谁执行运行时重定位？
谁执行运行时重定位？
提问于 2021-10-07 13:10:35
我试图更好地理解Linux中的运行时重定位，特别是在不同的情况下由谁来执行它们。下面是我目前的理解，准确吗？
- Position-dependent静态链接可执行程序没有运行时重定位needed
- Dynamically-linked可执行文件动态链接器(
ld.so)加载库，然后执行relocations - Statically链接饼libc启动代码执行relocations
- The动态链接器本身-
ld.so是一个自重新定位的二进制
。
谢谢
回答 1
Stack Overflow用户
回答已采纳
发布于 2021-10-08 04:19:22
你的答案(大部分)都是正确的。
您可以使用调试器观察重新定位的发生位置，并确认您的理解。
示例：
#include <stdio.h>
int main()
{
printf("%d\n", 123);
return 0;
}让我们从位置相关的动态链接二进制开始。
gcc -g -fno-pie -no-pie t.c
gdb -q ./a.out
Reading symbols from ./a.out...
(gdb) starti
Starting program: /tmp/a.out
Program stopped.
0x00007ffff7fd2090 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) disas main
Dump of assembler code for function main:
0x0000000000401126 <+0>: push %rbp
0x0000000000401127 <+1>: mov %rsp,%rbp
0x000000000040112a <+4>: mov $0x7b,%esi
0x000000000040112f <+9>: mov $0x402010,%edi
0x0000000000401134 <+14>: mov $0x0,%eax
0x0000000000401139 <+19>: call 0x401030 <printf@plt>
0x000000000040113e <+24>: mov $0x0,%eax
0x0000000000401143 <+29>: pop %rbp
0x0000000000401144 <+30>: ret
End of assembler dump.
(gdb) disas 0x401030
Dump of assembler code for function printf@plt:
0x0000000000401030 <+0>: jmp *0x2fe2(%rip) # 0x404018 <printf@got.plt>
0x0000000000401036 <+6>: push $0x0
0x000000000040103b <+11>: jmp 0x401020
End of assembler dump.在这里我们可以看到要重新定位的地址是0x404018。让我们看看地址是在哪里更新的：
(gdb) watch *(void**)0x404018
Hardware watchpoint 1: *(void**)0x404018
(gdb) c
Continuing.
Hardware watchpoint 1: *(void**)0x404018
Old value = (void *) 0x401036 <printf@plt+6>
New value = (void *) 0x7ffff7e54270 <printf>
0x00007ffff7fe0f10 in _dl_fixup (l=<optimized out>, reloc_arg=<optimized out>) at dl-runtime.c:146
146 dl-runtime.c: No such file or directory.
(gdb) bt
#0 0x00007ffff7fe0f10 in _dl_fixup (l=<optimized out>, reloc_arg=<optimized out>) at dl-runtime.c:146
#1 0x00007ffff7fe84fe in _dl_runtime_resolve_xsavec () at ../sysdeps/x86_64/dl-trampoline.h:126
#2 0x000000000040113e in main () at t.c:5
(gdb) info symbol $pc
_dl_fixup + 288 in section .text of /lib64/ld-linux-x86-64.so.2因此，在动态链接的非饼的情况下，它确实是动态加载器执行重定位。
注意:这是一个懒惰的迁移，并且正在按需进行，在main已经运行之后。
如果我们使它不懒惰，那么它将在调用main之前由ld.so执行：
gcc -g -fno-pie -no-pie t.c -Wl,-z,now重复上面的步骤，并观察在main之前发生的重定位
(gdb) run
Starting program: /tmp/a.out
Hardware watchpoint 1: *(void**)0x403fe8
Old value = (void *) 0x401036 <printf@plt+6>
New value = (void *) 0x7ffff7e54270
elf_machine_rela (skip_ifunc=<optimized out>, reloc_addr_arg=<optimized out>, version=<optimized out>, sym=<optimized out>, reloc=<optimized out>, map=<optimized out>)
at ../sysdeps/x86_64/dl-machine.h:464
464 ../sysdeps/x86_64/dl-machine.h: No such file or directory.
(gdb) bt
#0 elf_machine_rela (skip_ifunc=<optimized out>, reloc_addr_arg=<optimized out>, version=<optimized out>, sym=<optimized out>, reloc=<optimized out>, map=<optimized out>)
at ../sysdeps/x86_64/dl-machine.h:464
#1 elf_dynamic_do_Rela (skip_ifunc=<optimized out>, lazy=<optimized out>, nrelative=<optimized out>, relsize=<optimized out>, reladdr=<optimized out>, map=0x7ffff7ffe1a0)
at do-rel.h:137
#2 _dl_relocate_object (l=l@entry=0x7ffff7ffe1a0, scope=<optimized out>, reloc_mode=<optimized out>, consider_profiling=<optimized out>, consider_profiling@entry=0)
at dl-reloc.c:274
#3 0x00007ffff7fd555b in dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:2341
#4 0x00007ffff7fec1a2 in _dl_sysdep_start (start_argptr=start_argptr@entry=0x7fffffffe380, dl_main=dl_main@entry=0x7ffff7fd34c0 <dl_main>) at ../elf/dl-sysdep.c:252
#5 0x00007ffff7fd3021 in _dl_start_final (arg=0x7fffffffe380) at rtld.c:504
#6 _dl_start (arg=0x7fffffffe380) at rtld.c:597
#7 0x00007ffff7fd2098 in _start () from /lib64/ld-linux-x86-64.so.2重复其他构建组合。
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原文链接：
https://stackoverflow.com/questions/69481807
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