The ELFIO library consists of two independent parts: ELF File Reader - (IELFI) - and ELF Producer (IELFO). - Each is represented by its own set of interfaces. - The library does not contain any classes that need to be explicitly - instantiated. ELFIO itself provides the interfaces that - are used to access the library's functionality. -
-To make the program recognize all ELFIO interface classes, the ELFIO.h - header file is needed. This header file defines all - standard definitions from the TIS documentation. -
#include <ELFIO.h>- -
This chapter will explain how to work with the reader component - of the ELFIO library. The first step is to get a pointer - onto the ELF File Reader: -
IELFI* pReader; - ELFIO::GetInstance()->CreateELFI( &pReader );- -
Now, that there is a pointer on the IELFI - interface: initialize the object by loading the ELF file: -
char* filename = "file.o"; - pReader->Load( filename );- -
From here, there is access to the ELF header. This makes it possible to request file - parameters such as encoding, machine type, entry point, etc. - To get the encoding of the file use: -
unsigned char encoding = pReader->GetEncoding();- -
Please note: standard types and constants from the TIS document are defined - in the ELFTypes.h header file. This file is included automatically into the - project. For example: ELFDATA2LSB and ELFDATA2MSB constants - define a value for little and big endian encoding. -
-The source code for the ELF Dumping Utility can be found in the "Examples" - directory; included there are more examples on how to use different ELFIO - reader interfaces. -
-| ELFIO: Tutorial | ||
|---|---|---|
| Prev |
The ELFIO library can help you build a very short ELF executable file. - This chapter shows how to build an executable file that will run on - x86 Linux machines and print "Hello World!" on your console. -
-Just as with the reader, the first step is to get - a pointer onto the ELF File Writer (Producer): -
IELFO* pELFO; - ELFIO::GetInstance()->CreateELFO( &pELFO );- -
Before continuing, the library must be informed about the main - attributes of the executable file to be built. To do this, declare - that the executable ELF file will run on a 32 bit x86 machine; has little - endian encoding and uses the current version of the ELF file format: -
// You can't proceed without this function call! - pELFO->SetAttr( ELFCLASS32, ELFDATA2LSB, EV_CURRENT, - ET_EXEC, EM_386, EV_CURRENT, 0 );- -
Some sections of an ELF executable file should reside in the program - segments. To create this loadable segment call the - AddSegment() function. -
// Create a loadable segment - IELFOSegment* pSegment = pELFO->AddSegment( PT_LOAD, - 0x08040000, - 0x08040000, - PF_X | PF_R, - 0x1000 );- -
The following segment serves as a placeholder for our code section. To create - this code section call the AddSection() function: -
// Create code section - IELFOSection* pTextSec = pELFO->AddSection( ".text", - SHT_PROGBITS, - SHF_ALLOC | SHF_EXECINSTR, - 0, - 0x10, - 0 );- -
Then, add the executable code for the section: -
char text[] =
- { '\xB8', '\x04', '\x00', '\x00', '\x00', // mov eax, 4
- '\xBB', '\x01', '\x00', '\x00', '\x00', // mov ebx, 1
- '\xB9', '\xFD', '\x00', '\x04', '\x08', // 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
- '\x48', '\x65', '\x6C', '\x6C', '\x6F', // db 'Hello'
- '\x2C', '\x20', '\x57', '\x6F', '\x72', // db ', Wor'
- '\x6C', '\x64', '\x21', '\x0A' // db 'ld!', 10
- };
- pTextSec->SetData( text, sizeof( text ) );
-
- Next, this code section is put into the loadable segment: -
// Add code section into program segment - pSegment->AddSection( pTextSec ); - pTextSec->Release(); - pSegment->Release();- -
Finally, define the start address of the program - and create the result file: -
// Set program entry point - pELFO->SetEntry( 0x08040000 ); - // Create ELF file - pELFO->Save( "test.elf" ); - pELFO->Release();- -
Please note: Call the Release() functions - for each interface you have used. - This will free all resources the ELFIO library has created. -
-Now compile the program and run it. The result is a new ELF file - called "test.elf". The size of this working executable file is only - 267 bytes! Run it on your Linux machine with the following commands: -
[Writer]$ ./Writer - [Writer]$ chmod +x test.elf - [Writer]$ ./test.elf - Hello, World!- -
The full text for this program can be found in the "Writer" directory. - Also, in the "Examples" directory, two other programs "WriteObj" - and "WriteObj2" demonstrate the creation of ELF object files. -
-ELFIO is a C++ library for reading and generating files - in the ELF binary format. This library is unique and not based on - any other product. It is also platform independent. - The library uses standard ANSI C++ constructions and - runs on a wide variety of architectures. -
-While the library's implementation does make your work easier: - a basic knowledge of the ELF binary format is required. Information about ELF - is included in the TIS (Tool Interface Standards) documentation you received - with the library's source code. -
-| Next | ||
| Introduction |
ELF binary files consist of several sections. Each section has it's own - responsibility: some contain executable code; others describe program dependencies; - others symbol tables and so on. See the TIS documentation for a full description of each section. -
-To see how many sections the ELF file contains, including their - names and sizes, is demonstated in the following code: -
int nSecNo = pReader->GetSectionsNum();
- for ( int i = 0; i < nSecNo; ++i ) { // For all sections
- const IELFISection* pSec = pReader->GetSection( i );
- std::cout << pSec->GetName() << '' ''
- << pSec->GetSize() << std::endl;
- pSec->Release();
- }
-
- First, the number of sections are received; next, a pointer - on the IELFISection interface. Using this interface, - access is gained to the different section attributes: size, type, flags and address. - To get a buffer that contains the section's bytes use the - GetData() member function of this interface. - See the IELFISection declaration for a full - description of the IELFISection interface. -
-After the section data is received through the GetData() - function call, the data can be manipulated. - There are special sections - that provide information in predefined forms. The ELFIO library - processes these sections. The library provides a set of - section readers that understand these predefined formats and how to process their data. - The ELFIO.h header file currently defines the types of readers as: -
enum ReaderType {
- ELFI_STRING, // Strings reader
- ELFI_SYMBOL, // Symbol table reader
- ELFI_RELOCATION, // Relocation table reader
- ELFI_NOTE, // Notes reader
- ELFI_DYNAMIC, // Dynamic section reader
- ELFI_HASH // Hash
- };
-
- How to use the symbol table reader will be demonstated in the following example: -
-First, get the symbol section: -
const IELFISection* pSec = pReader->GetSection( ''.symtab'' );- -
Second, create a symbol section reader: -
IELFISymbolTable* pSymTbl = 0; - pReader->CreateSectionReader( IELFI::ELFI_SYMBOL, - pSec, - (void**)&pSymTbl );- -
And finally, use the section reader to process all entries - (print operations are omitted): -
std::string name;
- Elf32_Addr value;
- Elf32_Word size;
- unsigned char bind;
- unsigned char type;
- Elf32_Half section;
- int nSymNo = pSymTbl->GetSymbolNum();
- if ( 0 < nSymNo ) {
- for ( int i = 0; i < nSymNo; ++i ) {
- pSymTbl->GetSymbol( i, name, value, size,
- bind, type, section );
- }
- }
- pSymTbl->Release();
- pSec->Release();
-
- All interfaces from the ELFIO library should be freed after - use. Each interface has a Release() function. - It is not enough to only free the high level interface because - one of the sections or readers will still be held and its resources will not be cleared. -
-The interfaces are freed immediately after their use, in this - example we will free only the pReader object: -
pReader->Release();- -