我正在使用 valgrind/callgrind 来分析我的服务器代码以进行一些优化。callgrind 向我报告的两个最常用的调用（使用 kcachegrind 查看）是 _dl_lookup_symbol_x 和 do_lookup_x。但是，我不知道其中任何一个是什么，似乎也找不到任何关于它们的文档。
谁能告诉我这两个功能在哪里使用以及它们的作用？
问问题
8946 次
2 回答
16
_dl_lookup_symbol_x是 glibc C 运行时库中的一个内部函数。如果您浏览glibc 的源代码，您会在_dl_lookup_symbol_x定义上方找到以下注释：
/* Search loaded objects' symbol tables for a definition of the symbol
UNDEF_NAME, perhaps with a requested version for the symbol.
do_lookup_x只是在函数内调用的辅助_dl_lookup_symbol_x函数。
我不是 glibc 内部的专家，但据我所知，_dl_lookup_symbol_x在您的程序加载的共享库中查找符号（例如函数）。
我不知道为什么在您的分析中如此频繁地调用这些函数，但至少现在您对它们的作用有了一些线索。您的分析应该告诉您哪些函数负责_dl_lookup_symbol_x如此频繁地调用。
请注意，_dl_lookup_symbol_x在程序第一次启动时被多次调用是正常的，因为运行时会计算出具有给定名称的共享库函数的地址。如果您正在分析一个非常短暂的程序，那么您会发现大部分时间都花在了内部“管家”功能而不是您自己的代码上，这并不奇怪。
于 2012-08-02T00:40:03.247 回答
8
dl_lookup_symbol 从 _dl_fixup 调用，它从 _dl_runtime_resolve_sse 调用
在 gdb 中，如果您在第一个函数调用（任何调用）中中断，然后在惰性链接中（使用 gcc 时的默认设置），函数的地址实际上不会被解析，直到该调用几乎返回到 main 之后。用于解析函数地址的函数之一是 dl_lookup_symbol，因此对于每个使用延迟链接的程序调用的每个函数，几乎所有这些函数都应该看到一次。
源代码：
#include <stdio.h>
#include <string.h>
int main (void)
{
size_t a = strlen("Hello");
printf("%s is %d letters\n", "Hello", a);
if (a != sizeof("Hello") - 1)
{
perror("What?");
}
size_t b = strlen("Bye");
printf("%s is %d letters\n", "Bye", b);
if (b != sizeof("Bye") - 1)
{
perror("What?");
}
return 0;
}
去gdb：
(gdb) disas
Dump of assembler code for function main:
0x0000555555554700 <+0>: push rbp
0x0000555555554701 <+1>: mov rbp,rsp
0x0000555555554704 <+4>: sub rsp,0x10
0x0000555555554708 <+8>: mov QWORD PTR [rbp-0x8],0x5
0x0000555555554710 <+16>: mov rax,QWORD PTR [rbp-0x8]
0x0000555555554714 <+20>: mov rdx,rax
0x0000555555554717 <+23>: lea rsi,[rip+0xf6] # 0x555555554814
0x000055555555471e <+30>: lea rdi,[rip+0xf5] # 0x55555555481a
0x0000555555554725 <+37>: mov eax,0x0
=> 0x000055555555472a <+42>: call 0x5555555545a0 <printf@plt>
0x000055555555472f <+47>: cmp QWORD PTR [rbp-0x8],0x5
0x0000555555554734 <+52>: je 0x555555554742 <main+66>
0x0000555555554736 <+54>: lea rdi,[rip+0xef] # 0x55555555482c
0x000055555555473d <+61>: call 0x5555555545b0 <perror@plt>
0x0000555555554742 <+66>: mov QWORD PTR [rbp-0x10],0x3
0x000055555555474a <+74>: mov rax,QWORD PTR [rbp-0x10]
0x000055555555474e <+78>: mov rdx,rax
0x0000555555554751 <+81>: lea rsi,[rip+0xda] # 0x555555554832
0x0000555555554758 <+88>: lea rdi,[rip+0xbb] # 0x55555555481a
0x000055555555475f <+95>: mov eax,0x0
0x0000555555554764 <+100>: call 0x5555555545a0 <printf@plt>
0x0000555555554769 <+105>: cmp QWORD PTR [rbp-0x10],0x3
