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![Now, Let us write a ( hello.o) relocatable object file byte by byte! char text[] = { '\xB8', '\x04', '\x00', '\x00', '\x00', // mov eax, 4 '\xBB', '\x01', '\x00', '\x00', '\x00', // mov ebx, 1 '\xB9', '\x00', '\x00', '\x00', '\x00', // mov ecx, msg '\xBA', '\x0E', '\x00', '\x00', '\x00', // mov edx, 14 '\xCD', '\x80', // int 0x80 '\xB8', '\x01', '\x00', '\x00', '\x00', // mov eax, 1 '\xCD', '\x80' // int 0x80 }; char data[] = { '\x48', '\x65', '\x6C', '\x6C', '\x6F', '\x2C', '\x20', '\x57', '\x6F', '\x72', // Hello, World! In Hexadecimal. '\x6C', '\x64', '\x21', '\x0A' }; vector<char> vtext(&text[0], &text[29]); vector<char> vdata(&data[0], &data[14]); miasmaELF obj; sample.cpp](https://image.slidesharecdn.com/hareesh-oo1-090826153211-phpapp02/85/A-hands-on-introduction-to-the-ELF-Object-file-format-10-320.jpg)
![For more info on ELF A terrific article on ELF http://www.linuxjournal.com/article.php?sid=1059 The Specification http://x86.ddj.com/ftp/manuals/tools/elf.pdf For more info on ELF Pramod Kumar T [email_address] .co.in Hareesh Nagarajan [email_address] http://puggy.symonds.net/~hareesh miASMa is hosted at: http://freshmeat.net/miasma To Contact Us Thank You! Hope it was fun!](https://image.slidesharecdn.com/hareesh-oo1-090826153211-phpapp02/85/A-hands-on-introduction-to-the-ELF-Object-file-format-15-320.jpg)
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A hands-on introduction to the ELF Object file format
- 1. Hareesh Nagarajan PramodKumar Both from 7 th Semester CSE at RV College of Engineering A hands-on introduction to the ELF Object file format
- 2. Our experience withELF In our 6 th semester we developed miASMa - a 2 pass Macro Assembler for an x86 machine. miASMa generates Relocatable Object Files that conforming to the ELF Format. To achieve the latter we developed a simple code generation library called libmiASMaELF.
- 3. Types of ObjectFiles Relocatable File Executable File Shared Object File
- 4.
- 5.
- 6. Special Sections Various sections in ELF are predefined and hold program and control Information.
- 7. The Quick lookat the String table This is how most string tables in the ELF look like.
- 8. The Symbol Table(I'm bored, I thought this was talk made use of a hands-on approach :( )
- 9. The RelocationTable (Weare almost there !)
- 10. Now, Let uswrite a ( hello.o) relocatable object file byte by byte! char text[] = { '\xB8', '\x04', '\x00', '\x00', '\x00', // mov eax, 4 '\xBB', '\x01', '\x00', '\x00', '\x00', // mov ebx, 1 '\xB9', '\x00', '\x00', '\x00', '\x00', // mov ecx, msg '\xBA', '\x0E', '\x00', '\x00', '\x00', // mov edx, 14 '\xCD', '\x80', // int 0x80 '\xB8', '\x01', '\x00', '\x00', '\x00', // mov eax, 1 '\xCD', '\x80' // int 0x80 }; char data[] = { '\x48', '\x65', '\x6C', '\x6C', '\x6F', '\x2C', '\x20', '\x57', '\x6F', '\x72', // Hello, World! In Hexadecimal. '\x6C', '\x64', '\x21', '\x0A' }; vector<char> vtext(&text[0], &text[29]); vector<char> vdata(&data[0], &data[14]); miasmaELF obj; sample.cpp
- 11. obj.InitializeELFHeader(); obj.InitializeSymbolTable(); boolAddNewSection(string _SectionName , Elf32_Word shtype, Elf32_Word shflags, Elf32_Word shaddr, Elf32_Word shlink, Elf32_Word shinfo, Elf32_Word shaddralign, Elf32_Word shentsize); obj.AddNewSection(".shstrtab",SHT_STRTAB, 0,0,0,0,0,0); obj.AddNewSection(".text", SHT_PROGBITS,6,0,0,0,16,0); obj.AddNewSection(".data", SHT_PROGBITS,3,0,0,0,16,0); obj.AddNewSection(".symtab", SHT_SYMTAB, 0,0, obj.GetSectionIndexOfType(SHT_STRTAB, ".strtab"), 0, 4,sizeof(Elf32_Sym)); obj.AddNewSection(".rel.text",SHT_REL,0,0, obj.GetSectionIndexOfType(SHT_SYMTAB), obj.GetSectionIndexOfType(SHT_PROGBITS, ".text"), 4,sizeof(Elf32_Rel)); Prototype
- 12. bool AddContents(const vector<char>, int SectionIndex); obj.AddContents(vtext, obj.GetSectionIndexOfType(SHT_PROGBITS,".text")); obj.AddContents(vdata, obj.GetSectionIndexOfType(SHT_PROGBITS,".data")); bool AddSymbol(string _Symbol, Elf32_Addr stvalue, Elf32_Word stsize, unsigned char stbind, unsigned char sttype, Elf32_Half st_shndx); obj.AddSymbol( "_start" ,0,0, STB_WEAK, STT_FUNC, obj.GetSectionIndexOfType(SHT_PROGBITS, ".text")); obj.AddSymbol("myvariable",0,0, STB_GLOBAL, STT_OBJECT, obj.GetSectionIndexOfType(SHT_PROGBITS, ".data")); And then… Prototype Prototype
- 13. bool AddRelocationEntry(Elf32_Addr roffset, unsigned char rsym, unsigned char rtype, int SectionIndex); obj.AddRelocationEntry(11, obj.ReturnSymbolIndex("myvariable"), R_386_RELATIVE, obj.GetSectionIndexOfType(SHT_REL, ".rel.text")); obj.PrepareFile(); obj.WriteFile("hello.o"); //Yippie! The OBJECT FILE is created Finally… Prototype
- 14. Memory map ofhello.o $ g++ sample.cpp libmiasmaelf.o $./a.out ---> hello.o written!
- 15. For more infoon ELF A terrific article on ELF http://www.linuxjournal.com/article.php?sid=1059 The Specification http://x86.ddj.com/ftp/manuals/tools/elf.pdf For more info on ELF Pramod Kumar T [email_address] .co.in Hareesh Nagarajan [email_address] http://puggy.symonds.net/~hareesh miASMa is hosted at: http://freshmeat.net/miasma To Contact Us Thank You! Hope it was fun!