The Portable Executable File Format from Top to Bottom

Randy Kath
Microsoft Developer Network Technology Group

Abstract

The Windows NT version 3.1 operating system introduces a new executable file
format called the Portable Executable (PE) file format. The Portable Executable
File Format specification, though rather vague, has been made available to the
public and is included on the Microsoft Developer Network CD (Specs and
Strategy, Specifications, Windows NT File Format Specifications).

Yet this specification alone does not provide enough information to make it
easy, or even reasonable, for developers to understand the PE file format. This
article is meant to address that problem. In it you'll find a thorough
explanation of the entire PE file format, along with descriptions of all the
necessary structures and source code examples that demonstrate how to use this
information.

All of the source code examples that appear in this article are taken from a
dynamic-link library (DLL) called PEFILE.DLL. I wrote this DLL simply for the
purpose of getting at the important information contained within a PE file. The
DLL and its source code are also included on this CD as part of the PEFile
sample application; feel free to use the DLL in your own applications. Also,
feel free to take the source code and build on it for any specific purpose you
may have. At the end of this article, you'll find a brief list of the functions
exported from the PEFILE.DLL and an explanation of how to use them. I think
you'll find these functions make understanding the PE file format easier to
cope with.

Introduction

The recent addition of the Microsoft® Windows NT operating system to the
family of Windows operating systems brought many changes to the development
environment and more than a few changes to applications themselves. One of the
more significant changes is the introduction of the Portable Executable (PE)
file format. The new PE file format draws primarily from the COFF (Common
Object File Format) specification that is common to UNIX® operating systems.
Yet, to remain compatible with previous versions of the MS-DOS® and Windows
operating systems, the PE file format also retains the old familiar MZ header
from MS-DOS.

In this article, the PE file format is explained using a top-down approach.
This article discusses each of the components of the file as they occur when
you traverse the file's contents, starting at the top and working your way down
through the file.

Much of the definition of individual file components comes from the file
WINNT.H, a file included in the Microsoft Win32™ Software Development Kit (SDK)
for Windows NT. In it you will find structure type definitions for each of the
file headers and data directories used to represent various components in the
file. In other places in the file, WINNT.H lacks sufficient definition of the
file structure. In these places, I chose to define my own structures that can
be used to access the data from the file. You will find these structures
defined in PEFILE.H, a file used to create the PEFILE.DLL. The entire suite of
PEFILE.H development files is included in the PEFile sample application.

In addition to the PEFILE.DLL sample code, a separate Win32-based sample
application called EXEVIEW.EXE accompanies this article. This sample was
created for two purposes: First, I needed a way to be able to test the
PEFILE.DLL functions, which in some cases required multiple file views
simultaneously--hence the multiple view support. Second, much of the work of
figuring out PE file format involved being able to see the data interactively.
For example, to understand how the import address name table is structured, I
had to view the .idata section header, the import image data directory, the
optional header, and the actual .idata section body, all simultaneously.
EXEVIEW.EXE is the perfect sample for viewing that information.

Without further ado, let's begin.

Structure of PE Files

The PE file format is organized as a linear stream of data. It begins with an
MS-DOS header, a real-mode program stub, and a PE file signature. Immediately
following is a PE file header and optional header. Beyond that, all the section
headers appear, followed by all of the section bodies. Closing out the file are
a few other regions of miscellaneous information, including relocation
information, symbol table information, line number information, and string
table data. All of this is more easily absorbed by looking at it graphically,
as shown in Figure 1.

Figure 1. Structure of a Portable Executable file image

Starting with the MS-DOS file header structure, each of the components in the
PE file format is discussed below in the order in which it occurs in the file.
Much of this discussion is based on sample code that demonstrates how to get to
the information in the file. All of the sample code is taken from the file
PEFILE.C, the source module for PEFILE.DLL. Each of these examples takes
advantage of one of the coolest features of Windows NT, memory-mapped files.
Memory-mapped files permit the use of simple pointer dereferencing to access
the data contained within the file. Each of the examples uses memory-mapped
files for accessing data in PE files.

Note Refer to the section at the end of this article for a discussion on how to
use PEFILE.DLL.

MS-DOS/Real-Mode Header

As mentioned above, the first component in the PE file format is the MS-DOS
header. The MS-DOS header is not new for the PE file format. It is the same
MS-DOS header that has been around since version 2 of the MS-DOS operating
system. The main reason for keeping the same structure intact at the beginning
of the PE file format is so that, when you attempt to load a file created under
Windows version 3.1 or earlier, or MS DOS version 2.0 or later, the operating
system can read the file and understand that it is not compatible. In other
words, when you attempt to run a Windows NT executable on MS-DOS version 6.0,
you get this message: "This program cannot be run in DOS mode." If the MS-DOS
header was not included as the first part of the PE file format, the operating
system would simply fail the attempt to load the file and offer something
completely useless, such as: "The name specified is not recognized as an
internal or external command, operable program or batch file."

The MS-DOS header occupies the first 64 bytes of the PE file. A structure
representing its content is described below:

WINNT.H

typedef struct _IMAGE_DOS_HEADER {  // DOS .EXE header
     USHORT e_magic;         // Magic number
     USHORT e_cblp;          // Bytes on last page of file
     USHORT e_cp;            // Pages in file
     USHORT e_crlc;          // Relocations
     USHORT e_cparhdr;       // Size of header in paragraphs
     USHORT e_minalloc;      // Minimum extra paragraphs needed
     USHORT e_maxalloc;      // Maximum extra paragraphs needed
     USHORT e_ss;            // Initial (relative) SS value
     USHORT e_sp;            // Initial SP value
     USHORT e_csum;          // Checksum
     USHORT e_ip;            // Initial IP value
     USHORT e_cs;            // Initial (relative) CS value
     USHORT e_lfarlc;        // File address of relocation table
     USHORT e_ovno;          // Overlay number
     USHORT e_res[4];        // Reserved words
     USHORT e_oemid;         // OEM identifier (for e_oeminfo)
     USHORT e_oeminfo;       // OEM information; e_oemid specific
     USHORT e_res2[10];      // Reserved words
     LONG   e_lfanew;        // File address of new exe header
   } IMAGE_DOS_HEADER, *PIMAGE_DOS_HEADER;

The first field, e_magic, is the so-called magic number. This field is used to
identify an MS-DOS-compatible file type. All MS-DOS-compatible executable files
set this value to 0x54AD, which represents the ASCII characters MZ. MS-DOS
headers are sometimes referred to as MZ headers for this reason. Many other
fields are important to MS-DOS operating systems, but for Windows NT, there is
really one more important field in this structure. The final field, e_lfanew,
is a 4-byte offset into the file where the PE file header is located. It is
necessary to use this offset to locate the PE header in the file. For PE files
in Windows NT, the PE file header occurs soon after the MS-DOS header with only
the real-mode stub program between them.

Real-Mode Stub Program

The real-mode stub program is an actual program run by MS-DOS when the
executable is loaded. For an actual MS-DOS executable image file, the
application begins executing here. For successive operating systems, including
Windows, OS/2®, and Windows NT, an MS-DOS stub program is placed here that runs
instead of the actual application. The programs typically do no more than
output a line of text, such as: "This program requires Microsoft Windows v3.1
or greater." Of course, whoever creates the application is able to place any
stub they like here, meaning you may often see such things as: "You can't run a
Windows NT application on OS/2, it's simply not possible."

When building an application for Windows version 3.1, the linker links a
default stub program called WINSTUB.EXE into your executable. You can override
the default linker behavior by substituting your own valid MS-DOS-based program
in place of WINSTUB and indicating this to the linker with the STUB module
definition statement. Applications developed for Windows NT can do the same
thing by using the -STUB: linker option when linking the executable file.

PE File Header and Signature

The PE file header is located by indexing the e_lfanew field of the MS-DOS
header. The e_lfanew field simply gives the offset in the file, so add the
file's memory-mapped base address to determine the actual memory-mapped
address. For example, the following macro is included in the PEFILE.H source
file:

PEFILE.H

#define NTSIGNATURE(a) ((LPVOID)((BYTE *)a +    \
                         ((PIMAGE_DOS_HEADER)a)->e_lfanew))

When manipulating PE file information, I found that there were several
locations in the file that I needed to refer to often. Since these locations
are merely offsets into the file, it is easier to implement these locations as
macros because they provide much better performance than functions do.