0x000055555555476e <+110>: je 0x55555555477c <main+124>
0x0000555555554770 <+112>: lea rdi,[rip+0xb5] # 0x55555555482c
0x0000555555554777 <+119>: call 0x5555555545b0 <perror@plt>
0x000055555555477c <+124>: mov eax,0x0
0x0000555555554781 <+129>: leave
0x0000555555554782 <+130>: ret
下面，在进入 printf 之后，是惰性链接的神奇结果在完成之后出现的地方。在这里，我只是展示了，当我逐步执行此操作时，程序将通过，但是一旦 printf 被解决，如果我在
0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72]
该程序将直接进入 printf。所以，下次调用函数jmp QWORD PTR [rip+0x200a72]将直接带我们到 printf。此后直到函数返回 main 的所有代码都只是解析 printf 的地址，并且仅在第一次调用该函数时发生。
(gdb) disas
Dump of assembler code for function printf@plt:
=> 0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72] # 0x555555755018
0x00005555555545a6 <+6>: push 0x0
0x00005555555545ab <+11>: jmp 0x555555554590
(gdb) disas
Dump of assembler code for function printf@plt:
=> 0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72] # 0x555555755018
0x00005555555545a6 <+6>: push 0x0
0x00005555555545ab <+11>: jmp 0x555555554590
End of assembler dump.
(gdb) stepi
0x00005555555545a6 in printf@plt ()
(gdb) disas
Dump of assembler code for function printf@plt:
0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72] # 0x555555755018
=> 0x00005555555545a6 <+6>: push 0x0
0x00005555555545ab <+11>: jmp 0x555555554590
End of assembler dump.
(gdb) stepi
0x00005555555545ab in printf@plt ()
(gdb) disas
Dump of assembler code for function printf@plt:
0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72] # 0x555555755018
0x00005555555545a6 <+6>: push 0x0
=> 0x00005555555545ab <+11>: jmp 0x555555554590
介入。不知道为什么 gdb 不选择这些，它们只是 .plt 表中的说明。是的，地址不一样，我的系统在我们点击 _start 之前将 0x555555554000 添加到所有内容中
(gdb) stepi
0x0000555555554590 in ?? ()
(gdb) disas
No function contains program counter for selected frame.
(gdb) stepi
0x0000555555554596 in ?? ()
(gdb) disas
No function contains program counter for selected frame.
， 证明：
$ gdb -q ./simpletest
Reading symbols from ./simpletest...done.
(gdb) break _start
Breakpoint 1 at 0x5d0
(gdb) run
Starting program: /flerb/flerb/flerb/simpletest
Breakpoint 1, 0x00005555555545d0 in _start ()
(gdb)
来自 objdump：
Disassembly of section .plt:
0000000000000590 <.plt>:
590: ff 35 72 0a 20 00 push QWORD PTR [rip+0x200a72] # 201008 <_GLOBAL_OFFSET_TABLE_+0x8>
596: ff 25 74 0a 20 00 jmp QWORD PTR [rip+0x200a74] # 201010 <_GLOBAL_OFFSET_TABLE_+0x10>
59c: 0f 1f 40 00 nop DWORD PTR [rax+0x0]
数据库
(gdb) stepi
_dl_runtime_resolve_sse () at ../sysdeps/x86_64/dl-trampoline.h:164
164 ../sysdeps/x86_64/dl-trampoline.h: No such file or directory.
