$Id: pe.txt,v 1.7 1998/07/29 17:55:13 LUEVELSMEYER Exp $
The PE file format
==================
Purpose
-------
The PE ("portable executable") file format is the format of executable
binaries (DLLs and programs) for MS windows NT, windows 95 and
win32s
in windows NT, the drivers are in this format, too.
It can also be used for object files and libraries.
The format is designed by Microsoft, apparently based on a good
knowledge of COFF, the "common object file format" used for object files
and executables on several UNIXes and on VMS.
The win32 SDK includes a header file <winnt.h> containing #defines and
typedefs for the PE-format. I will mention the struct-member-names and
#defines as we go.
You may also find the DLL "imagehelp.dll" to be helpful. It is part of
windows NT, but documentation is rare. Some of its functions are
described in the "Developer Network".
General Layout
--------------
At the start of a PE file we find an MS-DOS executable ("stub")
this
makes any PE file a valid MS-DOS executable.
After the DOS-stub there is a 32-bit-signature with the magic number
0x00004550 (IMAGE_NT_SIGNATURE).
Then there is a file header (in the COFF-format) that tells on which
machine the binary is supposed to run, how many sections are in it, the
time it was linked, whether it is an executable or a DLL and so on. (The
difference between executable and DLL in this context is: a DLL can not
be started but only be used by another binary, and a binary cannot link
to an executable).
After that, we have an optional header (it is always there but still
called "optional" - COFF uses an "optional header" for libraries but not
for objects, that's why it is called "optional"). This tells us more
about how the binary should be loaded: The starting address, the amount
of stack to reserve, the size of the data segment etc..
An interesting part of the optional header is the trailing array of
'data directories'
these directories contain pointers to data in the
'sections'. If, for example, the binary has an export directory, you
will find a pointer to that directory in the array member
IMAGE_DIRECTORY_ENTRY_EXPORT, and it will point into one of the
sections.
Following the headers we find the 'sections', introduced by the 'section
headers'. Essentially, the sections' contents is what you really need to
execute a program, and all the header and directory stuff is just there
to help you find it.
Each section has some flags about alignment, what kind of data it
contains ("initialized data" and so on), whether it can be shared etc.,
and the data itself. Most, but not all, sections contain one or more
directories referenced through the entries of the optional header's
"data directory" array, like the directory of exported functions or the
directory of base relocations. Directoryless types of contents are, for
example, "executable code" or "initialized data".
+-------------------+
| DOS-stub |
+-------------------+
| file-header |
+-------------------+
| optional header |
|- - - - - - - - - -|
| |
| data directories |
| |
+-------------------+
| |
| section headers |
| |
+-------------------+
| |
| section 1 |
| |
+-------------------+
| |
| section 2 |
| |
+-------------------+
| |
| ... |
| |
+-------------------+
| |
| section n |
| |
+-------------------+
DOS-stub and Signature
----------------------
The concept of a DOS-stub is well-known from the 16-bit-windows-
executables (which were in the "NE" format). The stub is used for
OS/2-executables, self-extracting archives and other applications, too.
For PE-files, almost always this MS-DOS-executable consists of about 100
bytes that output an error message such as "this program needs windows
NT".
You recognize a DOS-stub by the validating the DOS-header, being a
struct IMAGE_DOS_HEADER. The first 2 bytes should be the sequence "MZ"
(there is a #define IMAGE_DOS_SIGNATURE for this WORD).
You distinguish a PE binary from other stubbed binaries by the trailing
signature, which you find at the offset given by the header member
'e_lfanew' (which is 32 bits long beginning at byte offset 60). For OS/2
and windows binaries, the signature is a 16-bit-word
for PE files, it
is a 32-bit-longword with the value IMAGE_NT_SIGNATURE #defined to be
0x00004550.
File Header
-----------
To get to the IMAGE_FILE_HEADER, validate the "MZ" of the DOS-header
(1st 2 bytes), then find the 'e_lfanew' member of the DOS-stub's header
and skip that many bytes from the beginning of the file. Verify the
signature you will find there. The file header, a struct
IMAGE_FILE_HEADER, begins immediatly after it
the members are described
top to bottom.
The first member is the 'Machine', a 16-bit-value indicating the system
the binary is intended to run on. Known legal values are
IMAGE_FILE_MACHINE_I386 (0x14c)
for Intel 80386 processor or better
0x014d
for Intel 80486 processor or better
0x014e
for Intel Pentium processor or better
0x0160
for R3000 (MIPS) processor, big endian
IMAGE_FILE_MACHINE_R3000 (0x162)
for R3000 (MIPS) processor, little endian
IMAGE_FILE_MACHINE_R4000 (0x166)
for R4000 (MIPS) processor, little endian
IMAGE_FILE_MACHINE_R10000 (0x168)
for R10000 (MIPS) processor, little endian
IMAGE_FILE_MACHINE_ALPHA (0x184)
for DEC Alpha AXP processor
IMAGE_FILE_MACHINE_POWERPC (0x1F0)
for IBM Power PC, little endian
Then we have the 'NumberOfSections', a 16-bit-value. It is the number of
sections that follow the headers. We will discuss the sections later.
