ELF文件解析

妖狐艹你老母 2023-02-12 12:21 54阅读 0赞

编译器编译源代码后生成的文件叫做**目标文件**，目标文件从结构上讲，已经是编译后的可执行文件格式，只是还没有经过链接的过程，其中有些符号还没有被调整。

现在PC平台主流的可执行文件格式有Windows下的PE和Linux下的ELF，它们都是COFF格式的变种。

不仅仅是可执行文件，动态链接库（win下的.dll，Linux下的.so）以及静态链接库（win下的.lib，Linux下的.a）都按照可执行文件格式存储。

通常c语言编译后机器代码保存在.text段，已初始化的全局变量和局部静态变量保存在.data段，未初始化的全局变量和局部静态变量一般放在.bss段里，.bss段只是为未初始化的全局变量和局部静态变量预留位置而已，它并没有内容，所在在文件中不占据空间。

## ELF Header ##

ELF文件格式的最前部是ELF文件头（ELF Header），它包含了描述整个文件的基本属性  
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// 32位机器
    typedef struct
    { 
      unsigned char	e_ident[EI_NIDENT];	/* Magic number and other info */
      Elf32_Half	e_type;			/* Object file type */
      Elf32_Half	e_machine;		/* Architecture */
      Elf32_Word	e_version;		/* Object file version */
      Elf32_Addr	e_entry;		/* Entry point virtual address */
      Elf32_Off	e_phoff;		/* Program header table file offset */
      Elf32_Off	e_shoff;		/* Section header table file offset */
      Elf32_Word	e_flags;		/* Processor-specific flags */
      Elf32_Half	e_ehsize;		/* ELF header size in bytes */
      Elf32_Half	e_phentsize;		/* Program header table entry size */
      Elf32_Half	e_phnum;		/* Program header table entry count */
      Elf32_Half	e_shentsize;		/* Section header table entry size */
      Elf32_Half	e_shnum;		/* Section header table entry count */
      Elf32_Half	e_shstrndx;		/* Section header string table index */
    } Elf32_Ehdr;
    
    // 64位机器
    typedef struct
    { 
      unsigned char	e_ident[EI_NIDENT];	/* Magic number and other info */
      Elf64_Half	e_type;			/* Object file type */
      Elf64_Half	e_machine;		/* Architecture */
      Elf64_Word	e_version;		/* Object file version */
      Elf64_Addr	e_entry;		/* Entry point virtual address */
      Elf64_Off	e_phoff;		/* Program header table file offset */
      Elf64_Off	e_shoff;		/* Section header table file offset */
      Elf64_Word	e_flags;		/* Processor-specific flags */
      Elf64_Half	e_ehsize;		/* ELF header size in bytes */
      Elf64_Half	e_phentsize;		/* Program header table entry size */
      Elf64_Half	e_phnum;		/* Program header table entry count */
      Elf64_Half	e_shentsize;		/* Section header table entry size */
      Elf64_Half	e_shnum;		/* Section header table entry count */
      Elf64_Half	e_shstrndx;		/* Section header string table index */
    } Elf64_Ehdr;

## program-headers ##

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/* Program segment header.  */
    
    typedef struct
    { 
      Elf32_Word	p_type;			/* Segment type */
      Elf32_Off	p_offset;		/* Segment file offset */
      Elf32_Addr	p_vaddr;		/* Segment virtual address */
      Elf32_Addr	p_paddr;		/* Segment physical address */
      Elf32_Word	p_filesz;		/* Segment size in file */
      Elf32_Word	p_memsz;		/* Segment size in memory */
      Elf32_Word	p_flags;		/* Segment flags */
      Elf32_Word	p_align;		/* Segment alignment */
    } Elf32_Phdr;
    
    typedef struct
    { 
      Elf64_Word	p_type;			/* Segment type */
      Elf64_Word	p_flags;		/* Segment flags */
      Elf64_Off	p_offset;		/* Segment file offset */
      Elf64_Addr	p_vaddr;		/* Segment virtual address */
      Elf64_Addr	p_paddr;		/* Segment physical address */
      Elf64_Xword	p_filesz;		/* Segment size in file */
      Elf64_Xword	p_memsz;		/* Segment size in memory */
      Elf64_Xword	p_align;		/* Segment alignment */
    } Elf64_Phdr

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## Section-Headers ##

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// 32位机器
    typedef struct
    { 
      Elf32_Word	sh_name;		/* Section name (string tbl index) */
      Elf32_Word	sh_type;		/* Section type */
      Elf32_Word	sh_flags;		/* Section flags */
      Elf32_Addr	sh_addr;		/* Section virtual addr at execution */
      Elf32_Off	sh_offset;		/* Section file offset */
      Elf32_Word	sh_size;		/* Section size in bytes */
      Elf32_Word	sh_link;		/* Link to another section */
      Elf32_Word	sh_info;		/* Additional section information */
      Elf32_Word	sh_addralign;		/* Section alignment */
      Elf32_Word	sh_entsize;		/* Entry size if section holds table */
    } Elf32_Shdr;
    
    // 64位机器
    typedef struct
    { 
      Elf64_Word	sh_name;		/* Section name (string tbl index) */
      Elf64_Word	sh_type;		/* Section type */
      Elf64_Xword	sh_flags;		/* Section flags */
      Elf64_Addr	sh_addr;		/* Section virtual addr at execution */
      Elf64_Off	sh_offset;		/* Section file offset */
      Elf64_Xword	sh_size;		/* Section size in bytes */
      Elf64_Word	sh_link;		/* Link to another section */
      Elf64_Word	sh_info;		/* Additional section information */
      Elf64_Xword	sh_addralign;		/* Section alignment */
      Elf64_Xword	sh_entsize;		/* Entry size if section holds table */
    } Elf64_Shdr;

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