嘿，_dl_runtime_resolve_sse，我们越来越近了。下面的内容可能看起来很难看。它不是。它只是保存寄存器，以便0x00007ffff7def11a <+154>: call 0x7ffff7de7970 <_dl_fixup>中途调用可以使用它需要的任何寄存器，而这确实是魔术发生的地方。在末尾 0x00007ffff7def1b0 <+304>: bnd jmp r11 将带我们到 printf 程序执行此特定功能的代码的这一次，因为这是程序执行此特定功能的代码的唯一时间。
(gdb) disas
Dump of assembler code for function _dl_runtime_resolve_sse:
=> 0x00007ffff7def080 <+0>: push rbx
0x00007ffff7def081 <+1>: mov rbx,rsp
0x00007ffff7def084 <+4>: and rsp,0xfffffffffffffff0
0x00007ffff7def088 <+8>: sub rsp,0x100
0x00007ffff7def08f <+15>: mov QWORD PTR [rsp+0xc0],rax
0x00007ffff7def097 <+23>: mov QWORD PTR [rsp+0xc8],rcx
0x00007ffff7def09f <+31>: mov QWORD PTR [rsp+0xd0],rdx
0x00007ffff7def0a7 <+39>: mov QWORD PTR [rsp+0xd8],rsi
0x00007ffff7def0af <+47>: mov QWORD PTR [rsp+0xe0],rdi
0x00007ffff7def0b7 <+55>: mov QWORD PTR [rsp+0xe8],r8
0x00007ffff7def0bf <+63>: mov QWORD PTR [rsp+0xf0],r9
0x00007ffff7def0c7 <+71>: movaps XMMWORD PTR [rsp],xmm0
0x00007ffff7def0cb <+75>: movaps XMMWORD PTR [rsp+0x10],xmm1
0x00007ffff7def0d0 <+80>: movaps XMMWORD PTR [rsp+0x20],xmm2
0x00007ffff7def0d5 <+85>: movaps XMMWORD PTR [rsp+0x30],xmm3
0x00007ffff7def0da <+90>: movaps XMMWORD PTR [rsp+0x40],xmm4
0x00007ffff7def0df <+95>: movaps XMMWORD PTR [rsp+0x50],xmm5
0x00007ffff7def0e4 <+100>: movaps XMMWORD PTR [rsp+0x60],xmm6
0x00007ffff7def0e9 <+105>: movaps XMMWORD PTR [rsp+0x70],xmm7
0x00007ffff7def0ee <+110>: bndmov [rsp+0x80],bnd0
0x00007ffff7def0f7 <+119>: bndmov [rsp+0x90],bnd1
0x00007ffff7def100 <+128>: bndmov [rsp+0xa0],bnd2
0x00007ffff7def109 <+137>: bndmov [rsp+0xb0],bnd3
0x00007ffff7def112 <+146>: mov rsi,QWORD PTR [rbx+0x10]
0x00007ffff7def116 <+150>: mov rdi,QWORD PTR [rbx+0x8]
0x00007ffff7def11a <+154>: call 0x7ffff7de7970 <_dl_fixup>
0x00007ffff7def11f <+159>: mov r11,rax
0x00007ffff7def122 <+162>: bndmov bnd3,[rsp+0xb0]
0x00007ffff7def12b <+171>: bndmov bnd2,[rsp+0xa0]
0x00007ffff7def134 <+180>: bndmov bnd1,[rsp+0x90]
0x00007ffff7def13d <+189>: bndmov bnd0,[rsp+0x80]
0x00007ffff7def146 <+198>: mov r9,QWORD PTR [rsp+0xf0]
0x00007ffff7def14e <+206>: mov r8,QWORD PTR [rsp+0xe8]
0x00007ffff7def156 <+214>: mov rdi,QWORD PTR [rsp+0xe0]
0x00007ffff7def15e <+222>: mov rsi,QWORD PTR [rsp+0xd8]
0x00007ffff7def166 <+230>: mov rdx,QWORD PTR [rsp+0xd0]
