Jos*_*ain 12 c++ profiling valgrind
我正在使用valgrind/callgrind来配置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.但是我不知道这些是什么,似乎无法找到任何关于它们的文档.
有谁能告诉我这两个功能在哪里使用,他们做了什么？
Emi*_*ier 15
_dl_lookup_symbol_x是glibc C运行时库中的内部函数.如果您浏览 glibc的源代码,您会在_dl_lookup_symbol_x定义上方找到此注释:
/* Search loaded objects' symbol tables for a definition of the symbol
UNDEF_NAME, perhaps with a requested version for the symbol.
do_lookup_x仅仅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,但是从我可以收集的内容中,_dl_lookup_symbol_x在程序加载的共享库中查找符号(例如函数).
我不知道为什么在你的分析中经常调用这些函数,但至少现在你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经常负责调用_dl_lookup_symbol_x.
请注意,_dl_lookup_symbol_x在程序首次启动时多次调用是正常的,因为运行时会计算出具有给定名称的共享库函数的地址.如果你正在分析一个非常短命的程序,那么你会发现大多数时间花在内部"内务"功能而不是你自己的代码上并不奇怪.
从_dl_fixup调用dl_lookup_symbol，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）时中断，然后在延迟链接中使用lacc链接（这是使用gcc时的默认设置），则在该调用返回到main之前，实际上不会解析函数的地址。用来解析函数地址的过程之一是dl_lookup_symbol，因此对于使用惰性链接的每个程序调用的每个函数，您应该都希望看到一次。
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;
}
前往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
进入printf之后，下面的内容是完成懒惰链接的神奇结果的地方。在这里，我只是展示了我逐步执行该程序的过程，但是一旦我解析了printf，就可以执行以下步骤：
0x00005555555545a0 <+0>: jmp QWORD PTR [rip+0x200a72]
该程序将直接进入printf。因此，下次调用该函数jmp QWORD PTR [rip + 0x200a72]将直接带我们到printf。此后直到函数返回main的所有代码都只是对printf地址的解析，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。不确定gdb为什么不选择这些内容，它们只是.plt表中的说明。是的，地址是不一样的，我的系统向所有内容添加0x555555554000，然后再点击_start
(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)
从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]
gdb
(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。以下内容可能很难看。不是。它只是保存寄存器，以便0x00007ffff7def11a <+154>: call 0x7ffff7de7970 <_dl_fixup>中途调用可以使用所需的任何寄存器，这才是真正发生魔术的地方。最后，0x00007ffff7def1b0 <+304>：bnd jmp r11将带我们去打印一次该程序针对该特定功能的代码，因为这是该程序针对该特定功能的唯一代码。
(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将带我们进入printf。
最后，我们进入_dl_fixup，在这里我选择了对_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.
到此为止。dl_lookup_symbol函数用于解析elf文件并使用符号，而pfm正是在此发生的。