Specializations > System programming & Algorithm > Linux Programming
Alexandre Gautier
01-21-2021 to 01-29-2021
Introduction to ELF or the Executable and Linkable Format, its uses, and structure.
Allowed Functions and System Calls
- Unless specified otherwise, you are allowed to use the C standard library
- You’re not allowed to use
system(3) - You’re not allowed to use
exec(2) andexec(3)
Tests
- Your program must be able to handle both 32-bit and 64-bit ELF files
- Your program must be able to handle both little and big endian ELF files
- Your program must be able to handle all types of ELF files
- Test ELF files:
Examine /usr/include/elf.h
Write a program that displays the information contained in the ELF file header of an ELF file.
- Usage:
0-hreadelf elf_filename - Your standard output, error output and status should be the exact same as
readelf -W -h
$ ./0-hreadelf ubuntu64
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: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400600
Start of program headers: 64 (bytes into file)
Start of section headers: 6936 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 9
Size of section headers: 64 (bytes)
Number of section headers: 31
Section header string table index: 28
$
$ ./0-hreadelf netbsd32
ELF Header:
Magic: 7f 45 4c 46 01 01 01 02 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - NetBSD
ABI Version: 0
Type: EXEC (Executable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x80484c0
Start of program headers: 52 (bytes into file)
Start of section headers: 2752 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 6
Size of section headers: 40 (bytes)
Number of section headers: 24
Section header string table index: 21
$
$ ./0-hreadelf sparcbigendian32
ELF Header:
Magic: 7f 45 4c 46 01 02 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, big endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Sparc
Version: 0x1
Entry point address: 0x10d20
Start of program headers: 52 (bytes into file)
Start of section headers: 84560 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 6
Size of section headers: 40 (bytes)
Number of section headers: 24
Section header string table index: 23
$ Compiled: make 0-hreadelf
Write a program that displays the information contained in the ELF sections' headers of an ELF file.
- Usage:
1-hreadelf elf_filename - Your standard output, error output and status should be the exact same as
readelf -W -S
$ ./1-hreadelf ubuntu64
There are 31 section headers, starting at offset 0x1b18:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 0000000000400238 000238 00001c 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000400254 000254 000020 00 A 0 0 4
[ 3] .note.gnu.build-id NOTE 0000000000400274 000274 000024 00 A 0 0 4
[ 4] .gnu.hash GNU_HASH 0000000000400298 000298 00001c 00 A 5 0 8
[ 5] .dynsym DYNSYM 00000000004002b8 0002b8 0000f0 18 A 6 1 8
[ 6] .dynstr STRTAB 00000000004003a8 0003a8 000076 00 A 0 0 1
[ 7] .gnu.version VERSYM 000000000040041e 00041e 000014 02 A 5 0 2
[ 8] .gnu.version_r VERNEED 0000000000400438 000438 000030 00 A 6 1 8
[ 9] .rela.dyn RELA 0000000000400468 000468 000018 18 A 5 0 8
[10] .rela.plt RELA 0000000000400480 000480 0000c0 18 AI 5 24 8
[11] .init PROGBITS 0000000000400540 000540 00001a 00 AX 0 0 4
[12] .plt PROGBITS 0000000000400560 000560 000090 10 AX 0 0 16
[13] .plt.got PROGBITS 00000000004005f0 0005f0 000008 00 AX 0 0 8
[14] .text PROGBITS 0000000000400600 000600 000382 00 AX 0 0 16
[15] .fini PROGBITS 0000000000400984 000984 000009 00 AX 0 0 4
[16] .rodata PROGBITS 0000000000400990 000990 000067 00 A 0 0 8
[17] .eh_frame_hdr PROGBITS 00000000004009f8 0009f8 000034 00 A 0 0 4
[18] .eh_frame PROGBITS 0000000000400a30 000a30 0000f4 00 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000600e10 000e10 000008 00 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000600e18 000e18 000008 00 WA 0 0 8