0x00007ffff7def16e <+238>: mov rcx,QWORD PTR [rsp+0xc8]
0x00007ffff7def176 <+246>: mov rax,QWORD PTR [rsp+0xc0]
0x00007ffff7def17e <+254>: movaps xmm0,XMMWORD PTR [rsp]
0x00007ffff7def182 <+258>: movaps xmm1,XMMWORD PTR [rsp+0x10]
0x00007ffff7def187 <+263>: movaps xmm2,XMMWORD PTR [rsp+0x20]
0x00007ffff7def18c <+268>: movaps xmm3,XMMWORD PTR [rsp+0x30]
0x00007ffff7def191 <+273>: movaps xmm4,XMMWORD PTR [rsp+0x40]
0x00007ffff7def196 <+278>: movaps xmm5,XMMWORD PTR [rsp+0x50]
0x00007ffff7def19b <+283>: movaps xmm6,XMMWORD PTR [rsp+0x60]
0x00007ffff7def1a0 <+288>: movaps xmm7,XMMWORD PTR [rsp+0x70]
0x00007ffff7def1a5 <+293>: mov rsp,rbx
0x00007ffff7def1a8 <+296>: mov rbx,QWORD PTR [rsp]
0x00007ffff7def1ac <+300>: add rsp,0x18
0x00007ffff7def1b0 <+304>: bnd jmp r11
End of assembler dump.
最后的 bnd jmp r11 将带我们到 printf。
最后，我们进入 _dl_fixup，我在其中挑出了对 _dl_lookup_symbol 的调用
(gdb) break *0x7ffff7def11a
Note: breakpoint 5 also set at pc 0x7ffff7def11a.
Breakpoint 8 at 0x7ffff7def11a: file ../sysdeps/x86_64/dl-trampoline.h, line 212.
(gdb) cont
Continuing.
Breakpoint 5, _dl_runtime_resolve_sse () at ../sysdeps/x86_64/dl-trampoline.h:212
212 in ../sysdeps/x86_64/dl-trampoline.h
(gdb) stepi
_dl_fixup (l=0x7ffff7ffe168, reloc_arg=0) at ../elf/dl-runtime.c:66
66 ../elf/dl-runtime.c: No such file or directory.
(gdb) disas
Dump of assembler code for function _dl_fixup:
=> 0x00007ffff7de7970 <+0>: push rbx
0x00007ffff7de7971 <+1>: mov r10,rdi
0x00007ffff7de7974 <+4>: mov esi,esi
0x00007ffff7de7976 <+6>: lea rdx,[rsi+rsi*2]
0x00007ffff7de797a <+10>: sub rsp,0x10
0x00007ffff7de797e <+14>: mov rax,QWORD PTR [rdi+0x68]
0x00007ffff7de7982 <+18>: mov rdi,QWORD PTR [rax+0x8]
0x00007ffff7de7986 <+22>: mov rax,QWORD PTR [r10+0xf8]
0x00007ffff7de798d <+29>: mov rax,QWORD PTR [rax+0x8]
0x00007ffff7de7991 <+33>: lea r8,[rax+rdx*8]
0x00007ffff7de7995 <+37>: mov rax,QWORD PTR [r10+0x70]
0x00007ffff7de7999 <+41>: mov rcx,QWORD PTR [r8+0x8]
0x00007ffff7de799d <+45>: mov rax,QWORD PTR [rax+0x8]
0x00007ffff7de79a1 <+49>: mov rdx,rcx
0x00007ffff7de79a4 <+52>: shr rdx,0x20
0x00007ffff7de79a8 <+56>: lea rsi,[rdx+rdx*2]
0x00007ffff7de79ac <+60>: lea rsi,[rax+rsi*8]
0x00007ffff7de79b0 <+64>: mov rax,QWORD PTR [r10]
0x00007ffff7de79b3 <+67>: mov QWORD PTR [rsp+0x8],rsi
0x00007ffff7de79b8 <+72>: mov rbx,rax
