ELF (1A) Young Won Lim 10/22/14

Transcription

1 ELF (1A)

2 Copyright (c) Young W. Lim. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". Please send corrections (or suggestions) to youngwlim@hotmail.com. This document was produced by using OpenOffice.

3 ELF Each ELF file is made up of one ELF header, followed by file data. The file data can include: Program header table, describing zero or more segments Section header table, describing zero or more sections Data referred to by entries in the program header table or section header table The segments contain information that is necessary for runtime execution of the file The sections contain important data for linking and relocation. Any byte in the entire file can be owned by at most one section, and there can be orphan bytes which are not owned by any section. ELF 3

4 ELF's two views Linking View Execution View ELF Header ELF Header Program Header Table (optional) Program Header Table Section 1 Segment 1 Section 2 *** Segment 2 Section n *** Section Header Table Section Header Table (optional) ELF 4

5 Relocatable Object Files Linking View ELF Header Program Header Table (optional) Section 1 Section 2 *** Section n.text.rodata.data.bss.symtab.rel.text.rel.data.debug.line.strtab machine code read only data initialized global data uninitialized data symbol table Section Header Table ELF 5

6 Example Program // main.c int a = 100; int b = 200; int main(void) { swap(); return 0; } // swap.c extern int a; extern int b; int *p1 = &a; int *p2; void swap(void) { Int temp; gcc -c main.c gcc -c swap.c gcc -o p main.o swap.o } p2 = &b; temp = *p1; *p1 = *p2; *p2 = temp; ELF 6

7 readelf -h ELF Header: Magic: 7f 45 4c Class: ELF32 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: REL (Relocatable file) Machine: Intel Version: 0x1 Entry point address: 0x0 Start of program headers: 0 (bytes into file) Start of section headers: 264 (bytes into file) Flags: 0x0 Size of this header: 52 (bytes) Size of program headers: 0 (bytes) Number of program headers: 0 Size of section headers: 40 (bytes) Number of section headers: 12 Section header string table index: 9 ELF 7

8 readelf -s ELF 8

9 readelf -S ELF 9

10 readelf -l ELF 10

11 Tools readelf is a Unix binary utility that displays information about one or more ELF files. A free software implementation is provided by GNU Binutils. elfutils provides alternative tools to GNU Binutils purely for Linux. elfdump is a command for viewing ELF information in an ELF file, available under Solaris and FreeBSD. objdump provides a wide range of information about ELF files and other object formats. objdump uses the Binary File Descriptor library as a back end to structure the ELF data. The Unix file utility can display some information about ELF files, including the instruction set architecture for which the code in a relocatable, executable, or shared object file is intended, or on which an ELF core dump was produced. ELF 11

12 tiny main.c // tiny.c int main(void) { return(42); } // tiny.asm BITS 32 GLOBAL main SECTION.text main: mov eax, 42 ret gcc -Wall tiny.c./a.out ; echo $? nasm -f elf tiny.asm gcc -Wall -s tiny.o./a.out ; echo $? gcc -Wall -s tiny.c gcc -Wall -s -O3 tiny.c gcc -Wall -s -nostartupfiles tiny.o ELF 12

13 tiny main.c // tiny.asm BITS 32 GLOBAL main SECTION.text _start: push dword, 42 call _exit // tiny.asm BITS 32 GLOBAL main SECTION.text _start: mov eax,, 1 mov ebx, 42 int 0x80 nasm -f elf tiny.asm gcc -Wall -s -nostartupfiles tiny.o./a.out ; echo $? nasm -f elf tiny.asm gcc -Wall -s -nostdlib tiny.o./a.out ; echo $? nasm -f elf tiny.asm ld -s tiny.o./a.out ; echo $? ELF 13

14 tiny main.c // tiny.s.global _start.text _start: xorl %eax, %eax incl %eax movb $42, %bl int $0x80 gcc -s -nostdlib tiny.s./a.out ; echo $? ELF 14

15 References [1] Essential C, Nick Parlante [2] Efficient C Programming, Mark A. Weiss [3] C A Reference Manual, Samuel P. Harbison & Guy L. Steele Jr. [4] C Language Express, I. K. Chun [5] A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux [6] [7] [8]