Notice that instead of retrieving the offset of the PE file header, this macro
retrieves the location of the PE file signature. Starting with Windows and OS/2
executables, .EXE files were given file signatures to specify the intended
target operating system. For the PE file format in Windows NT, this signature
occurs immediately before the PE file header structure. In versions of Windows
and OS/2, the signature is the first word of the file header. Also, for the PE
file format, Windows NT uses a DWORD for the signature.

The macro presented above returns the offset of where the file signature
appears, regardless of which type of executable file it is. So depending on
whether it's a Windows NT file signature or not, the file header exists either
after the signature DWORD or at the signature WORD. To resolve this confusion,
I wrote the ImageFileType function (following), which returns the type of image
file:

PEFILE.C

DWORD  WINAPI ImageFileType (
     LPVOID    lpFile)
{
     /* DOS file signature comes first. */
     if (*(USHORT *)lpFile == IMAGE_DOS_SIGNATURE)
         {
         /* Determine location of PE File header from
            DOS header. */
         if (LOWORD (*(DWORD *)NTSIGNATURE (lpFile)) ==
                                 IMAGE_OS2_SIGNATURE ||
             LOWORD (*(DWORD *)NTSIGNATURE (lpFile)) ==
                              IMAGE_OS2_SIGNATURE_LE)
             return (DWORD)LOWORD(*(DWORD *)NTSIGNATURE (lpFile));

         else if (*(DWORD *)NTSIGNATURE (lpFile) ==
                             IMAGE_NT_SIGNATURE)
             return IMAGE_NT_SIGNATURE;

         else
             return IMAGE_DOS_SIGNATURE;
         }

     else
         /* unknown file type */
         return 0;
}

The code listed above quickly shows how useful the NTSIGNATURE macro becomes.
The macro makes it easy to compare the different file types and return the
appropriate one for a given type of file. The four different file types defined
in WINNT.H are:

WINNT.H

#define IMAGE_DOS_SIGNATURE             0x5A4D      // MZ
#define IMAGE_OS2_SIGNATURE             0x454E      // NE
#define IMAGE_OS2_SIGNATURE_LE          0x454C      // LE
#define IMAGE_NT_SIGNATURE              0x00004550  // PE00

At first it seems curious that Windows executable file types do not appear on
this list. But then, after a little investigation, the reason becomes clear:
There really is no difference between Windows executables and OS/2 executables
other than the operating system version specification. Both operating systems
share the same executable file structure.

Turning our attention back to the Windows NT PE file format, we find that once
we have the location of the file signature, the PE file follows four bytes
later. The next macro identifies the PE file header:

PEFILE.C

#define PEFHDROFFSET(a) ((LPVOID)((BYTE *)a +  \
     ((PIMAGE_DOS_HEADER)a)->e_lfanew + SIZE_OF_NT_SIGNATURE))

The only difference between this and the previous macro is that this one adds
in the constant SIZE_OF_NT_SIGNATURE. Sad to say, this constant is not defined
in WINNT.H, but is instead one I defined in PEFILE.H as the size of a DWORD.

Now that we know the location of the PE file header, we can examine the data in
the header simply by assigning this location to a structure, as in the
following example:

PIMAGE_FILE_HEADER   pfh;

pfh = (PIMAGE_FILE_HEADER)PEFHDROFFSET (lpFile);

In this example, lpFile represents a pointer to the base of the memory-mapped
executable file, and therein lies the convenience of memory-mapped files. No
file I/O needs to be performed; simply dereference the pointer pfh to access
information in the file. The PE file header structure is defined as:

WINNT.H

typedef struct _IMAGE_FILE_HEADER {
     USHORT  Machine;
     USHORT  NumberOfSections;
     ULONG   TimeDateStamp;
     ULONG   PointerToSymbolTable;
     ULONG   NumberOfSymbols;
     USHORT  SizeOfOptionalHeader;
     USHORT  Characteristics;
} IMAGE_FILE_HEADER, *PIMAGE_FILE_HEADER;

#define IMAGE_SIZEOF_FILE_HEADER             20

Notice that the size of the file header structure is conveniently defined in
the include file. This makes it easy to get the size of the structure, but I
found it easier to use the sizeof function on the structure itself because it
does not require me to remember the name of the constant
IMAGE_SIZEOF_FILE_HEADER in addition to the IMAGE_FILE_HEADER structure name
itself. On the other hand, remembering the name of all the structures proved
challenging enough, especially since none of these structures is documented
anywhere except in the WINNT.H include file.

The information in the PE file is basically high-level information that is used
by the system or applications to determine how to treat the file. The first
field is used to indicate what type of machine the executable was built for,
such as the DEC® Alpha, MIPS R4000, Intel® x86, or some other processor. The
system uses this information to quickly determine how to treat the file before
going any further into the rest of the file data.

The Characteristics field identifies specific characteristics about the file.
For example, consider how separate debug files are managed for an executable.
It is possible to strip debug information from a PE file and store it in a
debug file (.DBG) for use by debuggers. To do this, a debugger needs to know
whether to find the debug information in a separate file or not and whether the
information has been stripped from the file or not. A debugger could find out
by drilling down into the executable file looking for debug information. To
save the debugger from having to search the file, a file characteristic that
indicates that the file has been stripped (IMAGE_FILE_DEBUG_STRIPPED) was
invented. Debuggers can look in the PE file header to quickly determine whether
the debug information is present in the file or not.

WINNT.H defines several other flags that indicate file header information much
the way the example described above does. I'll leave it as an exercise for the
reader to look up the flags to see if any of them are interesting or not. They
are located in WINNT.H immediately after the IMAGE_FILE_HEADER structure
described above.

One other useful entry in the PE file header structure is the NumberOfSections
field. It turns out that you need to know how many sections--more specifically,
how many section headers and section bodies--are in the file in order to
extract the information easily. Each section header and section body is laid
out sequentially in the file, so the number of sections is necessary to
determine where the section headers and bodies end. The following function
extracts the number of sections from the PE file header:

PEFILE.C

int   WINAPI NumOfSections (
     LPVOID    lpFile)
{
     /* Number of sections is indicated in file header. */
     return (int)((PIMAGE_FILE_HEADER)
                   PEFHDROFFSET (lpFile))->NumberOfSections);
}

As you can see, the PEFHDROFFSET and the other macros are pretty handy to have
around.

PE Optional Header

The next 224 bytes in the executable file make up the PE optional header.
Though its name is "optional header," rest assured that this is not an optional
entry in PE executable files. A pointer to the optional header is obtained with
the OPTHDROFFSET macro:

PEFILE.H

#define OPTHDROFFSET(a) ((LPVOID)((BYTE *)a                 + \
     ((PIMAGE_DOS_HEADER)a)->e_lfanew + SIZE_OF_NT_SIGNATURE + \
     sizeof (IMAGE_FILE_HEADER)))

The optional header contains most of the meaningful information about the
executable image, such as initial stack size, program entry point location,
preferred base address, operating system version, section alignment
information, and so forth. The IMAGE_OPTIONAL_HEADER structure represents the
optional header as follows:

WINNT.H

typedef struct _IMAGE_OPTIONAL_HEADER {
     //
     // Standard fields.
     //
     USHORT  Magic;
     UCHAR   MajorLinkerVersion;
     UCHAR   MinorLinkerVersion;
     ULONG   SizeOfCode;
     ULONG   SizeOfInitializedData;
     ULONG   SizeOfUninitializedData;
     ULONG   AddressOfEntryPoint;
     ULONG   BaseOfCode;
     ULONG   BaseOfData;
     //
     // NT additional fields.
     //
     ULONG   ImageBase;
     ULONG   SectionAlignment;
     ULONG   FileAlignment;
     USHORT  MajorOperatingSystemVersion;
     USHORT  MinorOperatingSystemVersion;
     USHORT  MajorImageVersion;
     USHORT  MinorImageVersion;
     USHORT  MajorSubsystemVersion;
     USHORT  MinorSubsystemVersion;
     ULONG   Reserved1;
     ULONG   SizeOfImage;
     ULONG   SizeOfHeaders;
     ULONG   CheckSum;
     USHORT  Subsystem;
     USHORT  DllCharacteristics;
     ULONG   SizeOfStackReserve;
     ULONG   SizeOfStackCommit;
     ULONG   SizeOfHeapReserve;
     ULONG   SizeOfHeapCommit;
     ULONG   LoaderFlags;
     ULONG   NumberOfRvaAndSizes;
     IMAGE_DATA_DIRECTORY DataDirectory[IMAGE_NUMBEROF_DIRECTORY_ENTRIES];
} IMAGE_OPTIONAL_HEADER, *PIMAGE_OPTIONAL_HEADER;

As you can see, the list of fields in this structure is rather lengthy. Rather
than bore you with descriptions of all of these fields, I'll simply discuss the
useful ones--that is, useful in the context of exploring the PE file format.

