tutorial.docbook

C++写的,提供操作ELF格式文件的API

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
]>

<book>
  <bookinfo>
    <date>2002-5-25</date>
    <title>ELFIO</title>
    <subtitle>Tutorial</subtitle>
    <authorgroup>
      <author>
        <firstname>Allan</firstname>
        <surname>Finch</surname>
      </author>
      <author>
        <firstname>Serge</firstname>
        <surname>Lamikhov-Center</surname>
      </author>
    </authorgroup>
  </bookinfo>
  <toc></toc>
  <preface id="introduction">
    <title>Introduction</title>
    <para>
      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.
    </para>
    <para>
      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.
    </para>
  </preface>
  <chapter id="get-started">
    <title>Getting Started With ELFIO</title>

    <sect1><title>Initialization</title>
      <para>
        The ELFIO library consists of two independent parts: ELF File Reader
        (<interfacename>IELFI</interfacename>)
        and ELF Producer (<interfacename>IELFO</interfacename>).
        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.
      </para>
      <para>
        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.
<programlisting><![CDATA[
    #include <ELFIO.h>
]]></programlisting>
      </para>
      <para>
        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:
<programlisting><![CDATA[
    IELFI* pReader;
    ELFIO::GetInstance()->CreateELFI( &pReader );
]]></programlisting>
      </para>
      <para>
        Now, that there is a pointer on the
        <interfacename>IELFI</interfacename> interface: initialize the
        object by loading the ELF file:
<programlisting><![CDATA[
    char* filename = "file.o";
    pReader->Load( filename );
]]></programlisting>
      </para>
      <para>
        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:
<programlisting><![CDATA[
    unsigned char encoding = pReader->GetEncoding();
]]></programlisting>
      </para>
      <para>
        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.
    </para>
    </sect1>

    <sect1><title>ELF File Sections</title>
      <para>
        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.
      </para>
      <para>
        To see how many sections the ELF file contains, including their
        names and sizes, is demonstated in the following code:
<programlisting><![CDATA[
    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();
    }
]]></programlisting>
      </para>
      <para>
        First, the number of sections are received; next, a pointer
        on the <interfacename>IELFISection</interfacename> 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
        <methodname>GetData()</methodname> member function of this interface.
        See the <interfacename>IELFISection</interfacename> declaration for
        a full description of the <interfacename>IELFISection</interfacename>
        interface.
      </para>
    </sect1>

    <sect1><title>Section Readers</title>
      <para>
        After the section data is received through the
        <methodname>GetData()</methodname> 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:
<programlisting><![CDATA[
    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
    };
]]></programlisting>
      </para>
      <para>
        How to use the symbol table reader will be demonstated in the
        following example:
      </para>
      <para>
        First, get the symbol section:
<programlisting><![CDATA[
    const IELFISection* pSec = pReader->GetSection( ''.symtab'' );
]]></programlisting>
      </para>
      <para>
        Second, create a symbol section reader:
<programlisting><![CDATA[
    IELFISymbolTable* pSymTbl = 0;
    pReader->CreateSectionReader( IELFI::ELFI_SYMBOL,
                                  pSec,
                                  (void**)&pSymTbl );
]]></programlisting>
      </para>
      <para>
        And finally, use the section reader to process all entries
        (print operations are omitted):
<programlisting><![CDATA[
    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();
]]></programlisting>
      </para>
    </sect1>

    <sect1><title>Finalization</title>
      <para>
        All interfaces from the ELFIO library should be freed after
        use. Each interface has a <methodname>Release()</methodname> 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.
      </para>
      <para>
        The interfaces are freed immediately after their use, in this
        example we will free only the pReader object:
<programlisting><![CDATA[
    pReader->Release();
]]></programlisting>
      </para>
    </sect1>
  </chapter>
  <chapter id="elfdump">
    <title>ELFDump Utility</title>
    <para>
      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.
    </para>
  </chapter>
  <chapter id="ielfo">
    <title>
      <interfacename>IELFO</interfacename> - ELF File Producer Interface
    </title>
    <para>
      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.
    </para>
    <para>
      Just as with the reader, the first step is to get
      a pointer onto the ELF File Writer (Producer):
<programlisting><![CDATA[
    IELFO* pELFO;
    ELFIO::GetInstance()->CreateELFO( &pELFO );
]]></programlisting>
    </para>
    <para>
      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:
<programlisting><![CDATA[
    // You can't proceed without this function call!
    pELFO->SetAttr( ELFCLASS32, ELFDATA2LSB, EV_CURRENT,
                    ET_EXEC, EM_386, EV_CURRENT, 0 );
]]></programlisting>
    </para>
    <para>
      Some sections of an ELF executable file should reside in the program
      segments. To create this loadable segment call the
      <methodname>AddSegment()</methodname> function.
<programlisting><![CDATA[
    // Create a loadable segment
    IELFOSegment* pSegment = pELFO->AddSegment( PT_LOAD,
                                                0x08040000,
                                                0x08040000,
                                                PF_X | PF_R,
                                                0x1000 );
]]></programlisting>
    </para>
    <para>
      The following segment serves as a placeholder for our code section.
      To create this code section call the AddSection() function:
<programlisting><![CDATA[
    // Create code section
    IELFOSection* pTextSec = pELFO->AddSection( ".text",
                                   SHT_PROGBITS,
                                   SHF_ALLOC | SHF_EXECINSTR,
                                   0,
                                   0x10,
                                   0 );
]]></programlisting>
    </para>
    <para>
      Then, add the executable code for the section:
<programlisting><![CDATA[
    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 ) );
]]></programlisting>
    </para>
    <para>
      Next, this code section is put into the loadable segment:
<programlisting><![CDATA[
    // Add code section into program segment
    pSegment->AddSection( pTextSec );
    pTextSec->Release();
    pSegment->Release();
]]></programlisting>
    </para>
    <para>
      Finally, define the start address of the program
      and create the result file:
<programlisting><![CDATA[
    // Set program entry point
    pELFO->SetEntry( 0x08040000 );
    // Create ELF file
    pELFO->Save( "test.elf" );
    pELFO->Release();
]]></programlisting>
    </para>
    <para>
      Please note: Call the <methodname>Release()</methodname> functions
      for each interface you have used.
      This will free all resources the ELFIO library has created.
    </para>
    <para>
      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:
<programlisting><![CDATA[
    [Writer]$ ./Writer
    [Writer]$ chmod +x test.elf
    [Writer]$ ./test.elf
    Hello, World!
]]></programlisting>
    </para>
    <para>
      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.
    </para>
  </chapter>
</book>