[21] .jcr PROGBITS 0000000000600e20 000e20 000008 00 WA 0 0 8
[22] .dynamic DYNAMIC 0000000000600e28 000e28 0001d0 10 WA 6 0 8
[23] .got PROGBITS 0000000000600ff8 000ff8 000008 08 WA 0 0 8
[24] .got.plt PROGBITS 0000000000601000 001000 000058 08 WA 0 0 8
[25] .data PROGBITS 0000000000601058 001058 000010 00 WA 0 0 8
[26] .bss NOBITS 0000000000601068 001068 000008 00 WA 0 0 1
[27] .comment PROGBITS 0000000000000000 001068 000034 01 MS 0 0 1
[28] .shstrtab STRTAB 0000000000000000 001a06 00010c 00 0 0 1
[29] .symtab SYMTAB 0000000000000000 0010a0 0006d8 18 30 47 8
[30] .strtab STRTAB 0000000000000000 001778 00028e 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
$
$ ./1-hreadelf netbsd32
There are 24 section headers, starting at offset 0xac0:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 080480f4 0000f4 000017 00 A 0 0 1
[ 2] .note.netbsd.ident NOTE 0804810c 00010c 000034 00 A 0 0 4
[ 3] .hash HASH 08048140 000140 0000a0 04 A 4 0 4
[ 4] .dynsym DYNSYM 080481e0 0001e0 000150 10 A 5 1 4
[ 5] .dynstr STRTAB 08048330 000330 0000bf 00 A 0 0 1
[ 6] .rel.got REL 080483f0 0003f0 000030 08 A 4 16 4
[ 7] .rel.plt REL 08048420 000420 000020 08 A 4 9 4
[ 8] .init PROGBITS 08048440 000440 000030 00 AX 0 0 4
[ 9] .plt PROGBITS 08048470 000470 000050 04 AX 0 0 4
[10] .text PROGBITS 080484c0 0004c0 000304 00 AX 0 0 4
[11] .fini PROGBITS 080487c4 0007c4 00001d 00 AX 0 0 4
[12] .rodata PROGBITS 08048800 000800 0000a7 00 A 0 0 32
[13] .data PROGBITS 080498a8 0008a8 000014 00 WA 0 0 4
[14] .ctors PROGBITS 080498bc 0008bc 000008 00 WA 0 0 4
[15] .dtors PROGBITS 080498c4 0008c4 000008 00 WA 0 0 4
[16] .got PROGBITS 080498cc 0008cc 000034 04 WA 0 0 4
[17] .dynamic DYNAMIC 08049900 000900 000088 08 WA 5 0 4
[18] .bss NOBITS 08049988 000988 00000c 00 WA 0 0 4
[19] .note NOTE 08049994 000988 000050 00 0 0 1
[20] .ident PROGBITS 080499e4 0009d8 000035 00 0 0 1
[21] .shstrtab STRTAB 00000000 000a0d 0000b1 00 0 0 1
[22] .symtab SYMTAB 00000000 000e80 000390 10 23 27 4
[23] .strtab STRTAB 00000000 001210 000125 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
$
$ ./1-hreadelf sparcbigendian32
There are 24 section headers, starting at offset 0x14a50:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 000100f4 0000f4 000013 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 00010108 000108 000020 00 A 0 0 4
[ 3] .hash HASH 00010128 000128 00019c 04 A 4 0 4
[ 4] .dynsym DYNSYM 000102c4 0002c4 000400 10 A 5 1 4
[ 5] .dynstr STRTAB 000106c4 0006c4 00025a 00 A 0 0 1
[ 6] .gnu.version VERSYM 0001091e 00091e 000080 02 A 4 0 2
[ 7] .gnu.version_r VERNEED 000109a0 0009a0 000060 00 A 5 2 4
[ 8] .rela.dyn RELA 00010a00 000a00 000054 0c A 4 0 4
[ 9] .rela.plt RELA 00010a54 000a54 00027c 0c A 4 20 4
[10] .init PROGBITS 00010ce0 000ce0 000038 00 AX 0 0 32
[11] .text PROGBITS 00010d20 000d20 01226c 00 AX 0 0 32
[12] .fini PROGBITS 00022fa0 012fa0 000030 00 AX 0 0 32
[13] .rodata PROGBITS 00022fd0 012fd0 0013a0 00 A 0 0 8
[14] .data PROGBITS 00034370 014370 000270 00 WA 0 0 4
[15] .eh_frame PROGBITS 000345e0 0145e0 000004 00 A 0 0 4
[16] .dynamic DYNAMIC 000345e4 0145e4 0000d0 08 WA 5 0 4
[17] .ctors PROGBITS 000346b4 0146b4 000008 00 WA 0 0 4
[18] .dtors PROGBITS 000346bc 0146bc 000008 00 WA 0 0 4
[19] .jcr PROGBITS 000346c4 0146c4 000004 00 WA 0 0 4
[20] .plt PROGBITS 000346c8 0146c8 0002b0 0c WAX 0 0 4
[21] .got PROGBITS 00034978 014978 000020 04 WA 0 0 4
[22] .bss NOBITS 00034998 014998 0442e0 00 WA 0 0 8
[23] .shstrtab STRTAB 00000000 014998 0000b7 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
$ Compiled: make 1-hreadelf
Write a program that displays the information contained in the ELF program headers of an ELF file.
