指向本模块的GOT表
movl (%eax),%eax # x@GOT表示符号x在GOT表中的索引
movl -4(%ebp),%ebx
leave
ret
.Lfe1:
.size test,.Lfe1-test

用gcc -shared testso.s -o testso.so生成共享库的反汇编的有关输出:

Disassembly of section .plt:

00000258 <.plt>:
258: ff b3 04 00 00 pushl 0x4(%ebx) #GOT表的第2个指针，对-fPIC编绎的函数，ebx总是指向GOT表
25d: 00
25e: ff a3 08 00 00 jmp *0x8(%ebx) #跳转到GOT表的第3个指针,调用__dl_linux_resolover
263: 00
264: 00 00 addb %al,(%eax)
266: 00 00 addb %al,(%eax)
268: ff a3 0c 00 00 jmp *0xc(%ebx) # 跳转到共享函数test()所在的GOT的指针
26d: 00
26e: 68 00 00 00 00 pushl x0 # test()所在GOT指针的初始入口
273: e9 e0 ff ff ff jmp 258 <_init+0x8> # 跳转到plt槽位1

Disassembly of section .text:

000002d8 :
2d8: 55 pushl %ebp
2d9: 89 e5 movl %esp,%ebp
2db: 53 pushl %ebx
2dc: e8 00 00 00 00 call 2e1
2e1: 5b popl %ebx
2e2: 81 c3 ab 10 00 addl x10ab,%ebx # 取GOT表指针
2e7: 00
2e8: 8b 83 10 00 00 movl 0x10(%ebx),%eax # 从GOT表中取变量x的地址
2ed: 00
2ee: 8b 00 movl (%eax),%eax
2f0: 8b 5d fc movl 0xfffffffc(%ebp),%ebx
2f3: c9 leave
2f4: c3 ret


引用testso的应用程序文件test.c:
main()
{
printf("%d\n",test());
}
用gcc test.c testso.so -o test生成可执行文件的反汇编输出：

Disassembly of section .plt:

08048398 <.plt>:
8048398: ff 35 54 95 04 pushl 0x8049554 # GOT表的第2个指针
804839d: 08
804839e: ff 25 58 95 04 jmp *0x8049558 #GOT表的第3个指针，运行_dl_linux_resolver
80483a3: 08
80483a4: 00 00 addb %al,(%eax)
80483a6: 00 00 addb %al,(%eax)
80483a8: ff 25 5c 95 04 jmp *0x804955c # printf()在plt的入口
80483ad: 08
80483ae: 68 00 00 00 00 pushl x0
80483b3: e9 e0 ff ff ff jmp 8048398 <_init+0x8>
80483b8: ff 25 60 95 04 jmp *0x8049560
80483bd: 08
80483be: 68 08 00 00 00 pushl x8
80483c3: e9 d0 ff ff ff jmp 8048398 <_init+0x8>
80483c8: ff 25 64 95 04 jmp *0x8049564 # test()在plt段的调用点
80483cd: 08 # [0x8048564]初始时指向0x80483ce
80483ce: 68 10 00 00 00 pushl x10 # test()在GOT表重定位表.rel.got中的索引
80483d3: e9 c0 ff ff ff jmp 8048398 <_init+0x8> # 跳转到plt的第1槽位
80483d8: ff 25 68 95 04 jmp *0x8049568
80483dd: 08
80483de: 68 18 00 00 00 pushl x18
80483e3: e9 b0 ff ff ff jmp 8048398 <_init+0x8>
80483e8: ff 25 6c 95 04 jmp *0x804956c
80483ed: 08
80483ee: 68 20 00 00 00 pushl x20
80483f3: e9 a0 ff ff ff jmp 8048398 <_init+0x8>
80483f8: ff 25 70 95 04 jmp *0x8049570
80483fd: 08
80483fe: 68 28 00 00 00 pushl x28
8048403: e9 90 ff ff ff jmp 8048398 <_init+0x8>

Disassembly of section .text:

