可执行C程序中的段

时间：2012-08-09 13:33:24

标签： c linux memory-management memory-mapping segments

我正在阅读有关部分和细分的信息。似乎您可以列出部分和细分之间的映射，如下所示。

$ readelf -l test

Elf file type is EXEC (Executable file)
Entry point 0x8048330
There are 9 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
  INTERP         0x000154 0x08048154 0x08048154 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.2]
  LOAD           0x000000 0x08048000 0x08048000 0x0065c 0x0065c R E 0x1000
  LOAD           0x000f14 0x08049f14 0x08049f14 0x00104 0x00110 RW  0x1000
  DYNAMIC        0x000f28 0x08049f28 0x08049f28 0x000c8 0x000c8 RW  0x4
  NOTE           0x000168 0x08048168 0x08048168 0x00044 0x00044 R   0x4
  GNU_EH_FRAME   0x000564 0x08048564 0x08048564 0x00034 0x00034 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
  GNU_RELRO      0x000f14 0x08049f14 0x08049f14 0x000ec 0x000ec R   0x1

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame 
   03     .ctors .dtors .jcr .dynamic .got .got.plt .data .bss 
   04     .dynamic 
   05     .note.ABI-tag .note.gnu.build-id 
   06     .eh_frame_hdr 
   07     
   08     .ctors .dtors .jcr .dynamic .got

我的问题，

  1. 我无法理解程序标题的含义是什么？它们与细分市场有什么关系？
  2. 段到段映射是明确的。但有人可以命名吗？我只看到数字。我确定了代码seg（03），data seg（02）和stack（07）。

2 个答案:

答案 0 :(得分：12)

要了解readelf的输出，它将有助于您了解ELF文件的格式。请参考this document.

至于理解如何解释readelf .bss This section holds uninitialized data that contributes to the program's memory image. By definition, the system initializes the data with zeros when the program begins to run. This section is of type SHT_NOBITS. The attribute types are SHF_ALLOC and SHF_WRITE. .comment This section holds version control information. This section is of type SHT_PROGBITS. No attribute types are used. .ctors This section holds initialized pointers to the C++ constructor functions. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE. .data This section holds initialized data that contribute to the program's memory image. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE. .data1 This section holds initialized data that contribute to the program's memory image. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE. .debug This section holds information for symbolic debugging. The contents are unspecified. This section is of type SHT_PROGBITS. No attribute types are used. .dtors This section holds initialized pointers to the C++ destructor functions. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE. .dynamic This section holds dynamic linking information. The section's attributes will include the SHF_ALLOC bit. Whether the SHF_WRITE bit is set is processor-specific. This section is of type SHT_DYNAMIC. See the attributes above. .dynstr This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries. This section is of type SHT_STRTAB. The attribute type used is SHF_ALLOC. .dynsym This section holds the dynamic linking symbol table. This section is of type SHT_DYNSYM. The attribute used is SHF_ALLOC. .fini This section holds executable instructions that contribute to the process termination code. When a program exits normally the system arranges to execute the code in this section. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR. .gnu.version This section holds the version symbol table, an array of ElfN_Half elements. This section is of type SHT_GNU_versym. The attribute type used is SHF_ALLOC. .gnu.version_d This section holds the version symbol definitions, a table of ElfN_Verdef structures. This section is of type SHT_GNU_verdef. The attribute type used is SHF_ALLOC. .gnu.version_r This section holds the version symbol needed elements, a table of ElfN_Verneed structures. This section is of type SHT_GNU_versym. The attribute type used is SHF_ALLOC. .got This section holds the global offset table. This section is of type SHT_PROGBITS. The attributes are processor specific. .hash This section holds a symbol hash table. This section is of type SHT_HASH. The attribute used is SHF_ALLOC. .init This section holds executable instructions that contribute to the process initialization code. When a program starts to run the system arranges to execute the code in this section before calling the main program entry point. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR. .interp This section holds the pathname of a program interpreter. If the file has a loadable segment that includes the section, the section's attributes will include the SHF_ALLOC bit. Otherwise, that bit will be off. This section is of type SHT_PROGBITS. .line This section holds line number information for symbolic debugging, which describes the correspondence between the program source and the machine code. The contents are unspecified. This section is of type SHT_PROGBITS. No attribute types are used. .note This section holds information in the "Note Section" format. This section is of type SHT_NOTE. No attribute types are used. OpenBSD native executables usually contain a .note.openbsd.ident section to identify themselves, for the kernel to bypass any compatibility ELF binary emulation tests when loading the file. .note.GNU-stack This section is used in Linux object files for declaring stack attributes. This section is of type SHT_PROGBITS. The only attribute used is SHF_EXECINSTR. This indicates to the GNU linker that the object file requires an executable stack. .plt This section holds the procedure linkage table. This section is of type SHT_PROGBITS. The attributes are processor specific. .relNAME This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, "NAME" is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rel.text. This section is of type SHT_REL. .relaNAME This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, "NAME" is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rela.text. This section is of type SHT_RELA. .rodata This section holds read-only data that typically contributes to a nonwritable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC. .rodata1 This section holds read-only data that typically contributes to a nonwritable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC. .shstrtab This section holds section names. This section is of type SHT_STRTAB. No attribute types are used. .strtab This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. This section is of type SHT_STRTAB. .symtab This section holds a symbol table. If the file has a loadable segment that includes the symbol table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. This section is of type SHT_SYMTAB. .text This section holds the "text", or executable instructions, of a program. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR. 的输出可能会有所帮助。

关于您的问题2，link描述了细分。在该文档中搜索“各个部分保存程序和控制信息：”以查找描述段名称的区域。

该文件描述的段如下：

各个部分包含程序和控制信息：

{{1}}

答案 1 :(得分：3)

ELF二进制文件中的程序头描述了如何运行二进制文件。有趣的部分是LOAD头，它将部分二进制文件加载到内存中的不同位置。二进制文件中可能有几乎任意数量的LOAD标头，但通常链接器将所有只读和可执行文件放入一个，并将所有内容读/写到另一个中。有些操作系统将具有只读数据LOAD头，读写数据和只读可执行代码，以提高安全性。

这里的段只是指在内存中不同位置加载的二进制文件的一部分。所以基本上是不同的LOAD标题。

部分是链接期间数据的组织方式。出于各种原因，您希望更好地组织粒度而不仅仅是数据/代码。有些数据是只读的，在你的例子中放在“.rodata”中。代码在“.text”中，初始化数据在“.data”中，而在程序启动时归零的变量中的数据在“.bss”中。

“段到段映射”告诉您哪些段在哪些段（不同的LOAD头）。因此，“。text”和“.rodata”位于第一个LOAD标题（第三个程序标题）中，“。data”位于第二个LOAD标题中（第四个程序标题）。

堆栈是操作系统在执行时为您提供的，而不是由ELF二进制文件描述的。

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