Standard Fields

First, note that the structure is divided into "Standard fields" and "NT
additional fields." The standard fields are those common to the Common Object
File Format (COFF), which most UNIX executable files use. Though the standard
fields retain the names defined in COFF, Windows NT actually uses some of them
for different purposes that would be better described with other names.

    * Magic. I was unable to track down what this field is used for. For the
      EXEVIEW.EXE sample application, the value is 0x010B or 267.
    * MajorLinkerVersion, MinorLinkerVersion. Indicates version of the linker
      that linked this image. The preliminary Windows NT Software Development
      Kit (SDK), which shipped with build 438 of Windows NT, includes linker
      version 2.39 (2.27 hex).
    * SizeOfCode. Size of executable code.
    * SizeOfInitializedData. Size of initialized data.
    * SizeOfUninitializedData. Size of uninitialized data.
    * AddressOfEntryPoint. Of the standard fields, the AddressOfEntryPoint field
      is the most interesting for the PE file format. This field indicates the
      location of the entry point for the application and, perhaps more
      importantly to system hackers, the location of the end of the Import
      Address Table (IAT). The following function demonstrates how to retrieve
      the entry point of a Windows NT executable image from the optional header.

      PEFILE.C

           LPVOID  WINAPI GetModuleEntryPoint (
               LPVOID    lpFile)
           {
               PIMAGE_OPTIONAL_HEADER   poh;

               poh = (PIMAGE_OPTIONAL_HEADER)OPTHDROFFSET (lpFile);

               if (poh != NULL)
                   return (LPVOID)poh->AddressOfEntryPoint;
               else
                   return NULL;
           }

    * BaseOfCode. Relative offset of code (".text" section) in loaded image.
    * BaseOfData. Relative offset of uninitialized data (".bss" section) in
      loaded image.

Windows NT Additional Fields

The additional fields added to the Windows NT PE file format provide loader
support for much of the Windows NT-specific process behavior. Following is a
summary of these fields.

    * ImageBase. Preferred base address in the address space of a process to map
      the executable image to. The linker that comes with the Microsoft Win32
      SDK for Windows NT defaults to 0x00400000, but you can override the
      default with the -BASE: linker switch.
    * SectionAlignment. Each section is loaded into the address space of a
      process sequentially, beginning at ImageBase. SectionAlignment dictates
      the minimum amount of space a section can occupy when loaded--that is,
      sections are aligned on SectionAlignment boundaries.

      Section alignment can be no less than the page size (currently 4096 bytes
      on the x86 platform) and must be a multiple of the page size as dictated
      by the behavior of Windows NT's virtual memory manager. 4096 bytes is the
      x86 linker default, but this can be set using the -ALIGN: linker switch.

    * FileAlignment. Minimum granularity of chunks of information within the
      image file prior to loading. For example, the linker zero-pads a section
      body (raw data for a section) up to the nearest FileAlignment boundary in
      the file. Version 2.39 of the linker mentioned earlier aligns image files
      on a 0x200-byte granularity. This value is constrained to be a power of 2
      between 512 and 65,535.
    * * MajorOperatingSystemVersion. Indicates the major version of the Windows
      NT operating system, currently set to 1 for Windows NT version 1.0.
    * MinorOperatingSystemVersion. Indicates the minor version of the Windows NT
      operating system, currently set to 0 for Windows NT version 1.0
    * MajorImageVersion. Used to indicate the major version number of the
      application; in Microsoft Excel version 4.0, it would be 4.
    * MinorImageVersion. Used to indicate the minor version number of the
      application; in Microsoft Excel version 4.0, it would be 0.
    * MajorSubsystemVersion. Indicates the Windows NT Win32 subsystem major
      version number, currently set to 3 for Windows NT version 3.10.
    * MinorSubsystemVersion. Indicates the Windows NT Win32 subsystem minor
      version number, currently set to 10 for Windows NT version 3.10.
    * Reserved1. Unknown purpose, currently not used by the system and set to
      zero by the linker.
    * * SizeOfImage. Indicates the amount of address space to reserve in the
      address space for the loaded executable image. This number is influenced
      greatly by SectionAlignment. For example, consider a system having a fixed
      page size of 4096 bytes. If you have an executable with 11 sections, each
      less than 4096 bytes, aligned on a 65,536-byte boundary, the SizeOfImage
      field would be set to 11 * 65,536 = 720,896 (176 pages). The same file
      linked with 4096-byte alignment would result in 11 * 4096 = 45,056 (11
      pages) for the SizeOfImage field. This is a simple example in which each
      section requires less than a page of memory. In reality, the linker
      determines the exact SizeOfImage by figuring each section individually. It
      first determines how many bytes the section requires, then it rounds up to
      the nearest page boundary, and finally it rounds page count to the nearest
      SectionAlignment boundary. The total is then the sum of each section's
      individual requirement.
    * SizeOfHeaders. This field indicates how much space in the file is used for
      representing all the file headers, including the MS-DOS header, PE file
      header, PE optional header, and PE section headers. The section bodies
      begin at this location in the file.
    * CheckSum. A checksum value is used to validate the executable file at load
      time. The value is set and verified by the linker. The algorithm used for
      creating these checksum values is proprietary information and will not be
      published.
    * Subsystem. Field used to identify the target subsystem for this
      executable. Each of the possible subsystem values are listed in the
      WINNT.H file immediately after the IMAGE_OPTIONAL_HEADER structure.
    * DllCharacteristics. Flags used to indicate if a DLL image includes entry
      points for process and thread initialization and termination.
    * SizeOfStackReserve, SizeOfStackCommit, SizeOfHeapReserve,
      SizeOfHeapCommit. These fields control the amount of address space to
      reserve and commit for the stack and default heap. Both the stack and heap
      have default values of 1 page committed and 16 pages reserved. These
      values are set with the linker switches -STACKSIZE: and -HEAPSIZE:.
    * LoaderFlags. Tells the loader whether to break on load, debug on load, or
      the default, which is to let things run normally.
    * NumberOfRvaAndSizes. This field identifies the length of the DataDirectory
      array that follows. It is important to note that this field is used to
      identify the size of the array, not the number of valid entries in the
      array.
    * DataDirectory. The data directory indicates where to find other important
      components of executable information in the file. It is really nothing
      more than an array of IMAGE_DATA_DIRECTORY structures that are located at
      the end of the optional header structure. The current PE file format
      defines 16 possible data directories, 11 of which are now being used.

Data Directories

As defined in WINNT.H, the data directories are:

WINNT.H

// Directory Entries

// Export Directory
#define IMAGE_DIRECTORY_ENTRY_EXPORT         0
// Import Directory
#define IMAGE_DIRECTORY_ENTRY_IMPORT         1
// Resource Directory
#define IMAGE_DIRECTORY_ENTRY_RESOURCE       2
// Exception Directory
#define IMAGE_DIRECTORY_ENTRY_EXCEPTION      3
// Security Directory
#define IMAGE_DIRECTORY_ENTRY_SECURITY       4
// Base Relocation Table
#define IMAGE_DIRECTORY_ENTRY_BASERELOC      5
// Debug Directory
#define IMAGE_DIRECTORY_ENTRY_DEBUG          6
// Description String
#define IMAGE_DIRECTORY_ENTRY_COPYRIGHT      7
// Machine Value (MIPS GP)
#define IMAGE_DIRECTORY_ENTRY_GLOBALPTR      8
// TLS Directory
#define IMAGE_DIRECTORY_ENTRY_TLS            9
// Load Configuration Directory
#define IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG    10

Each data directory is basically a structure defined as an
IMAGE_DATA_DIRECTORY. And although data directory entries themselves are the
same, each specific directory type is entirely unique. The definition of each
defined data directory is described in "Predefined Sections" later in this
article.

