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NAME="ielfo"
>Chapter 3. <FONT
COLOR="RED"
>IELFO</FONT
> - ELF File Producer Interface
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>
<P
> 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.
</P
>
<P
> Just as with the reader, the first step is to get
a pointer onto the ELF File Writer (Producer):
<PRE
CLASS="PROGRAMLISTING"
> IELFO* pELFO;
ELFIO::GetInstance()->CreateELFO( &pELFO );</PRE
>
</P
>
<P
> 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:
<PRE
CLASS="PROGRAMLISTING"
> // You can't proceed without this function call!
pELFO->SetAttr( ELFCLASS32, ELFDATA2LSB, EV_CURRENT,
ET_EXEC, EM_386, EV_CURRENT, 0 );</PRE
>
</P
>
<P
> Some sections of an ELF executable file should reside in the program
segments. To create this loadable segment call the
<TT
CLASS="METHODNAME"
>AddSegment()</TT
> function.
<PRE
CLASS="PROGRAMLISTING"
> // Create a loadable segment
IELFOSegment* pSegment = pELFO->AddSegment( PT_LOAD,
0x08040000,
0x08040000,
PF_X | PF_R,
0x1000 );</PRE
>
</P
>
<P
> The following segment serves as a placeholder for our code section. To create
this code section call the AddSection() function:
<PRE
CLASS="PROGRAMLISTING"
> // Create code section
IELFOSection* pTextSec = pELFO->AddSection( ".text",
SHT_PROGBITS,
SHF_ALLOC | SHF_EXECINSTR,
0,
0x10,
0 );</PRE
>
</P
>
<P
> Then, add the executable code for the section:
<PRE
CLASS="PROGRAMLISTING"
> 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 ) );</PRE
>
</P
>
<P
> Next, this code section is put into the loadable segment:
<PRE
CLASS="PROGRAMLISTING"
> // Add code section into program segment
pSegment->AddSection( pTextSec );
pTextSec->Release();
pSegment->Release();</PRE
>
</P
>
<P
> Finally, define the start address of the program
and create the result file:
<PRE
CLASS="PROGRAMLISTING"
> // Set program entry point
pELFO->SetEntry( 0x08040000 );
// Create ELF file
pELFO->Save( "test.elf" );
pELFO->Release();</PRE
>
</P
>
<P
> Please note: Call the <TT
CLASS="METHODNAME"
>Release()</TT
> functions
for each interface you have used.
This will free all resources the ELFIO library has created.
</P
>
<P
> 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:
<PRE
CLASS="PROGRAMLISTING"
> [Writer]$ ./Writer
[Writer]$ chmod +x test.elf
[Writer]$ ./test.elf
Hello, World!</PRE
>
</P
>
<P
> 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.
</P
>
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