Question

标签是数字常量符号，被链接器转换成对应的内存地址。

global _start

section .rodata
  hello  : db `hello, world!\n`

section .text  
  main:
    mov   rax, 1
    mov   rdi, 1
    mov   rsi, hello
    mov   rdx, 14
    syscall    
    jmp   exit 

  exit:
    mov   rax, 60
    xor   rdi, rdi
    syscall

  _start:
    jmp   main

$ objdump -d -M intel ./test

0000000000401000 <main>:
  401000:  b8 01 00 00 00       mov  eax,0x1
  401005:  bf 01 00 00 00       mov  edi,0x1
  40100a:  48 be 00 20 40 00 00   movabs rsi,0x402000
  401011:  00 00 00 
  401014:  ba 0e 00 00 00       mov  edx,0xe
  401019:  0f 05          syscall 
  40101b:  eb 00          jmp  40101d <exit>

000000000040101d <exit>:
  40101d:  b8 3c 00 00 00       mov  eax,0x3c
  401022:  48 31 ff         xor  rdi,rdi
  401025:  0f 05          syscall 

0000000000401027 <_start>:
  401027:  eb d7          jmp  401000 <main>

$ readelf -s test.o   # 编译

Symbol table '.symtab' contains 8 entries:
   Num:  Value      Size Type  Bind   Vis    Ndx Name
   0: 0000000000000000   0 NOTYPE  LOCAL  DEFAULT  UND 
   1: 0000000000000000   0 FILE  LOCAL  DEFAULT  ABS test.s
   2: 0000000000000000   0 SECTION LOCAL  DEFAULT  1 
   3: 0000000000000000   0 SECTION LOCAL  DEFAULT  2 
   4: 0000000000000000   1 OBJECT  LOCAL  DEFAULT  1 hello
   5: 0000000000000000   0 NOTYPE  LOCAL  DEFAULT  2 main
   6: 000000000000001d   0 NOTYPE  LOCAL  DEFAULT  2 exit
   7: 0000000000000027   0 NOTYPE  GLOBAL DEFAULT  2 _start

$ readelf -s test  # 链接

Symbol table '.symtab' contains 13 entries:
   Num:  Value      Size Type  Bind   Vis    Ndx Name
   0: 0000000000000000   0 NOTYPE  LOCAL  DEFAULT  UND 
   1: 0000000000401000   0 SECTION LOCAL  DEFAULT  1 
   2: 0000000000402000   0 SECTION LOCAL  DEFAULT  2 
   3: 0000000000000000   0 SECTION LOCAL  DEFAULT  3 
   4: 0000000000000000   0 SECTION LOCAL  DEFAULT  4 
   5: 0000000000000000   0 FILE  LOCAL  DEFAULT  ABS test.s
   6: 0000000000402000   1 OBJECT  LOCAL  DEFAULT  2 hello
   7: 0000000000401000   0 NOTYPE  LOCAL  DEFAULT  1 main
   8: 000000000040101d   0 NOTYPE  LOCAL  DEFAULT  1 exit
   9: 0000000000401027   0 NOTYPE  GLOBAL DEFAULT  1 _start
  10: 000000000040300e   0 NOTYPE  GLOBAL DEFAULT  2 __bss_start
  11: 000000000040300e   0 NOTYPE  GLOBAL DEFAULT  2 _edata
  12: 0000000000403010   0 NOTYPE  GLOBAL DEFAULT  2 _end

本地标签

以点（.）开头的标签被称作本地标签（local label），会自动添加前缀。这有助于将大段代码分成多个片段。且不用担心命名冲突，类似名字空间机制。

按源码顺序，前缀为上一个非本地标签名称。本地标签是一种符号命名机制，也是 LOCAL 符号。

global _start

section .rodata
  hello  : db `hello, world!\n`

section .text  
  main:
    jmp   .print

  .print:
    mov   rax, 1
    mov   rdi, 1
    mov   rsi, hello
    mov   rdx, 14
    syscall    
    jmp   .exit 

  .exit:
    mov   rax, 60
    xor   rdi, rdi
    syscall

  _start:
    mov   rax, main.print   ; 完整标签。
    jmp   rax

$ objdump -d -M intel ./test

0000000000401000 <main>:
  401000:  eb 00          jmp  401002 <main.print>

0000000000401002 <main.print>:
  401002:  b8 01 00 00 00       mov  eax,0x1
  401007:  bf 01 00 00 00       mov  edi,0x1
  40100c:  48 be 00 20 40 00 00   movabs rsi,0x402000
  401013:  00 00 00 
  401016:  ba 0e 00 00 00       mov  edx,0xe
  40101b:  0f 05          syscall 
  40101d:  eb 00          jmp  40101f <main.exit>

000000000040101f <main.exit>:
  40101f:  b8 3c 00 00 00       mov  eax,0x3c
  401024:  48 31 ff         xor  rdi,rdi
  401027:  0f 05          syscall 

0000000000401029 <_start>:
  401029:  48 b8 02 10 40 00 00   movabs rax,0x401002
  401030:  00 00 00 
  401033:  ff e0          jmp  rax

除标签外，还可使用 $ 表达地址。

• $ : 当前内存地址。
• $$ : 当前所在段（section）起始地址。

可在任意段（.data, .text）内使用。