WINNT.H

typedef struct _IMAGE_DATA_DIRECTORY {
     ULONG   VirtualAddress;
     ULONG   Size;
} IMAGE_DATA_DIRECTORY, *PIMAGE_DATA_DIRECTORY;

Each data directory entry specifies the size and relative virtual address of
the directory. To locate a particular directory, you determine the relative
address from the data directory array in the optional header. Then use the
virtual address to determine which section the directory is in. Once you
determine which section contains the directory, the section header for that
section is then used to find the exact file offset location of the data
directory.

So to get a data directory, you first need to know about sections, which are
described next. An example of how to locate data directories immediately
follows this discussion.

PE File Sections

The PE file specification consists of the headers defined so far and a generic
object called a section. Sections contain the content of the file, including
code, data, resources, and other executable information. Each section has a
header and a body (the raw data). Section headers are described below, but
section bodies lack a rigid file structure. They can be organized in almost any
way a linker wishes to organize them, as long as the header is filled with
enough information to be able to decipher the data.

Section Headers

Section headers are located sequentially right after the optional header in the
PE file format. Each section header is 40 bytes with no padding between them.
Section headers are defined as in the following structure:

WINNT.H

#define IMAGE_SIZEOF_SHORT_NAME              8

typedef struct _IMAGE_SECTION_HEADER {
     UCHAR   Name[IMAGE_SIZEOF_SHORT_NAME];
     union {
             ULONG   PhysicalAddress;
             ULONG   VirtualSize;
     } Misc;
     ULONG   VirtualAddress;
     ULONG   SizeOfRawData;
     ULONG   PointerToRawData;
     ULONG   PointerToRelocations;
     ULONG   PointerToLinenumbers;
     USHORT  NumberOfRelocations;
     USHORT  NumberOfLinenumbers;
     ULONG   Characteristics;
} IMAGE_SECTION_HEADER, *PIMAGE_SECTION_HEADER;

How do you go about getting section header information for a particular
section? Since section headers are organized sequentially in no specific order,
section headers must be located by name. The following function shows how to
retrieve a section header from a PE image file given the name of the section:

PEFILE.C

BOOL    WINAPI GetSectionHdrByName (
     LPVOID                   lpFile,
     IMAGE_SECTION_HEADER     *sh,
     char                     *szSection)
{
     PIMAGE_SECTION_HEADER    psh;
     int                      nSections = NumOfSections (lpFile);
     int                      i;

     if ((psh = (PIMAGE_SECTION_HEADER)SECHDROFFSET (lpFile)) !=
          NULL)
         {
         /* find the section by name */
         for (i=0; i<nSections; i++)
             {
             if (!strcmp (psh->Name, szSection))
                 {
                 /* copy data to header */
                 CopyMemory ((LPVOID)sh,
                             (LPVOID)psh,
                             sizeof (IMAGE_SECTION_HEADER));
                 return TRUE;
                 }
             else
                 psh++;
             }
         }

     return FALSE;
}

The function simply locates the first section header via the SECHDROFFSET
macro. Then the function loops through each section, comparing each section's
name with the name of the section it's looking for, until it finds the right
one. When the section is found, the function copies the data from the
memory-mapped file to the structure passed in to the function. The fields of
the IMAGE_SECTION_HEADER structure can then be accessed directly from the
structure.

Section Header Fields

    * Name. Each section header has a name field up to eight characters long,
      for which the first character must be a period.
    * PhysicalAddress or VirtualSize. The second field is a union field that is
      not currently used.
    * VirtualAddress. This field identifies the virtual address in the process's
      address space to which to load the section. The actual address is created
      by taking the value of this field and adding it to the ImageBase virtual
      address in the optional header structure. Keep in mind, though, that if
      this image file represents a DLL, there is no guarantee that the DLL will
      be loaded to the ImageBase location requested. So once the file is loaded
      into a process, the actual ImageBase value should be verified
      programmatically using GetModuleHandle.
    * SizeOfRawData. This field indicates the FileAlignment-relative size of the
      section body. The actual size of the section body will be less than or
      equal to a multiple of FileAlignment in the file. Once the image is loaded
      into a process's address space, the size of the section body becomes less
      than or equal to a multiple of SectionAlignment.
    * PointerToRawData. This is an offset to the location of the section body in
      the file.
    * PointerToRelocations, PointerToLinenumbers, NumberOfRelocations,
      NumberOfLinenumbers. None of these fields are used in the PE file format.
    * Characteristics. Defines the section characteristics. These values are
      found both in WINNT.H and in the Portable Executable Format specification
      located on this CD.

  Value               Definition

  0x00000020          Code section

  0x00000040          Initialized data section

  0x00000080          Uninitialized data section

  0x04000000          Section cannot be cached

  0x08000000          Section is not pageable

  0x10000000          Section is shared

  0x20000000          Executable section

  0x40000000          Readable section

  0x80000000          Writable section

Locating Data Directories

Data directories exist within the body of their corresponding data section.
Typically, data directories are the first structure within the section body,
but not out of necessity. For that reason, you need to retrieve information
from both the section header and optional header to locate a specific data
directory.

To make this process easier, the following function was written to locate the
data directory for any of the directories defined in WINNT.H:

PEFILE.C

LPVOID  WINAPI ImageDirectoryOffset (
         LPVOID    lpFile,
         DWORD     dwIMAGE_DIRECTORY)
{
     PIMAGE_OPTIONAL_HEADER   poh;
     PIMAGE_SECTION_HEADER    psh;
     int                      nSections = NumOfSections (lpFile);
     int                      i = 0;
     LPVOID                   VAImageDir;

     /* Must be 0 thru (NumberOfRvaAndSizes-1). */
     if (dwIMAGE_DIRECTORY >= poh->NumberOfRvaAndSizes)
         return NULL;

     /* Retrieve offsets to optional and section headers. */
     poh = (PIMAGE_OPTIONAL_HEADER)OPTHDROFFSET (lpFile);
     psh = (PIMAGE_SECTION_HEADER)SECHDROFFSET (lpFile);

     /* Locate image directory's relative virtual address. */
     VAImageDir = (LPVOID)poh->DataDirectory
                        [dwIMAGE_DIRECTORY].VirtualAddress;

     /* Locate section containing image directory. */
     while (i++<nSections)
         {
         if (psh->VirtualAddress <= (DWORD)VAImageDir &&
             psh->VirtualAddress +
                  psh->SizeOfRawData > (DWORD)VAImageDir)
             break;
         psh++;
         }

     if (i > nSections)
         return NULL;

     /* Return image import directory offset. */
     return (LPVOID)(((int)lpFile +
                      (int)VAImageDir. psh->VirtualAddress) +
                     (int)psh->PointerToRawData);
}

The function begins by validating the requested data directory entry number.
Then it retrieves pointers to the optional header and first section header.
From the optional header, the function determines the data directory's virtual
address, and it uses this value to determine within which section body the data
directory is located. Once the appropriate section body has been identified,
the specific location of the data directory is found by translating the
relative virtual address of the data directory to a specific address into the
file.

Predefined Sections

An application for Windows NT typically has the nine predefined sections named
.text, .bss, .rdata, .data, .rsrc, .edata, .idata, .pdata, and .debug. Some
applications do not need all of these sections, while others may define still
more sections to suit their specific needs. This behavior is similar to code
and data segments in MS-DOS and Windows version 3.1. In fact, the way an
application defines a unique section is by using the standard compiler
directives for naming code and data segments or by using the name segment
compiler option -NT--exactly the same way in which applications defined unique
code and data segments in Windows version 3.1.

The following is a discussion of some of the more interesting sections common
to typical Windows NT PE files.

Executable code section, .text

One difference between Windows version 3.1 and Windows NT is that the default
behavior combines all code segments (as they are referred to in Windows version
3.1) into a single section called ".text" in Windows NT. Since Windows NT uses
a page-based virtual memory management system, there is no advantage to
separating code into distinct code segments. Consequently, having one large
code section is easier to manage for both the operating system and the
application developer.

