启动期间生成的 ELF 可执行段错误答案

【问题标题】：Generated ELF executable segfaults during startup启动期间生成的 ELF 可执行段错误
【发布时间】：2016-11-27 20:51:21
【问题描述】：

我正在生成一个 ELF 可执行文件，其中 .text 部分加载到 LOAD 段中。它可以很好地反汇编，但尝试在gdb 下运行它会得到During startup program terminated with signal SIGSEGV, Segmentation fault.

readelf 给出：

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x400056
  Start of program headers:          759 (bytes into file)
  Start of section headers:          503 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         1
  Size of section headers:           64 (bytes)
  Number of section headers:         4
  Section header string table index: 2

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000400040  00000040
       000000000000005b  0000000000000000  AX       0     0     16
  [ 2] .strtab           STRTAB           0000000000000000  0000009b
       000000000000009c  0000000000000000           0     0     4
  [ 3] .symtab           SYMTAB           0000000000000000  00000137
       00000000000000c0  0000000000000018           2     1     4
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

There are no section groups in this file.

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000040 0x0000000000400000 0x0000000000400000
                 0x000000000000009b 0x000000000000009b  R E    200000

 Section to Segment mapping:
  Segment Sections...
   00     .text 

There is no dynamic section in this file.

There are no relocations in this file.

The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.

Symbol table '.symtab' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT    1 
     2: 0000000000400040    16 FUNC    GLOBAL DEFAULT    1 _R_Potato
     3: 0000000000400050     6 FUNC    GLOBAL DEFAULT    1 _R_Main
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     5: 0000000000400056    18 FUNC    GLOBAL DEFAULT    1 _start
     6: 0000000000400068    25 FUNC    GLOBAL DEFAULT    1 GetStringLength
     7: 0000000000400081    26 FUNC    GLOBAL DEFAULT    1 _R_Print

如果需要，可以在 here 找到 hexdump。我假设问题出在内核试图从 LOAD 段映射内容，但我没有看到问题。这是怎么回事？

【问题讨论】：

标签： compiler-construction posix elf

【解决方案1】：

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000040 0x0000000000400000 0x0000000000400000
                 0x000000000000009b 0x000000000000009b  R E    200000

这就是你的问题：你要求内核执行 mmap(0x400000, 0x9b, PROT_READ|PROT_EXEC, MAP_FIXED, $fd, 0x40) 的等效操作，但这样的 mmap 调用总是会失败并返回 EINVAL，因为地址和偏移量不是等效的模对齐。

您希望文件偏移量为0，而不是0x40。也就是说，您的LOAD 段应该涵盖both Elf64_Ehdr 和程序 .text 一起（LOAD 段的大小已经 em> 两者都包含）。

【讨论】：