0x00007ffff7de79bb <+75>: add rbx,QWORD PTR [r8]
0x00007ffff7de79be <+78>: cmp ecx,0x7
0x00007ffff7de79c1 <+81>: jne 0x7ffff7de7b17 <_dl_fixup+423>
0x00007ffff7de79c7 <+87>: test BYTE PTR [rsi+0x5],0x3
0x00007ffff7de79cb <+91>: jne 0x7ffff7de7a67 <_dl_fixup+247>
0x00007ffff7de79d1 <+97>: mov rax,QWORD PTR [r10+0x1c8]
0x00007ffff7de79d8 <+104>: test rax,rax
0x00007ffff7de79db <+107>: je 0x7ffff7de7a90 <_dl_fixup+288>
0x00007ffff7de79e1 <+113>: mov rax,QWORD PTR [rax+0x8]
0x00007ffff7de79e5 <+117>: movzx eax,WORD PTR [rax+rdx*2]
0x00007ffff7de79e9 <+121>: and eax,0x7fff
0x00007ffff7de79ee <+126>: lea rdx,[rax+rax*2]
0x00007ffff7de79f2 <+130>: mov rax,QWORD PTR [r10+0x2e0]
0x00007ffff7de79f9 <+137>: lea r8,[rax+rdx*8]
0x00007ffff7de79fd <+141>: mov eax,0x0
0x00007ffff7de7a02 <+146>: mov r9d,DWORD PTR [r8+0x8]
0x00007ffff7de7a06 <+150>: test r9d,r9d
0x00007ffff7de7a09 <+153>: cmove r8,rax
0x00007ffff7de7a0d <+157>: mov edx,DWORD PTR fs:0x18
0x00007ffff7de7a15 <+165>: test edx,edx
0x00007ffff7de7a17 <+167>: mov eax,0x1
0x00007ffff7de7a1c <+172>: jne 0x7ffff7de7ab0 <_dl_fixup+320>
0x00007ffff7de7a22 <+178>: mov esi,DWORD PTR [rsi]
0x00007ffff7de7a24 <+180>: mov rcx,QWORD PTR [r10+0x380]
0x00007ffff7de7a2b <+187>: lea rdx,[rsp+0x8]
0x00007ffff7de7a30 <+192>: push 0x0
0x00007ffff7de7a32 <+194>: push rax
0x00007ffff7de7a33 <+195>: mov r9d,0x1
0x00007ffff7de7a39 <+201>: add rdi,rsi
0x00007ffff7de7a3c <+204>: mov rsi,r10
0x00007ffff7de7a3f <+207>: call 0x7ffff7de2d10 <_dl_lookup_symbol_x>
0x00007ffff7de7a44 <+212>: mov r8,rax
0x00007ffff7de7a47 <+215>: mov eax,DWORD PTR fs:0x18
0x00007ffff7de7a4f <+223>: test eax,eax
0x00007ffff7de7a51 <+225>: pop rcx
0x00007ffff7de7a52 <+226>: pop rsi
0x00007ffff7de7a53 <+227>: jne 0x7ffff7de7ad0 <_dl_fixup+352>
0x00007ffff7de7a55 <+229>: mov rsi,QWORD PTR [rsp+0x8]
0x00007ffff7de7a5a <+234>: test rsi,rsi
0x00007ffff7de7a5d <+237>: je 0x7ffff7de7aa0 <_dl_fixup+304>
0x00007ffff7de7a5f <+239>: test r8,r8
0x00007ffff7de7a62 <+242>: je 0x7ffff7de7aa8 <_dl_fixup+312>
0x00007ffff7de7a64 <+244>: mov rax,QWORD PTR [r8]
0x00007ffff7de7a67 <+247>: movzx edx,BYTE PTR [rsi+0x4]
0x00007ffff7de7a6b <+251>: add rax,QWORD PTR [rsi+0x8]
0x00007ffff7de7a6f <+255>: and edx,0xf
0x00007ffff7de7a72 <+258>: cmp dl,0xa