The .text section also contains the entry point mentioned earlier. The IAT also
lives in the .text section immediately before the module entry point. (The
IAT's presence in the .text section makes sense because the table is really a
series of jump instructions, for which the specific location to jump to is the
fixed-up address.) When Windows NT executable images are loaded into a
process's address space, the IAT is fixed up with the location of each imported
function's physical address. In order to find the IAT in the .text section, the
loader simply locates the module entry point and relies on the fact that the
IAT occurs immediately before the entry point. And since each entry is the same
size, it is easy to walk backward in the table to find its beginning.

Data sections, .bss, .rdata, .data

The .bss section represents uninitialized data for the application, including
all variables declared as static within a function or source module.

The .rdata section represents read-only data, such as literal strings,
constants, and debug directory information.

All other variables (except automatic variables, which appear on the stack) are
stored in the .data section. Basically, these are application or module global
variables.

Resources section, .rsrc

The .rsrc section contains resource information for a module. It begins with a
resource directory structure like most other sections, but this section's data
is further structured into a resource tree. The IMAGE_RESOURCE_DIRECTORY, shown
below, forms the root and nodes of the tree.

WINNT.H

typedef struct _IMAGE_RESOURCE_DIRECTORY {
     ULONG   Characteristics;
     ULONG   TimeDateStamp;
     USHORT  MajorVersion;
     USHORT  MinorVersion;
     USHORT  NumberOfNamedEntries;
     USHORT  NumberOfIdEntries;
} IMAGE_RESOURCE_DIRECTORY, *PIMAGE_RESOURCE_DIRECTORY;

Looking at the directory structure, you won't find any pointer to the next
nodes. Instead, there are two fields, NumberOfNamedEntries and
NumberOfIdEntries, used to indicate how many entries are attached to the
directory. By attached, I mean the directory entries follow immediately after
the directory in the section data. The named entries appear first in ascending
alphabetical order, followed by the ID entries in ascending numerical order.

A directory entry consists of two fields, as described in the following
IMAGE_RESOURCE_DIRECTORY_ENTRY structure:

WINNT.H

typedef struct _IMAGE_RESOURCE_DIRECTORY_ENTRY {
     ULONG   Name;
     ULONG   OffsetToData;
} IMAGE_RESOURCE_DIRECTORY_ENTRY, *PIMAGE_RESOURCE_DIRECTORY_ENTRY;

The two fields are used for different things depending on the level of the
tree. The Name field is used to identify either a type of resource, a resource
name, or a resource's language ID. The OffsetToData field is always used to
point to a sibling in the tree, either a directory node or a leaf node.

Leaf nodes are the lowest node in the resource tree. They define the size and
location of the actual resource data. Each leaf node is represented using the
following IMAGE_RESOURCE_DATA_ENTRY structure:

WINNT.H

typedef struct _IMAGE_RESOURCE_DATA_ENTRY {
     ULONG   OffsetToData;
     ULONG   Size;
     ULONG   CodePage;
     ULONG   Reserved;
} IMAGE_RESOURCE_DATA_ENTRY, *PIMAGE_RESOURCE_DATA_ENTRY;

The two fields OffsetToData and Size indicate the location and size of the
actual resource data. Since this information is used primarily by functions
once the application has been loaded, it makes more sense to make the
OffsetToData field a relative virtual address. This is precisely the case.
Interestingly enough, all other offsets, such as pointers from directory
entries to other directories, are offsets relative to the location of the root
node.

To make all of this a little clearer, consider Figure 2.

  [PEF2034C  12787 bytes ]

Figure 2. A simple resource tree structure

Figure 2 depicts a very simple resource tree containing only two resource
objects, a menu, and a string table. Further, the menu and string table have
only one item each. Yet, you can see how complicated the resource tree
becomes--even with as few resources as this.

At the root of the tree, the first directory has one entry for each type of
resource the file contains, no matter how many of each type there are. In
Figure 2, there are two entries identified by the root, one for the menu and
one for the string table. If there had been one or more dialog resources
included in the file, the root node would have had one more entry and,
consequently, another branch for the dialog resources.

The basic resource types are identified in the file WINUSER.H and are listed
below:

WINUSER.H

/*
  * Predefined Resource Types
  */
#define RT_CURSOR           MAKEINTRESOURCE(1)
#define RT_BITMAP           MAKEINTRESOURCE(2)
#define RT_ICON             MAKEINTRESOURCE(3)
#define RT_MENU             MAKEINTRESOURCE(4)
#define RT_DIALOG           MAKEINTRESOURCE(5)
#define RT_STRING           MAKEINTRESOURCE(6)
#define RT_FONTDIR          MAKEINTRESOURCE(7)
#define RT_FONT             MAKEINTRESOURCE(8)
#define RT_ACCELERATOR      MAKEINTRESOURCE(9)
#define RT_RCDATA           MAKEINTRESOURCE(10)
#define RT_MESSAGETABLE     MAKEINTRESOURCE(11)

At the top level of the tree, the MAKEINTRESOURCE values listed above are
placed in the Name field of each type entry, identifying the different
resources by type.

Each of the entries in the root directory points to a sibling node in the
second level of the tree. These nodes are directories, too, each having their
own entries. At this level, the directories are used to identify the name of
each resource within a given type. If you had multiple menus defined in your
application, there would be an entry for each one here at the second level of
the tree.

As you are probably already aware, resources can be identified by name or by
integer. They are distinguished in this level of the tree via the Name field in
the directory structure. If the most significant bit of the Name field is set,
the other 31 bits are used as an offset to an IMAGE_RESOURCE_DIR_STRING_U
structure.

WINNT.H

typedef struct _IMAGE_RESOURCE_DIR_STRING_U {
     USHORT  Length;
     WCHAR   NameString[ 1 ];
} IMAGE_RESOURCE_DIR_STRING_U, *PIMAGE_RESOURCE_DIR_STRING_U;

This structure is simply a 2-byte Length field followed by Length UNICODE
characters.

On the other hand, if the most significant bit of the Name field is clear, the
lower 31 bits are used to represent the integer ID of the resource. Figure 2
shows the menu resource as a named resource and the string table as an ID
resource.

If there were two menu resources, one identified by name and one by resource,
they would both have entries immediately after the menu resource directory. The
named resource entry would appear first, followed by the integer-identified
resource. The directory fields NumberOfNamedEntries and NumberOfIdEntries would
each contain the value 1, indicating the presence of one entry.

Below level two, the resource tree does not branch out any further. Level one
branches into directories representing each type of resource, and level two
branches into directories representing each resource by identifier. Level three
maps a one-to-one correspondence between the individually identified resources
and their respective language IDs. To indicate the language ID of a resource,
the Name field of the directory entry structure is used to indicate both the
primary language and sublanguage ID for the resource. The Win32 SDK for Windows
NT lists the default value resources. For the value 0x0409, 0x09 represents the
primary language as LANG_ENGLISH, and 0x04 is defined as SUBLANG_ENGLISH_CAN
for the sublanguage. The entire set of language IDs is defined in the file
WINNT.H, included as part of the Win32 SDK for Windows NT.

Since the language ID node is the last directory node in the tree, the
OffsetToData field in the entry structure is an offset to a leaf node--the
IMAGE_RESOURCE_DATA_ENTRY structure mentioned earlier.

Referring back to Figure 2, you can see one data entry node for each language
directory entry. This node simply indicates the size of the resource data and
the relative virtual address where the resource data is located.

