Some interesting portions of the file
Typedefs
/* Type for a 16-bit quantity. */
typedef uint16_t Elf64_Half;
/* Types for signed and unsigned 32-bit quantities. */
typedef uint32_t Elf64_Word;
typedef int32_t Elf64_Sword;
/* Types for signed and unsigned 64-bit quantities. */
typedef uint64_t Elf64_Xword;
typedef int64_t Elf64_Sxword;
/* Type of addresses. */
typedef uint64_t Elf64_Addr;
/* Type of file offsets. */
typedef uint64_t Elf64_Off;
/* Type for section indices, which are 16-bit quantities. */
typedef uint16_t Elf64_Section;
/* Type for version symbol information. */
typedef Elf64_Half Elf64_Versym;
Elf64_Ehdr (File header)
Elf64_Ehdr represents the ELF file header which appears at the start of every ELF file.
#define EI_NIDENT (16)
typedef struct
{
unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
Elf64_Half e_type; /* Object file type see ET_ macros */
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;
Legal values for e_type
#define ET_NONE 0 /* No file type */
#define ET_REL 1 /* Relocatable file */
#define ET_EXEC 2 /* Executable file */
#define ET_DYN 3 /* Shared object file */
#define ET_CORE 4 /* Core file */
#define ET_NUM 5 /* Number of defined types */
#define ET_LOOS 0xfe00 /* OS-specific range start */
#define ET_HIOS 0xfeff /* OS-specific range end */
#define ET_LOPROC 0xff00 /* Processor-specific range start */
#define ET_HIPROC 0xffff /* Processor-specific range end */
Legal values for e_machine
Some interesting values for e_machine:
#define EM_NONE 0 /* No machine */
…
#define EM_386 3 /* Intel 80386 */
…
#define EM_X86_64 62 /* AMD x86-64 architecture */
…
#define EM_MMIX 80 /* Donald Knuth's educational 64-bit proc */
…
#define EM_AARCH64 183 /* ARM AARCH64 */
…
#define EM_CUDA 190 /* NVIDIA CUDA */
…
#define EM_BPF 247 /* Linux BPF -- in-kernel virtual machine */
#define EM_CSKY 252 /* C-SKY */
#define EM_NUM 253
Legal values for e_version
#define EV_NONE 0 /* Invalid ELF version */
#define EV_CURRENT 1 /* Current version, will be increased if versin changes */
#define EV_NUM 2
Displaying the ELF header
An ELF object file's header can be displayed with readelf -h
$ readelf -h unit.o
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: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 648 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 12
Section header string table index: 11
EI_…
#define EI_OSABI 7 /* OS ABI identification */
#define ELFOSABI_SYSV 0 /* Alias. */
…
#define ELFOSABI_GNU 3 /* Object uses GNU ELF extensions. */
#define ELFOSABI_LINUX ELFOSABI_GNU /* Compatibility alias. */
…
#define ELFOSABI_STANDALONE 255 /* Standalone (embedded) application */
Elf64_Shdr (Section header)
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;
Legel values for sh_type
#define SHT_NULL 0 /* Section header table entry unused */
#define SHT_PROGBITS 1 /* Program data */
#define SHT_SYMTAB 2 /* Symbol table, se also SHT_DYNSYM */
#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, see also SHT_SYMTAB */
#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 */
Legal values for sh_flags
#define SHF_WRITE ( 1 << 0) /* Writable */
#define SHF_ALLOC ( 1 << 1) /* Occupies memory during execution */
#define SHF_EXECINSTR ( 1 << 2) /* Executable */
#define SHF_MERGE ( 1 << 4) /* Might be merged */
#define SHF_STRINGS ( 1 << 5) /* Contains nul-terminated strings */
#define SHF_INFO_LINK ( 1 << 6) /* `sh_info' contains SHT index */
#define SHF_LINK_ORDER ( 1 << 7) /* Preserve order after combining */
#define SHF_OS_NONCONFORMING ( 1 << 8) /* Non-standard OS specific handling required */
#define SHF_GROUP ( 1 << 9) /* Section is member of a group. */
#define SHF_TLS ( 1 << 10) /* Section hold thread-local data. */
#define SHF_COMPRESSED ( 1 << 11) /* Section with compressed data. */
#define SHF_MASKOS 0x0ff00000 /* OS-specific. */
#define SHF_MASKPROC 0xf0000000 /* Processor-specific */
#define SHF_ORDERED ( 1 << 30) /* Special ordering requirement (Solaris). */
#define SHF_EXCLUDE (1U << 31) /* Section is excluded unless referenced or allocated (Solaris).*/
Elf64_Section (Symbol table entry)
