ELF文件类型：
1. 待重定位文件 (relocatable file)
2. 共享目标文件 (shared object file)
3. 可执行文件 (executable file)
ELF文件分为文件头和文件体两部分,文件头用来描述整个程序文件的组织结构，文件体则是真正的程序文件。
同时，ELF文件又将程序体分为不同的段(segment)和节(section)。程序中有很多段，如代码段和数据段等，同样也有很多节，段是由节组成的，多个节经过链接之后就被合并成一个段。
ELF文件使用程序头表(program header table)来描述段信息，使用节头表(section header table)来描述节信息。由于段和节信息不是固定不变的，所以两表在文件中位置也不固定。因此，需要一个固定位置，固定大小的数据结构，来描述程序头表和节头表，这个数据就是ELF文件头。
ELF文件结构具体布局如下:
overview.png
ELF文件的相关定义可在linux系统/usr/include/elf.h找到
文件头结构
/* Type for a 16-bit quantity. */
typedef uint16_t Elf32_Half;
/* Types for signed and unsigned 32-bit quantities. */
typedef uint32_t Elf32_Word;
/* Type of addresses. */
typedef uint32_t Elf32_Addr;
/* Type of file offsets. */
typedef uint32_t Elf32_Off;
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 */
}
-
e_ident[EI_NIDENT]，16个字节
e_ident.png
-
e_type,2个字节，指定elf文件类型
type.png
-
e_machine， 2字节 , 表示在哪种硬件平台执行
e_machine.png
-
e_version, 4字节，表示版本信息 -
e_entry，4字节，指明操作系统运行该程序时，将控制权转交到的虚拟地址 -
e_phoff，4字节，指明程序头表在文件中的字节偏移量。0表示没有 -
e_shoff，4字节，指明节头标偏移，0表示没有 -
e_flags，4字节，表示处理器相关标志 -
e_ehsize，2字节，指明文件头大小 -
e_phentsize，2字节，指明程序头表中每个条目的大小 -
e_phnum，2字节，指明程序头表中条目数量，即段个数 -
e_shentsize，2字节，指明节头表中每个条目的大小 -
e_shnum，2字节，指明节头标中条目的数量，即节个数 -
e_shstrndx，用来指明string name table 在节头表中的索引index
程序头表结构(program header table)
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;
p_type:
/* Legal values for p_type (segment type). */
#define PT_NULL 0 /* Program header table entry unused */
#define PT_LOAD 1 /* Loadable program segment */
#define PT_DYNAMIC 2 /* Dynamic linking information */
#define PT_INTERP 3 /* Program interpreter */
#define PT_NOTE 4 /* Auxiliary information */
#define PT_SHLIB 5 /* Reserved */
#define PT_PHDR 6 /* Entry for header table itself */
#define PT_TLS 7 /* Thread-local storage segment */
#define PT_NUM 8 /* Number of defined types */
#define PT_LOOS 0x60000000 /* Start of OS-specific */
#define PT_GNU_EH_FRAME 0x6474e550 /* GCC .eh_frame_hdr segment */
#define PT_GNU_STACK 0x6474e551 /* Indicates stack executability */
#define PT_GNU_RELRO 0x6474e552 /* Read-only after relocation */
#define PT_LOSUNW 0x6ffffffa
#define PT_SUNWBSS 0x6ffffffa /* Sun Specific segment */
#define PT_SUNWSTACK 0x6ffffffb /* Stack segment */
#define PT_HISUNW 0x6fffffff
#define PT_HIOS 0x6fffffff /* End of OS-specific */
#define PT_LOPROC 0x70000000 /* Start of processor-specific */
#define PT_HIPROC 0x7fffffff /* End of processor-specific */
节头表(section header table)
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;
sh_type:
/* Legal values for sh_type (section type). */
#define SHT_NULL 0 /* Section header table entry unused */
#define SHT_PROGBITS 1 /* Program data */
#define SHT_SYMTAB 2 /* Symbol table */
#define SHT_STRTAB 3 /* String table */
#define SHT_RELA 4 /* Relocation entries with addends */
#define SHT_HASH 5 /* Symbol hash table */
#define SHT_DYNAMIC 6 /* Dynamic linking information */
#define SHT_NOTE 7 /* Notes */
#define SHT_NOBITS 8 /* Program space with no data (bss) */
#define SHT_REL 9 /* Relocation entries, no addends */
#define SHT_SHLIB 10 /* Reserved */
#define SHT_DYNSYM 11 /* Dynamic linker symbol table */
#define SHT_INIT_ARRAY 14 /* Array of constructors */
#define SHT_FINI_ARRAY 15 /* Array of destructors */
#define SHT_PREINIT_ARRAY 16 /* Array of pre-constructors */
#define SHT_GROUP 17 /* Section group */
#define SHT_SYMTAB_SHNDX 18 /* Extended section indeces */
#define SHT_NUM 19 /* Number of defined types. */
#define SHT_LOOS 0x60000000 /* Start OS-specific. */
#define SHT_GNU_ATTRIBUTES 0x6ffffff5 /* Object attributes. */
#define SHT_GNU_HASH 0x6ffffff6 /* GNU-style hash table. */
#define SHT_GNU_LIBLIST 0x6ffffff7 /* Prelink library list */
#define SHT_CHECKSUM 0x6ffffff8 /* Checksum for DSO content. */
#define SHT_LOSUNW 0x6ffffffa /* Sun-specific low bound. */
#define SHT_SUNW_move 0x6ffffffa
#define SHT_SUNW_COMDAT 0x6ffffffb
#define SHT_SUNW_syminfo 0x6ffffffc
#define SHT_GNU_verdef 0x6ffffffd /* Version definition section. */
#define SHT_GNU_verneed 0x6ffffffe /* Version needs section. */
#define SHT_GNU_versym 0x6fffffff /* Version symbol table. */
#define SHT_HISUNW 0x6fffffff /* Sun-specific high bound. */
#define SHT_HIOS 0x6fffffff /* End OS-specific type */
#define SHT_LOPROC 0x70000000 /* Start of processor-specific */
#define SHT_HIPROC 0x7fffffff /* End of processor-specific */
#define SHT_LOUSER 0x80000000 /* Start of application-specific */
#define SHT_HIUSER 0x8fffffff /* End of application-specific */
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