One advantage to having so much structure to the resource data section, .rsrc,
is that you can glean a great deal of information from the section without
accessing the resources themselves. For example, you can find out how many
there are of each type of resource, what resources--if any--use a particular
language ID, whether a particular resource exists or not, and the size of
individual types of resources. To demonstrate how to make use of this
information, the following function shows how to determine the different types
of resources a file includes:

PEFILE.C

int     WINAPI GetListOfResourceTypes (
     LPVOID    lpFile,
     HANDLE    hHeap,
     char      **pszResTypes)
{
     PIMAGE_RESOURCE_DIRECTORY          prdRoot;
     PIMAGE_RESOURCE_DIRECTORY_ENTRY    prde;
     char                               *pMem;
     int                                nCnt, i;

     /* Get root directory of resource tree. */
     if ((prdRoot = PIMAGE_RESOURCE_DIRECTORY)ImageDirectoryOffset
            (lpFile, IMAGE_DIRECTORY_ENTRY_RESOURCE)) == NULL)
         return 0;

     /* Allocate enough space from heap to cover all types. */
     nCnt = prdRoot->NumberOfIdEntries * (MAXRESOURCENAME + 1);
     *pszResTypes = (char *)HeapAlloc (hHeap,
                                       HEAP_ZERO_MEMORY,
                                       nCnt);
     if ((pMem = *pszResTypes) == NULL)
         return 0;

     /* Set pointer to first resource type entry. */
     prde = (PIMAGE_RESOURCE_DIRECTORY_ENTRY)((DWORD)prdRoot +
                sizeof (IMAGE_RESOURCE_DIRECTORY));

     /* Loop through all resource directory entry types. */
     for (i=0; i<prdRoot->NumberOfIdEntries; i++)
         {
         if (LoadString (hDll, prde->Name, pMem, MAXRESOURCENAME))
             pMem += strlen (pMem) + 1;

         prde++;
         }

     return nCnt;
}

This function returns a list of resource type names in the string identified by
pszResTypes. Notice that, at the heart of this function, LoadString is called
using the Name field of each resource type directory entry as the string ID. If
you look in the PEFILE.RC, you'll see that I defined a series of resource type
strings whose IDs are defined the same as the type specifiers in the directory
entries. There is also a function in PEFILE.DLL that returns the total number
of resource objects in the .rsrc section. It would be rather easy to expand on
these functions or write new functions that extracted other information from
this section.

Export data section, .edata

The .edata section contains export data for an application or DLL. When
present, this section contains an export directory for getting to the export
information.

WINNT.H

typedef struct _IMAGE_EXPORT_DIRECTORY {
     ULONG   Characteristics;
     ULONG   TimeDateStamp;
     USHORT  MajorVersion;
     USHORT  MinorVersion;
     ULONG   Name;
     ULONG   Base;
     ULONG   NumberOfFunctions;
     ULONG   NumberOfNames;
     PULONG  *AddressOfFunctions;
     PULONG  *AddressOfNames;
     PUSHORT *AddressOfNameOrdinals;
} IMAGE_EXPORT_DIRECTORY, *PIMAGE_EXPORT_DIRECTORY;

The Name field in the export directory identifies the name of the executable
module. NumberOfFunctions and NumberOfNames fields indicate how many functions
and function names are being exported from the module.

The AddressOfFunctions field is an offset to a list of exported function entry
points. The AddressOfNames field is the address of an offset to the beginning
of a null-separated list of exported function names. AddressOfNameOrdinals is
an offset to a list of ordinal values (each 2 bytes long) for the same exported
functions.

The three AddressOf... fields are relative virtual addresses into the address
space of a process once the module has been loaded. Once the module is loaded,
the relative virtual address should be added to the module base address to get
the exact location in the address space of the process. Before the file is
loaded, however, the address can be determined by subtracting the section
header virtual address (VirtualAddress) from the given field address, adding
the section body offset (PointerToRawData) to the result, and then using this
value as an offset into the image file. The following example illustrates this
technique:

PEFILE.C

int  WINAPI GetExportFunctionNames (
     LPVOID    lpFile,
     HANDLE    hHeap,
     char      **pszFunctions)
{
     IMAGE_SECTION_HEADER       sh;
     PIMAGE_EXPORT_DIRECTORY    ped;
     char                       *pNames, *pCnt;
     int                        i, nCnt;

     /* Get section header and pointer to data directory
        for .edata section. */
     if ((ped = (PIMAGE_EXPORT_DIRECTORY)ImageDirectoryOffset
             (lpFile, IMAGE_DIRECTORY_ENTRY_EXPORT)) == NULL)
         return 0;
     GetSectionHdrByName (lpFile, &sh, ".edata");

     /* Determine the offset of the export function names. */
     pNames = (char *)(*(int *)((int)ped->AddressOfNames -
                                (int)sh.VirtualAddress   +
                                (int)sh.PointerToRawData +
                                (int)lpFile)    -
                       (int)sh.VirtualAddress   +
                       (int)sh.PointerToRawData +
                       (int)lpFile);

     /* Figure out how much memory to allocate for all strings. */
     pCnt = pNames;
     for (i=0; i<(int)ped->NumberOfNames; i++)
         while (*pCnt++);
     nCnt = (int)(pCnt. pNames);

     /* Allocate memory off heap for function names. */
     *pszFunctions = HeapAlloc (hHeap, HEAP_ZERO_MEMORY, nCnt);

     /* Copy all strings to buffer. */
     CopyMemory ((LPVOID)*pszFunctions, (LPVOID)pNames, nCnt);

     return nCnt;
}

Notice that in this function the variable pNames is assigned by determining
first the address of the offset and then the actual offset location. Both the
address of the offset and the offset itself are relative virtual addresses and
must be translated before being used, as the function demonstrates. You could
write a similar function to determine the ordinal values or entry points of the
functions, but why bother when I already did this for you? The
GetNumberOfExportedFunctions, GetExportFunctionEntryPoints, and
GetExportFunctionOrdinals functions also exist in the PEFILE.DLL.

Import data section, .idata

The .idata section is import data, including the import directory and import
address name table. Although an IMAGE_DIRECTORY_ENTRY_IMPORT directory is
defined, no corresponding import directory structure is included in the file
WINNT.H. Instead, there are several other structures called
IMAGE_IMPORT_BY_NAME, IMAGE_THUNK_DATA, and IMAGE_IMPORT_DESCRIPTOR.
Personally, I couldn't make heads or tails of how these structures are supposed
to correlate to the .idata section, so I spent several hours deciphering the
.idata section body and came up with a much simpler structure. I named this
structure IMAGE_IMPORT_MODULE_DIRECTORY.

PEFILE.H

typedef struct tagImportDirectory
     {
     DWORD    dwRVAFunctionNameList;
     DWORD    dwUseless1;
     DWORD    dwUseless2;
     DWORD    dwRVAModuleName;
     DWORD    dwRVAFunctionAddressList;
     }IMAGE_IMPORT_MODULE_DIRECTORY,
      * PIMAGE_IMPORT_MODULE_DIRECTORY;

Unlike the data directories of other sections, this one repeats one after
another for each imported module in the file. Think of it as an entry in a list
of module data directories, rather than a data directory to the entire section
of data. Each entry is a directory to the import information for a specific
module.

One of the fields in the IMAGE_IMPORT_MODULE_DIRECTORY structure is
dwRVAModuleName, a relative virtual address pointing to the name of the module.
There are also two dwUseless parameters in the structure that serve as padding
to keep the structure aligned properly within the section. The PE file format
specification mentions something about import flags, a time/date stamp, and
major/minor versions, but these two fields remained empty throughout my
experimentation, so I still consider them useless.

Based on the definition of this structure, you can retrieve the names of
modules and all functions in each module that are imported by an executable
file. The following function demonstrates how to retrieve all the module names
imported by a particular PE file:

PEFILE.C

int  WINAPI GetImportModuleNames (
     LPVOID    lpFile,
     HANDLE    hHeap,
     char      **pszModules)
{
     PIMAGE_IMPORT_MODULE_DIRECTORY  pid;
     IMAGE_SECTION_HEADER            idsh;
     BYTE                            *pData;
     int                             nCnt = 0, nSize = 0, i;
     char                            *pModule[1024];
     char                            *psz;

     pid = (PIMAGE_IMPORT_MODULE_DIRECTORY)ImageDirectoryOffset
              (lpFile, IMAGE_DIRECTORY_ENTRY_IMPORT);
     pData = (BYTE *)pid;

     /* Locate section header for ".idata" section. */
     if (!GetSectionHdrByName (lpFile, &idsh, ".idata"))
         return 0;

     /* Extract all import modules. */
     while (pid->dwRVAModuleName)
         {
         /* Allocate buffer for absolute string offsets. */
         pModule[nCnt] = (char *)(pData +
                (pid->dwRVAModuleName-idsh.VirtualAddress));
         nSize += strlen (pModule[nCnt]) + 1;

         /* Increment to the next import directory entry. */
         pid++;
         nCnt++;
         }

     /* Copy all strings to one chunk of heap memory. */
     *pszModules = HeapAlloc (hHeap, HEAP_ZERO_MEMORY, nSize);
     psz = *pszModules;
     for (i=0; i<nCnt; i++)
         {
         strcpy (psz, pModule[i]);
         psz += strlen (psz) + 1;
         }

     return nCnt;
}

The function is pretty straightforward. However, one thing is worth pointing
out--notice the while loop. This loop is terminated when pid->dwRVAModuleName
is 0. Implied here is that at the end of the list of
IMAGE_IMPORT_MODULE_DIRECTORY structures is a null structure that has a value
of 0 for at least the dwRVAModuleName field. This is the behavior I observed in
my experimentation with the file and later confirmed in the PE file format
specification.

