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Libelf sample programs

The first source example is a demo that copies an elf to an new one. The result it produce from a running executable is an non-running Elf executable.

elf_copy.c (source file) (build : gcc -o elf_copy -lelf elf_copy.c)

elf_obj_loc.c (souce file) (build : gcc -o elf_obj_loc -lelf elf_obj_loc.c)

And have a look on objdump disassembly output :

shell$ > objdump -d sample.o sample.o: file format elf32-i386 Disassembly of section .text: 00000000 <whois>: 0: 55 push %ebp 1: 89 e5 mov %esp,%ebp
3: a1 00 00 00 00 mov 0x0,%eax	variable "who"
8: 5d pop %ebp 9: c3 ret 0000000a <what>: a: 55 push %ebp b: 89 e5 mov %esp,%ebp
d: a1 00 00 00 00 mov 0x0,%eax	variable "hello"
12: 5d pop %ebp 13: c3 ret 00000014 <main>: 14: 8d 4c 24 04 lea 0x4(%esp),%ecx 18: 83 e4 f0 and $0xfffffff0,%esp 1b: ff 71 fc pushl 0xfffffffc(%ecx) 1e: 55 push %ebp 1f: 89 e5 mov %esp,%ebp 21: 53 push %ebx 22: 51 push %ecx 23: 83 ec 10 sub $0x10,%esp
26: e8 fc ff ff ff call 27 <main+0x13>	defined function (whois)
2b: 89 c3 mov %eax,%ebx
2d: e8 fc ff ff ff call 2e <main+0x1a>	defined function (what)
32: 89 5c 24 08 mov %ebx,0x8(%esp) 36: 89 44 24 04 mov %eax,0x4(%esp) 3a: c7 04 24 0c 00 00 00 movl $0xc,(%esp)
41: e8 fc ff ff ff call 42 <main+0x2e>	undefine function (printf)
46: b8 00 00 00 00 mov $0x0,%eax 4b: 83 c4 10 add $0x10,%esp 4e: 59 pop %ecx 4f: 5b pop %ebx 50: 5d pop %ebp 51: 8d 61 fc lea 0xfffffffc(%ecx),%esp 54: c3 ret

All adresses, including variables and functions, are left unchanged from the binary relocatable object which does not contain an address at all (value is set to 0 for variables and 0xFFFFFFFC for functions).

elf_obj_loc applies .rel sections to the corresponding sections as whould do a linker. In addition, it maps undefined symbols to adress 0x2500000 incrementing by 0x10 at each symbol. The output of objdump clearly shows that it is happier than it was with the raw object.

shell$ > ./elf_obj_loc sample.o sample.reloc shell$ > objdump -d sample.reloc sample.reloc: file format elf32-i386 Disassembly of section .text: 08048000 <whois>: 8048000: 55 push %ebp 8048001: 89 e5 mov %esp,%ebp
8048003: a1 5c 80 04 08 mov 0x804805c,%eax	variable who
8048008: 5d pop %ebp 8048009: c3 ret 0804800a <what>: 804800a: 55 push %ebp 804800b: 89 e5 mov %esp,%ebp
804800d: a1 58 80 04 08 mov 0x8048058,%eax	variable hello
8048012: 5d pop %ebp 8048013: c3 ret 08048014 <main>: 8048014: 8d 4c 24 04 lea 0x4(%esp),%ecx 8048018: 83 e4 f0 and $0xfffffff0,%esp 804801b: ff 71 fc pushl 0xfffffffc(%ecx) 804801e: 55 push %ebp 804801f: 89 e5 mov %esp,%ebp 8048021: 53 push %ebx 8048022: 51 push %ecx 8048023: 83 ec 10 sub $0x10,%esp
8048026: e8 d5 ff ff ff call 8048000 <whois>	defined function
804802b: 89 c3 mov %eax,%ebx
804802d: e8 d8 ff ff ff call 804800a <what>	defined function
8048032: 89 5c 24 08 mov %ebx,0x8(%esp) 8048036: 89 44 24 04 mov %eax,0x4(%esp)
804803a: c7 04 24 60 80 04 08 movl $0x8048060,(%esp)
8048041: e8 ca 7f 4b fa call 2500010 <printf>	undefined function
8048046: b8 00 00 00 00 mov $0x0,%eax 804804b: 83 c4 10 add $0x10,%esp 804804e: 59 pop %ecx 804804f: 5b pop %ebx 8048050: 5d pop %ebp 8048051: 8d 61 fc lea 0xfffffffc(%ecx),%esp 8048054: c3 ret

Hosted by the courtesy of    GitHub

The stars ASAP Durée du voyage intersidéral Résolutions de l'ONU en HTML Bussard Ramjet

DWARF : dwarf2xml ELF : libelf examples Code presentation : ctoohtml