0x00007ffff7de7a75 <+261>: je 0x7ffff7de7b10 <_dl_fixup+416>
0x00007ffff7de7a7b <+267>: mov edx,DWORD PTR [rip+0x215267] # 0x7ffff7ffcce8 <_rtld_global_ro+72>
0x00007ffff7de7a81 <+273>: test edx,edx
0x00007ffff7de7a83 <+275>: jne 0x7ffff7de7a88 <_dl_fixup+280>
0x00007ffff7de7a85 <+277>: mov QWORD PTR [rbx],rax
0x00007ffff7de7a88 <+280>: add rsp,0x10
0x00007ffff7de7a8c <+284>: pop rbx
0x00007ffff7de7a8d <+285>: ret
0x00007ffff7de7a8e <+286>: xchg ax,ax
0x00007ffff7de7a90 <+288>: xor r8d,r8d
0x00007ffff7de7a93 <+291>: jmp 0x7ffff7de7a0d <_dl_fixup+157>
0x00007ffff7de7a98 <+296>: nop DWORD PTR [rax+rax*1+0x0]
0x00007ffff7de7aa0 <+304>: xor eax,eax
0x00007ffff7de7aa2 <+306>: jmp 0x7ffff7de7a7b <_dl_fixup+267>
0x00007ffff7de7aa4 <+308>: nop DWORD PTR [rax+0x0]
0x00007ffff7de7aa8 <+312>: xor eax,eax
0x00007ffff7de7aaa <+314>: jmp 0x7ffff7de7a67 <_dl_fixup+247>
0x00007ffff7de7aac <+316>: nop DWORD PTR [rax+0x0]
0x00007ffff7de7ab0 <+320>: mov DWORD PTR fs:0x1c,0x1
0x00007ffff7de7abc <+332>: mov eax,0x5
0x00007ffff7de7ac1 <+337>: jmp 0x7ffff7de7a22 <_dl_fixup+178>
0x00007ffff7de7ac6 <+342>: nop WORD PTR cs:[rax+rax*1+0x0]
0x00007ffff7de7ad0 <+352>: xor eax,eax
0x00007ffff7de7ad2 <+354>: xchg DWORD PTR fs:0x1c,eax
0x00007ffff7de7ada <+362>: cmp eax,0x2
0x00007ffff7de7add <+365>: jne 0x7ffff7de7a55 <_dl_fixup+229>
0x00007ffff7de7ae3 <+371>: mov rdi,QWORD PTR fs:0x10
0x00007ffff7de7aec <+380>: xor r10d,r10d
0x00007ffff7de7aef <+383>: add rdi,0x1c
0x00007ffff7de7af3 <+387>: mov edx,0x1
0x00007ffff7de7af8 <+392>: mov esi,0x81
0x00007ffff7de7afd <+397>: mov eax,0xca
0x00007ffff7de7b02 <+402>: syscall
0x00007ffff7de7b04 <+404>: jmp 0x7ffff7de7a55 <_dl_fixup+229>
0x00007ffff7de7b09 <+409>: nop DWORD PTR [rax+0x0]
0x00007ffff7de7b10 <+416>: call rax
0x00007ffff7de7b12 <+418>: jmp 0x7ffff7de7a7b <_dl_fixup+267>
0x00007ffff7de7b17 <+423>: lea rcx,[rip+0x10862] # 0x7ffff7df8380 <__PRETTY_FUNCTION__.10645>
0x00007ffff7de7b1e <+430>: lea rsi,[rip+0xea8f] # 0x7ffff7df65b4
0x00007ffff7de7b25 <+437>: lea rdi,[rip+0x1081c] # 0x7ffff7df8348
0x00007ffff7de7b2c <+444>: mov edx,0x4f
0x00007ffff7de7b31 <+449>: call 0x7ffff7df1210 <__GI___assert_fail>
End of assembler dump.
这就是这次旅程的全部内容。dl_lookup_symbol 函数用于解析 elf 文件和使用符号，这就是 pfm 发生的地方。
于 2017-06-22T01:57:13.027 回答