The first field in the structure, dwRVAFunctionNameList, is a relative virtual
address to a list of relative virtual addresses that each point to the function
names within the file. As shown in the following data, the module and function
names of all imported modules are listed in the .idata section data:

E6A7 0000 F6A7 0000  08A8 0000 1AA8 0000  ................
28A8 0000 3CA8 0000  4CA8 0000 0000 0000  (...<...L.......
0000 4765 744F 7065  6E46 696C 654E 616D  ..GetOpenFileNam
6541 0000 636F 6D64  6C67 3332 2E64 6C6C  eA..comdlg32.dll
0000 2500 4372 6561  7465 466F 6E74 496E  ..%.CreateFontIn
6469 7265 6374 4100  4744 4933 322E 646C  directA.GDI32.dl
6C00 A000 4765 7444  6576 6963 6543 6170  l...GetDeviceCap
7300 C600 4765 7453  746F 636B 4F62 6A65  s...GetStockObje
6374 0000 D500 4765  7454 6578 744D 6574  ct....GetTextMet
7269 6373 4100 1001  5365 6C65 6374 4F62  ricsA...SelectOb
6A65 6374 0000 1601  5365 7442 6B43 6F6C  ject....SetBkCol
6F72 0000 3501 5365  7454 6578 7443 6F6C  or..5.SetTextCol
6F72 0000 4501 5465  7874 4F75 7441 0000  or..E.TextOutA..

The above data is a portion taken from the .idata section of the EXEVIEW.EXE
sample application. This particular section represents the beginning of the
list of import module and function names. If you begin examining the right
section part of the data, you should recognize the names of familiar Win32 API
functions and the module names they are found in. Reading from the top down,
you get GetOpenFileNameA, followed by the module name COMDLG32.DLL. Shortly
after that, you get CreateFontIndirectA, followed by the module GDI32.DLL and
then the functions GetDeviceCaps, GetStockObject, GetTextMetrics, and so forth.

This pattern repeats throughout the .idata section. The first module name is
COMDLG32.DLL and the second is GDI32.DLL. Notice that only one function is
imported from the first module, while many functions are imported from the
second module. In both cases, the function names and the module name to which
they belong are ordered such that a function name appears first, followed by
the module name and then by the rest of the function names, if any.

The following function demonstrates how to retrieve the function names for a
specific module:

PEFILE.C

int  WINAPI GetImportFunctionNamesByModule (
     LPVOID    lpFile,
     HANDLE    hHeap,
     char      *pszModule,
     char      **pszFunctions)
{
     PIMAGE_IMPORT_MODULE_DIRECTORY  pid;
     IMAGE_SECTION_HEADER     idsh;
     DWORD                    dwBase;
     int                      nCnt = 0, nSize = 0;
     DWORD                    dwFunction;
     char                     *psz;

     /* Locate section header for ".idata" section. */
     if (!GetSectionHdrByName (lpFile, &idsh, ".idata"))
         return 0;

     pid = (PIMAGE_IMPORT_MODULE_DIRECTORY)ImageDirectoryOffset
              (lpFile, IMAGE_DIRECTORY_ENTRY_IMPORT);

     dwBase = ((DWORD)pid. idsh.VirtualAddress);

     /* Find module's pid. */
     while (pid->dwRVAModuleName &&
            strcmp (pszModule,
                   (char *)(pid->dwRVAModuleName+dwBase)))
         pid++;

     /* Exit if the module is not found. */
     if (!pid->dwRVAModuleName)
         return 0;

     /* Count number of function names and length of strings. */
     dwFunction = pid->dwRVAFunctionNameList;
     while (dwFunction                      &&
            *(DWORD *)(dwFunction + dwBase) &&
            *(char *)((*(DWORD *)(dwFunction + dwBase)) +
             dwBase+2))
         {
         nSize += strlen ((char *)((*(DWORD *)(dwFunction +
              dwBase)) + dwBase+2)) + 1;
         dwFunction += 4;
         nCnt++;
         }

     /* Allocate memory off heap for function names. */
     *pszFunctions = HeapAlloc (hHeap, HEAP_ZERO_MEMORY, nSize);
     psz = *pszFunctions;

     /* Copy function names to memory pointer. */
     dwFunction = pid->dwRVAFunctionNameList;
     while (dwFunction                      &&
            *(DWORD *)(dwFunction + dwBase) &&
            *((char *)((*(DWORD *)(dwFunction + dwBase)) +
             dwBase+2)))
         {
         strcpy (psz, (char *)((*(DWORD *)(dwFunction + dwBase)) +
                 dwBase+2));
         psz += strlen((char *)((*(DWORD *)(dwFunction + dwBase))+
                 dwBase+2)) + 1;
         dwFunction += 4;
         }

     return nCnt;
}

Like the GetImportModuleNames function, this function relies on the end of each
list of information to have a zeroed entry. In this case, the list of function
names ends with one that is zero.

The final field, dwRVAFunctionAddressList, is a relative virtual address to a
list of virtual addresses that will be placed in the section data by the loader
when the file is loaded. Before the file is loaded, however, these virtual
addresses are replaced by relative virtual addresses that correspond exactly to
the list of function names. So before the file is loaded, there are two
identical lists of relative virtual addresses pointing to imported function
names.

Debug information section, .debug

Debug information is initially placed in the .debug section. The PE file format
also supports separate debug files (normally identified with a .DBG extension)
as a means of collecting debug information in a central location. The debug
section contains the debug information, but the debug directories live in the
.rdata section mentioned earlier. Each of those directories references debug
information in the .debug section. The debug directory structure is defined as
an IMAGE_DEBUG_DIRECTORY, as follows:

WINNT.H

typedef struct _IMAGE_DEBUG_DIRECTORY {
     ULONG   Characteristics;
     ULONG   TimeDateStamp;
     USHORT  MajorVersion;
     USHORT  MinorVersion;
     ULONG   Type;
     ULONG   SizeOfData;
     ULONG   AddressOfRawData;
     ULONG   PointerToRawData;
} IMAGE_DEBUG_DIRECTORY, *PIMAGE_DEBUG_DIRECTORY;

The section is divided into separate portions of data representing different
types of debug information. For each one there is a debug directory described
above. The different types of debug information are listed below:

WINNT.H

#define IMAGE_DEBUG_TYPE_UNKNOWN          0
#define IMAGE_DEBUG_TYPE_COFF             1
#define IMAGE_DEBUG_TYPE_CODEVIEW         2
#define IMAGE_DEBUG_TYPE_FPO              3
#define IMAGE_DEBUG_TYPE_MISC             4

The Type field in each directory indicates which type of debug information the
directory represents. As you can see in the list above, the PE file format
supports many different types of debug information, as well as some other
informational fields. Of those, the IMAGE_DEBUG_TYPE_MISC information is
unique. This information was added to represent miscellaneous information about
the executable image that could not be added to any of the more structured data
sections in the PE file format. This is the only location in the image file
where the image name is sure to appear. If an image exports information, the
export data section will also include the image name.

Each type of debug information has its own header structure that defines its
data. Each of these is listed in the file WINNT.H. One nice thing about the
IMAGE_DEBUG_DIRECTORY structure is that it includes two fields that identify
the debug information. The first of these, AddressOfRawData, is the relative
virtual address of the data once the file is loaded. The other,
PointerToRawData, is an actual offset within the PE file, where the data is
located. This makes it easy to locate specific debug information.

