ELF header

Running:

readelf -h hello_world.o

outputs:

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:          64 (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:         7
Section header string table index: 3

Running:

readelf -h hello_world.out

outputs:

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:               0x4000b0
Start of program headers:          64 (bytes into file)
Start of section headers:          272 (bytes into file)
Flags:                             0x0
Size of this header:               64 (bytes)
Size of program headers:           56 (bytes)
Number of program headers:         2
Size of section headers:           64 (bytes)
Number of section headers:         6
Section header string table index: 3

Bytes in the object file:

00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  01 00 3e 00 01 00 00 00  00 00 00 00 00 00 00 00  |..>.............|
00000020  00 00 00 00 00 00 00 00  40 00 00 00 00 00 00 00  |........@.......|
00000030  00 00 00 00 40 00 00 00  00 00 40 00 07 00 03 00  |....@.....@.....|

Executable:

00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  02 00 3e 00 01 00 00 00  b0 00 40 00 00 00 00 00  |..>.......@.....|
00000020  40 00 00 00 00 00 00 00  10 01 00 00 00 00 00 00  |@...............|
00000030  00 00 00 00 40 00 38 00  02 00 40 00 06 00 03 00  |....@.8...@.....|

Structure represented:

# define EI_NIDENT 16

typedef struct {
    unsigned char   e_ident[EI_NIDENT];
    Elf64_Half      e_type;
    Elf64_Half      e_machine;
    Elf64_Word      e_version;
    Elf64_Addr      e_entry;
    Elf64_Off       e_phoff;
    Elf64_Off       e_shoff;
    Elf64_Word      e_flags;
    Elf64_Half      e_ehsize;
    Elf64_Half      e_phentsize;
    Elf64_Half      e_phnum;
    Elf64_Half      e_shentsize;
    Elf64_Half      e_shnum;
    Elf64_Half      e_shstrndx;
} Elf64_Ehdr;

Manual breakdown:

0 0: EI_MAG = 7f 45 4c 46 = 0x7f 'E', 'L', 'F': ELF magic number
0 4: EI_CLASS = 02 = ELFCLASS64: 64 bit elf
0 5: EI_DATA = 01 = ELFDATA2LSB: little endian data
0 6: EI_VERSION = 01: format version
0 7: EI_OSABI (only in 2003 Update) = 00 = ELFOSABI_NONE: no extensions.
0 8: EI_PAD = 8x 00: reserved bytes. Must be set to 0.
1 0: e_type = 01 00 = 1 (big endian) = ET_REl: relocatable format
On the executable it is 02 00 for ET_EXEC.
Another important possibility for the executable is ET_DYN for PIE executables and shared libraries.
ET_DYN tells the Linux kernel that the code is position independent, and can loaded at a random memory location with ASLR.
This is explained further at:
- stackoverflow.com/questions/2463150/what-is-the-fpie-option-for-position-independent-executables-in-gcc-and-ld/51308031#51308031
- stackoverflow.com/questions/34519521/why-does-gcc-create-a-shared-object-instead-of-an-executable-binary-according-to/55704865#55704865
1 2: e_machine = 3e 00 = 62 = EM_X86_64: AMD64 architecture
1 4: e_version = 01 00 00 00: must be 1
1 8: e_entry = 8x 00: execution address entry point, or 0 if not applicable like for the object file since there is no entry point.
On the executable, it is b0 00 40 00 00 00 00 00. The kernel puts the RIP directly on that value when executing. It can be configured by the linker script or -e. But it will segfault if you set it too low: stackoverflow.com/questions/2187484/why-is-the-elf-execution-entry-point-virtual-address-of-the-form-0x80xxxxx-and-n
2 0: e_phoff = 8x 00: program header table offset, 0 if not present.
40 00 00 00 on the executable, i.e. it starts immediately after the ELF header.
2 8: e_shoff = 40 7x 00 = 0x40: section header table file offset, 0 if not present.
3 0: e_flags = 00 00 00 00 Arch specific. i386 docs say:
The Intel386 architecture defines no flags; so this member contains zero.
3 4: e_ehsize = 40 00: size of this elf header. TODO why this field needed? Isn't the size fixed?
3 6: e_phentsize = 00 00: size of each program header, 0 if not present.
38 00 on executable: it is 56 bytes long
3 8: e_phnum = 00 00: number of program header entries, 0 if not present.
02 00 on executable: there are 2 entries.
3 A: e_shentsize and e_shnum = 40 00 07 00: section header size and number of entries
3 E: e_shstrndx (Section Header STRing iNDeX) = 03 00: index of the .shstrtab section.

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