- Usage:
2-hreadelf elf_filename - Your standard output, error output and status should be the exact same as
readelf -W -l
$ ./2-hreadelf ubuntu64
Elf file type is EXEC (Executable file)
Entry point 0x400600
There are 9 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000040 0x0000000000400040 0x0000000000400040 0x0001f8 0x0001f8 R E 0x8
INTERP 0x000238 0x0000000000400238 0x0000000000400238 0x00001c 0x00001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x000b24 0x000b24 R E 0x200000
LOAD 0x000e10 0x0000000000600e10 0x0000000000600e10 0x000258 0x000260 RW 0x200000
DYNAMIC 0x000e28 0x0000000000600e28 0x0000000000600e28 0x0001d0 0x0001d0 RW 0x8
NOTE 0x000254 0x0000000000400254 0x0000000000400254 0x000044 0x000044 R 0x4
GNU_EH_FRAME 0x0009f8 0x00000000004009f8 0x00000000004009f8 0x000034 0x000034 R 0x4
GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW 0x10
GNU_RELRO 0x000e10 0x0000000000600e10 0x0000000000600e10 0x0001f0 0x0001f0 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 .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame
03 .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag .note.gnu.build-id
06 .eh_frame_hdr
07
08 .init_array .fini_array .jcr .dynamic .got
$
$ ./2-hreadelf netbsd32
Elf file type is EXEC (Executable file)
Entry point 0x80484c0
There are 6 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x000c0 0x000c0 R E 0x4
INTERP 0x0000f4 0x080480f4 0x080480f4 0x00017 0x00017 R 0x1
[Requesting program interpreter: /usr/libexec/ld.elf_so]
LOAD 0x000000 0x08048000 0x08048000 0x008a7 0x008a7 R E 0x1000
LOAD 0x0008a8 0x080498a8 0x080498a8 0x000e0 0x000ec RW 0x1000
DYNAMIC 0x000900 0x08049900 0x08049900 0x00088 0x00088 RW 0x4
NOTE 0x00010c 0x0804810c 0x0804810c 0x00034 0x00034 R 0x4
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.netbsd.ident .hash .dynsym .dynstr .rel.got .rel.plt .init .plt .text .fini .rodata
03 .data .ctors .dtors .got .dynamic .bss
04 .dynamic
05 .note.netbsd.ident
$
$ ./2-hreadelf sparcbigendian32
Elf file type is EXEC (Executable file)
Entry point 0x10d20
There are 6 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x00010034 0x00010034 0x000c0 0x000c0 R E 0x4
INTERP 0x0000f4 0x000100f4 0x000100f4 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x00010000 0x00010000 0x14370 0x14370 R E 0x10000
LOAD 0x014370 0x00034370 0x00034370 0x00628 0x44908 RWE 0x10000
DYNAMIC 0x0145e4 0x000345e4 0x000345e4 0x000d0 0x000d0 RW 0x4
NOTE 0x000108 0x00010108 0x00010108 0x00020 0x00020 R 0x4
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .text .fini .rodata
03 .data .eh_frame .dynamic .ctors .dtors .jcr .plt .got .bss
04 .dynamic
05 .note.ABI-tag
$ Compiled: make 2-hreadelf
Write a program that displays the information contained in the ELF symbol tables of an ELF file.
- Usage:
100-hreadelf elf_filename - Your standard output, error output and status should be the exact same as
readelf -W -s
$ ./100-hreadelf ubuntu64
Symbol table '.dynsym' contains 10 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND write@GLIBC_2.2.5 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@GLIBC_2.4 (3)
3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND close@GLIBC_2.2.5 (2)
4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND dprintf@GLIBC_2.2.5 (2)
5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND read@GLIBC_2.2.5 (2)
6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2)
7: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
8: 0000000000000000 0 FUNC GLOBAL DEFAULT UND open@GLIBC_2.2.5 (2)
9: 0000000000000000 0 FUNC GLOBAL DEFAULT UND exit@GLIBC_2.2.5 (2)
Symbol table '.symtab' contains 73 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000400238 0 SECTION LOCAL DEFAULT 1
2: 0000000000400254 0 SECTION LOCAL DEFAULT 2
3: 0000000000400274 0 SECTION LOCAL DEFAULT 3
4: 0000000000400298 0 SECTION LOCAL DEFAULT 4
5: 00000000004002b8 0 SECTION LOCAL DEFAULT 5