As a last example, consider the following function, which extracts the image
name from the IMAGE_DEBUG_MISC structure:

PEFILE.C

int    WINAPI RetrieveModuleName (
     LPVOID    lpFile,
     HANDLE    hHeap,
     char      **pszModule)
{

     PIMAGE_DEBUG_DIRECTORY    pdd;
     PIMAGE_DEBUG_MISC         pdm = NULL;
     int                       nCnt;

     if (!(pdd = (PIMAGE_DEBUG_DIRECTORY)ImageDirectoryOffset
                (lpFile, IMAGE_DIRECTORY_ENTRY_DEBUG)))
         return 0;

     while (pdd->SizeOfData)
         {
         if (pdd->Type == IMAGE_DEBUG_TYPE_MISC)
             {
             pdm = (PIMAGE_DEBUG_MISC)
                 ((DWORD)pdd->PointerToRawData + (DWORD)lpFile);

             nCnt = lstrlen (pdm->Data)*(pdm->Unicode?2:1);
             *pszModule = (char *)HeapAlloc (hHeap,
                                             HEAP_ZERO_MEMORY,
                                             nCnt+1;
             CopyMemory (*pszModule, pdm->Data, nCnt);

             break;
             }

         pdd ++;
         }

     if (pdm != NULL)
         return nCnt;
     else
         return 0;
}

As you can see, the structure of the debug directory makes it relatively easy
to locate a specific type of debug information. Once the IMAGE_DEBUG_MISC
structure is located, extracting the image name is as simple as invoking the
CopyMemory function.

As mentioned above, debug information can be stripped into separate .DBG files.
The Windows NT SDK includes a utility called REBASE.EXE that serves this
purpose. For example, in the following statement an executable image named
TEST.EXE is being stripped of debug information:

rebase -b 40000 -x c:\samples\testdir test.exe

The debug information is placed in a new file called TEST.DBG and located in
the path specified, in this case c:\samples\testdir. The file begins with a
single IMAGE_SEPARATE_DEBUG_HEADER structure, followed by a copy of the section
headers that exist in the stripped executable image. Then the .debug section
data follows the section headers. So, right after the section headers are the
series of IMAGE_DEBUG_DIRECTORY structures and their associated data. The debug
information itself retains the same structure as described above for normal
image file debug information.

Summary of the PE File Format

The PE file format for Windows NT introduces a completely new structure to
developers familiar with the Windows and MS-DOS environments. Yet developers
familiar with the UNIX environment will find that the PE file format is similar
to, if not based on, the COFF specification.

The entire format consists of an MS-DOS MZ header, followed by a real-mode stub
program, the PE file signature, the PE file header, the PE optional header, all
of the section headers, and finally, all of the section bodies.

The optional header ends with an array of data directory entries that are
relative virtual addresses to data directories contained within section bodies.
Each data directory indicates how a specific section body's data is structured.

The PE file format has eleven predefined sections, as is common to applications
for Windows NT, but each application can define its own unique sections for
code and data.

The .debug predefined section also has the capability of being stripped from
the file into a separate debug file. If so, a special debug header is used to
parse the debug file, and a flag is specified in the PE file header to indicate
that the debug data has been stripped.

PEFILE.DLL Function Descriptions

PEFILE.DLL consists mainly of functions that either retrieve an offset into a
given PE file or copy a portion of the file data to a specific structure. Each
function has a single requirement--the first parameter is a pointer to the
beginning of the PE file. That is, the file must first be memory-mapped into
the address space of your process, and the base location of the file mapping is
the value lpFile that you pass as the first parameter to every function.

The function names are meant to be self-explanatory, and each function is
listed with a brief comment describing its purpose. If, after reading through
the list of functions, you cannot determine what a function is for, refer to
the EXEVIEW.EXE sample application to find an example of how the function is
used. The following list of function prototypes can also be found in PEFILE.H:

PEFILE.H

/* Retrieve a pointer offset to the MS-DOS MZ header. */
BOOL WINAPI GetDosHeader (LPVOID, PIMAGE_DOS_HEADER);

/* Determine the type of an .EXE file. */
DWORD WINAPI ImageFileType (LPVOID);

/* Retrieve a pointer offset to the PE file header. */
BOOL WINAPI GetPEFileHeader (LPVOID, PIMAGE_FILE_HEADER);

/* Retrieve a pointer offset to the PE optional header .*/
BOOL WINAPI GetPEOptionalHeader (LPVOID,
                                   PIMAGE_OPTIONAL_HEADER);

/* Return the address of the module entry point. */
LPVOID WINAPI GetModuleEntryPoint (LPVOID);

/* Return a count of the number of sections in the file. */
int  WINAPI NumOfSections (LPVOID);

/* Return the desired base address of the executable when
    it is loaded into a process's address space. */
LPVOID WINAPI GetImageBase (LPVOID);

/* Determine the location within the file of a specific
    image data directory.  */
LPVOID WINAPI ImageDirectoryOffset (LPVOID, DWORD);

/* Function retrieve names of all the sections in the file. */
int WINAPI GetSectionNames (LPVOID, HANDLE, char **);

/* Copy the section header information for a specific section. */
BOOL WINAPI GetSectionHdrByName (LPVOID,
                                   PIMAGE_SECTION_HEADER, char *);

/* Get null-separated list of import module names. */
int WINAPI GetImportModuleNames (LPVOID, HANDLE, char  **);

/* Get null-separated list of import functions for a module. */
int WINAPI GetImportFunctionNamesByModule (LPVOID, HANDLE,
                                            char *, char  **);

/* Get null-separated list of exported function names. */
int WINAPI GetExportFunctionNames (LPVOID, HANDLE, char **);

/* Get number of exported functions. */
int WINAPI GetNumberOfExportedFunctions (LPVOID);

/* Get list of exported function virtual address entry points. */
LPVOID WINAPI GetExportFunctionEntryPoints (LPVOID);

/* Get list of exported function ordinal values. */
LPVOID WINAPI GetExportFunctionOrdinals (LPVOID);

/* Determine total number of resource objects. */
int WINAPI GetNumberOfResources (LPVOID);

/* Return list of all resource object types used in file. */
int WINAPI GetListOfResourceTypes (LPVOID, HANDLE, char **);

/* Determine if debug information has been removed from file. */
BOOL WINAPI IsDebugInfoStripped (LPVOID);

/* Get name of image file. */
int WINAPI RetrieveModuleName (LPVOID, HANDLE, char **);

/* Function determines if the file is a valid debug file. */
BOOL WINAPI IsDebugFile (LPVOID);

/* Function returns debug header from debug file. */
BOOL WINAPI GetSeparateDebugHeader(LPVOID,
                                    PIMAGE_SEPARATE_DEBUG_HEADER);

In addition to the functions listed above, the macros mentioned earlier in this
article are also defined in the PEFILE.H file. The complete list is as follows:

/* Offset to PE file signature                              */
#define NTSIGNATURE(a) ((LPVOID)((BYTE *)a                +  \
                         ((PIMAGE_DOS_HEADER)a)->e_lfanew))

/* MS-OS header identifies the NT PEFile signature dword;
    the PEFILE header exists just after that dword.           */
#define PEFHDROFFSET(a) ((LPVOID)((BYTE *)a               +  \
                          ((PIMAGE_DOS_HEADER)a)->e_lfanew +  \
                              SIZE_OF_NT_SIGNATURE))

/* PE optional header is immediately after PEFile header.    */
#define OPTHDROFFSET(a) ((LPVOID)((BYTE *)a               +  \
                          ((PIMAGE_DOS_HEADER)a)->e_lfanew +  \
                            SIZE_OF_NT_SIGNATURE           +  \
                            sizeof (IMAGE_FILE_HEADER)))

/* Section headers are immediately after PE optional header. */
#define SECHDROFFSET(a) ((LPVOID)((BYTE *)a               +  \
                          ((PIMAGE_DOS_HEADER)a)->e_lfanew +  \
                            SIZE_OF_NT_SIGNATURE           +  \
                            sizeof (IMAGE_FILE_HEADER)     +  \
                            sizeof (IMAGE_OPTIONAL_HEADER)))

To use PEFILE.DLL, simply include the header file PEFILE.H and link the DLL to
your application. All of the functions are mutually exclusive functions, but
some were written as much to support others as for the information they
provide. For example, the function GetSectionNames is useful for getting the
exact names of all sections. Yet to be able to retrieve the section header for
a unique section name (one defined by the application developer during
compile), you would first have to get the list of names and then call the
function GetSectionHeaderByName with the exact name of the section. Enjoy!

1997 Microsoft Corporation. All rights reserved. Legal Notices

第一篇己翻译：
http://bbs.pediy.com//showthread.php?s=&threadid=19618

最初由 kanxue 发布
第一篇己翻译：
http://bbs.pediy.com//showthread.php?s=&threadid=19618

谢谢坛主

其实关于PE的文章有很多了,为什么一定要看翻译的呢