080484c8 :
80484c8: 55 pushl %ebp
80484c9: 89 e5 movl %esp,%ebp
80484cb: e8 f8 fe ff ff call 80483c8 <_init+0x38> #
80484d0: 89 c0 movl %eax,%eax
80484d2: 50 pushl %eax
80484d3: 68 38 85 04 08 pushl x8048538
80484d8: e8 cb fe ff ff call 80483a8 <_init+0x18>
80484dd: 83 c4 08 addl x8,%esp
80484e0: c9 leave
80484e1: c3 ret

ld.so-1.9.9/d-link/i386/resolve.S

#define ALIGN 4
#define RUN linux_run
#define RESOLVE _dl_linux_resolve
#define RESOLVER _dl_linux_resolver
#define EXIT _interpreter_exit
#define INIT __loader_bootstrap

.text
.align ALIGN
.align 16

.globl RESOLVE
.type RESOLVE,@function
RESOLVE:
pusha
lea 0x20(%esp),%eax /* eax = tpnt and reloc_entry params */
pushl 4(%eax) /* push copy of reloc_entry param */
pushl (%eax) /* push copy of tpnt param */
pushl %eax /* _dl_linux_resolver expects a dummy
* param - this could be removed */
#ifdef __PIC__
call .L24
.L24:
popl %ebx
addl $_GLOBAL_OFFSET_TABLE_+[.-.L24],%ebx
movl RESOLVER@GOT(%ebx),%ebx /* eax = resolved func */
call *%ebx
#else
call RESOLVER
#endif
movl %eax,0x2C(%esp) /* store func addr over original
* tpnt param */
addl xC,%esp /* remove copy parameters */
popa /* restore regs */
ret /* jump to func removing original
* reloc_entry param from stack */
.LFE2:
.size RESOLVE,.LFE2-RESOLVE

d-link/i386/elfinterp.c:

unsigned int _dl_linux_resolver(int dummy, int i)
{
unsigned int * sp;
int reloc_entry;
int reloc_type;
struct elf32_rel * this_reloc;
char * strtab;
struct elf32_sym * symtab;
struct elf32_rel * rel_addr;
struct elf_resolve * tpnt;
int symtab_index;
char * new_addr;
char ** got_addr;
unsigned int instr_addr;
sp = &i;
reloc_entry = sp[1];
tpnt = (struct elf_resolve *) sp[0];

rel_addr = (struct elf32_rel *) (tpnt->dynamic_info[DT_JMPREL] +
tpnt->loadaddr); 取可执行程序的GOT重定位表

this_reloc = rel_addr + (reloc_entry >> 3);
reloc_type = ELF32_R_TYPE(this_reloc->r_info);
symtab_index = ELF32_R_SYM(this_reloc->r_info);

symtab = (struct elf32_sym *) (tpnt->dynamic_info[DT_SYMTAB] + tpnt->loadaddr);
strtab = (char *) (tpnt->dynamic_info[DT_STRTAB] + tpnt->loadaddr);


if (reloc_type != R_386_JMP_SLOT) {
_dl_fdprintf(2, "%s: Incorrect relocation type in jump relocations\n",
_dl_progname);
_dl_exit(1);
};

/* Address of jump instruction to fix up */
instr_addr = ((int)this_reloc->r_offset + (int)tpnt->loadaddr);
got_addr = (char **) instr_addr;

#ifdef DEBUG
_dl_fdprintf(2, "Resolving symbol %s\n",
strtab + symtab[symtab_index].st_name);
#endif

/* Get the address of the GOT entry */
new_addr = _dl_find_hash(strtab + symtab[symtab_index].st_name,
tpnt->symbol_scope, (int) got_addr, tpnt, 0);
if(!new_addr) {
_dl_fdprintf(2, "%s: can't resolve symbol '%s'\n",
_dl_progname, strtab + symtab[symtab_index].st_name);
_dl_exit(1);
};
/* #define DEBUG_LIBRARY */
#ifdef DEBUG_LIBRARY
if((unsigned int) got_addr < 0x40000000) {
_dl_fdprintf(2, "Calling library function: %s\n",
strtab + symtab[symtab_index].st_name);
} else {
*got_addr = new_addr;
}
#else
*got_addr = new_addr; 更新GOT函数指针
#endif
return (unsigned int) new_addr;
}

原文转自：http://www.ltesting.net