6: 00000000004003a8 0 SECTION LOCAL DEFAULT 6
7: 000000000040041e 0 SECTION LOCAL DEFAULT 7
8: 0000000000400438 0 SECTION LOCAL DEFAULT 8
9: 0000000000400468 0 SECTION LOCAL DEFAULT 9
10: 0000000000400480 0 SECTION LOCAL DEFAULT 10
11: 0000000000400540 0 SECTION LOCAL DEFAULT 11
12: 0000000000400560 0 SECTION LOCAL DEFAULT 12
13: 00000000004005f0 0 SECTION LOCAL DEFAULT 13
14: 0000000000400600 0 SECTION LOCAL DEFAULT 14
15: 0000000000400984 0 SECTION LOCAL DEFAULT 15
16: 0000000000400990 0 SECTION LOCAL DEFAULT 16
17: 00000000004009f8 0 SECTION LOCAL DEFAULT 17
18: 0000000000400a30 0 SECTION LOCAL DEFAULT 18
19: 0000000000600e10 0 SECTION LOCAL DEFAULT 19
20: 0000000000600e18 0 SECTION LOCAL DEFAULT 20
21: 0000000000600e20 0 SECTION LOCAL DEFAULT 21
22: 0000000000600e28 0 SECTION LOCAL DEFAULT 22
23: 0000000000600ff8 0 SECTION LOCAL DEFAULT 23
24: 0000000000601000 0 SECTION LOCAL DEFAULT 24
25: 0000000000601058 0 SECTION LOCAL DEFAULT 25
26: 0000000000601068 0 SECTION LOCAL DEFAULT 26
27: 0000000000000000 0 SECTION LOCAL DEFAULT 27
28: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
29: 0000000000600e20 0 OBJECT LOCAL DEFAULT 21 __JCR_LIST__
30: 0000000000400630 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones
31: 0000000000400670 0 FUNC LOCAL DEFAULT 14 register_tm_clones
32: 00000000004006b0 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux
33: 0000000000601068 1 OBJECT LOCAL DEFAULT 26 completed.7585
34: 0000000000600e18 0 OBJECT LOCAL DEFAULT 20 __do_global_dtors_aux_fini_array_entry
35: 00000000004006d0 0 FUNC LOCAL DEFAULT 14 frame_dummy
36: 0000000000600e10 0 OBJECT LOCAL DEFAULT 19 __frame_dummy_init_array_entry
37: 0000000000000000 0 FILE LOCAL DEFAULT ABS 3-cp.c
38: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
39: 0000000000400b20 0 OBJECT LOCAL DEFAULT 18 __FRAME_END__
40: 0000000000600e20 0 OBJECT LOCAL DEFAULT 21 __JCR_END__
41: 0000000000000000 0 FILE LOCAL DEFAULT ABS
42: 0000000000600e18 0 NOTYPE LOCAL DEFAULT 19 __init_array_end
43: 0000000000600e28 0 OBJECT LOCAL DEFAULT 22 _DYNAMIC
44: 0000000000600e10 0 NOTYPE LOCAL DEFAULT 19 __init_array_start
45: 00000000004009f8 0 NOTYPE LOCAL DEFAULT 17 __GNU_EH_FRAME_HDR
46: 0000000000601000 0 OBJECT LOCAL DEFAULT 24 _GLOBAL_OFFSET_TABLE_
47: 0000000000400980 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini
48: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTable
49: 0000000000601058 0 NOTYPE WEAK DEFAULT 25 data_start
50: 0000000000000000 0 FUNC GLOBAL DEFAULT UND write@@GLIBC_2.2.5
51: 0000000000601068 0 NOTYPE GLOBAL DEFAULT 25 _edata
52: 0000000000400984 0 FUNC GLOBAL DEFAULT 15 _fini
53: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@@GLIBC_2.4
54: 0000000000000000 0 FUNC GLOBAL DEFAULT UND close@@GLIBC_2.2.5
55: 0000000000000000 0 FUNC GLOBAL DEFAULT UND dprintf@@GLIBC_2.2.5
56: 0000000000000000 0 FUNC GLOBAL DEFAULT UND read@@GLIBC_2.2.5
57: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_2.2.5
58: 0000000000601058 0 NOTYPE GLOBAL DEFAULT 25 __data_start
59: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
60: 0000000000601060 0 OBJECT GLOBAL HIDDEN 25 __dso_handle
61: 0000000000400990 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used
62: 0000000000400910 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init
63: 0000000000601070 0 NOTYPE GLOBAL DEFAULT 26 _end
64: 0000000000400600 42 FUNC GLOBAL DEFAULT 14 _start
65: 0000000000601068 0 NOTYPE GLOBAL DEFAULT 26 __bss_start
66: 00000000004006f6 534 FUNC GLOBAL DEFAULT 14 main
67: 0000000000000000 0 FUNC GLOBAL DEFAULT UND open@@GLIBC_2.2.5
68: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
69: 0000000000000000 0 FUNC GLOBAL DEFAULT UND exit@@GLIBC_2.2.5
70: 0000000000601068 0 OBJECT GLOBAL HIDDEN 25 __TMC_END__
71: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable
72: 0000000000400540 0 FUNC GLOBAL DEFAULT 11 _init
$ Compiled: make 100-hreadelf
- Samuel Pomeroy - allelomorph