typedef struct
{
Elf64_Word st_name; /* Symbol name (string tbl index) */
unsigned char st_info; /* Symbol type and binding */
unsigned char st_other; /* Symbol visibility */
Elf64_Section st_shndx; /* Section index */
Elf64_Addr st_value; /* Symbol value */
Elf64_Xword st_size; /* Symbol size */
} Elf64_Sym;
typedef struct
{
Elf64_Half si_boundto; /* Direct bindings, symbol bound to */
Elf64_Half si_flags; /* Per symbol flags */
} Elf64_Syminfo;
Possible values for si_boundto
#define SYMINFO_BT_SELF 0xffff /* Symbol bound to self */
#define SYMINFO_BT_PARENT 0xfffe /* Symbol bound to parent */
#define SYMINFO_BT_LOWRESERVE 0xff00 /* Beginning of reserved entries */
Possible values for si_flags
#define SYMINFO_FLG_DIRECT 0x0001 /* Direct bound symbol */
#define SYMINFO_FLG_PASSTHRU 0x0002 /* Pass-thru symbol for translator */
#define SYMINFO_FLG_COPY 0x0004 /* Symbol is a copy-reloc */
#define SYMINFO_FLG_LAZYLOAD 0x0008 /* Symbol bound to object to be lazy
Syminfo version values
#define SYMINFO_NONE 0
#define SYMINFO_CURRENT 1
#define SYMINFO_NUM 2
Legal values for the ST_BIND subfield of st_info (symbol binding)
#define STB_LOCAL 0 /* Local symbol */
#define STB_GLOBAL 1 /* Global symbol */
#define STB_WEAK 2 /* Weak symbol */
#define STB_NUM 3 /* Number of defined types. */
#define STB_LOOS 10 /* Start of OS-specific */
#define STB_GNU_UNIQUE 10 /* Unique symbol. */
#define STB_HIOS 12 /* End of OS-specific */
#define STB_LOPROC 13 /* Start of processor-specific */
#define STB_HIPROC 15 /* End of processor-specific */
Legal value for the ST_TYPE subfield of st_info (symbol type)
#define STT_NOTYPE 0 /* Symbol type is unspecified */
#define STT_OBJECT 1 /* Symbol is a data object */
#define STT_FUNC 2 /* Symbol is a code object */
#define STT_SECTION 3 /* Symbol associated with a section */
#define STT_FILE 4 /* Symbol's name is file name */
#define STT_COMMON 5 /* Symbol is a common data object */
#define STT_TLS 6 /* Symbol is thread-local data object*/
#define STT_NUM 7 /* Number of defined types. */
#define STT_LOOS 10 /* Start of OS-specific */
#define STT_GNU_IFUNC 10 /* Symbol is indirect code object */
#define STT_HIOS 12 /* End of OS-specific */
#define STT_LOPROC 13 /* Start of processor-specific */
#define STT_HIPROC 15 /* End of processor-specific */
Elf64_auxv_t (Auxiliary vector)
An auxiliary vector is normally only used by the program interpreter.
The usual definition in an ABI supplement uses the name auxv_t and is usually not defined in <elf.h>.
GNU(?) includ it in <elf.h> because it cannot hurt and renamed it (likely from auxv_t to Elf64_auxv_t) to avoid conflicts.
typedef struct
{
uint64_t a_type; /* Entry type */
union
{
uint64_t a_val; /* Integer value */
/* We use to have pointer elements added here. We cannot do that,
though, since it does not work when using 32-bit definitions
on 64-bit platforms and vice versa. */
} a_un;
} Elf64_auxv_t;
TODO: Is the auxiliary vector used to implement atexit()?
The dynamic linker can be asked to show some debug information about the auxiliary vector by setting the variable LD_SHOW_AUXV=1.
See also
-
man 7 vdso
-
man 3 getauxval
-
/proc/$PID/auxv
Elf64_Phdr (Program header)
The program header is meaningful only for executable and shared object files.
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;
Legal values for p_flags
#define PF_X (1 << 0) /* Segment is executable */
#define PF_W (1 << 1) /* Segment is writable */
#define PF_R (1 << 2) /* Segment is readable */
#define PF_MASKOS 0x0ff00000 /* OS-specific */
#define PF_MASKPROC 0xf0000000 /* Processor-specific */
Legal values for p_type
#define PT_NULL 0 /* Program header table entry unused */
#define PT_LOAD 1 /* Loadable program segment, described by p_filesz and p_memsz */
#define PT_DYNAMIC 2 /* Dynamic linking information */
#define PT_INTERP 3 /* Program interpreter, may occur at most once */
#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 */
PT_LOAD describes .code and .data sections that come from the executable file and the file of a .bss section.
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