Next is a timestamp 'TimeDateStamp' (32 bit), giving the time the file
was created. You can distinguish several versions of the same file by
this value, even if the "official" version number was not altered. (The
format of the timestamp is not documented except that it should be
somewhat unique among versions of the same file, but apparently it is
'seconds since January 1 1970 00:00:00' in UTC - the format used by most
C compilers for the time_t.)
This timestamp is used for the binding of import directories, which will
be discussed later.
Warning: some compilers tend to set this timestamp to absurd values.
The members 'PointerToSymbolTable' and 'NumberOfSymbols' (both 32 bit)
are used for debugging information. I don't know how to decipher them,
and I've found the pointer to be always 0.
'SizeOfOptionalHeader' (16 bit) is simply sizeof(IMAGE_OPTIONAL_HEADER).
You can use it to validate the correctness of the PE file's structure.
'Characteristics' is 16 bits and consists of a collection of flags, most
of them being valid only for object files and libraries:
Bit 0 (IMAGE_FILE_RELOCS_STRIPPED) is set if there is no relocation
information in the file. This refers to relocation information per
section in the sections themselves
it is not used for executables,
which have relocation information in the 'base relocation' directory
described below.
Bit 1 (IMAGE_FILE_EXECUTABLE_IMAGE) is set if the file is executable
(i.e. it is not an object file or a library).
Bit 2 (IMAGE_FILE_LINE_NUMS_STRIPPED) is set if the line number
information is stripped
this is not used for executable files.
Bit 3 (IMAGE_FILE_LOCAL_SYMS_STRIPPED) is set if there is no
information about local symbols in the file (this is not used
for executable files).
Bit 4 (IMAGE_FILE_AGGRESIVE_WS_TRIM) is set if the operating system
is supposed to trim the working set of the running process (the
amount of RAM the process uses) aggressivly by paging it out. This
should be set if it is a demon-like application that waits most of
the time and only wakes up once a day, or the like.
Bits 7 (IMAGE_FILE_BYTES_REVERSED_LO) and 15
(IMAGE_FILE_BYTES_REVERSED_HI) are set if the endianess of the file is
not what the machine would expect, so it must swap bytes before
reading. This is unreliable for executable files (the OS expects
executables to be correctly byte-ordered).
Bit 8 (IMAGE_FILE_32BIT_MACHINE) is set if the machine is expected
to be a 32 bit machine. This is always set.
Bit 9 (IMAGE_FILE_DEBUG_STRIPPED) is set if there is no debugging
information in the file. This is unused for executable files.
Bit 10 (IMAGE_FILE_REMOVABLE_RUN_FROM_SWAP) is set if the application
may not run from a removable medium such as a floppy or a CD-ROM. In
this case, the operating system is advised to copy the file to the
swapfile and execute it from there.
Bit 11 (IMAGE_FILE_NET_RUN_FROM_SWAP) is set if the application may
not run from the network. In this case, the operating system is
advised to copy the file to the swapfile and execute it from there.
Bit 12 (IMAGE_FILE_SYSTEM) is set if the file is a system file such
as a driver. This is unused for executable files.
Bit 13 (IMAGE_FILE_DLL) is set if the file is a DLL.
Bit 14 (IMAGE_FILE_UP_SYSTEM_ONLY) is set if the file is not
designed to run on multiprocessor systems (that is, it will crash
there because it relies in some way on exactly one processor).
Relative Virtual Addresses
--------------------------
The PE format makes heavy use of so-called RVAs. An RVA, aka "relative
virtual address", is used to describe a memory address if you don't know
the base address. It is the value you need to add to the base address to
get the linear address.
The base address is the address the PE image is loaded to, and may vary
from one invocation to the next.
Example: suppose an executable file is loaded to address 0x400000 and
execution starts at RVA 0x1560. The effective execution start will then
be at the address 0x401560. If the executable were loaded to 0x100000,
the execution start would be 0x101560.
Things become complicated because the parts of the PE-file (the
sections) are not necessarily aligned the same way the loaded image is.
For example, the sections of the file are often aligned to
512-byte-borders, but the loaded image is perhaps aligned to
4096-byte-borders. See 'SectionAlignment' and 'FileAlignment' below.
So to find a piece of information in a PE-file for a specific RVA,
you must calculate the offsets as if the file were loaded, but skip
according to the file-offsets.
As an example, suppose you knew the execution starts at RVA 0x1560, and
want to diassemble the code starting there. To find the address in the
file, you will have to find out that sections in RAM are aligned to 4096
bytes and the ".code"-section starts at RVA 0x1000 in RAM and is 16384
bytes long
then you know that RVA 0x1560 is at offset 0x560 in that
section. Find out that the sections are aligned to 512-byte-borders in
the file and that ".code" begins at offset 0x800 in the file, and you
know that the code execution start is at byte 0x800+0x560=0xd60 in the
file.
Easy if you know how it works
Optional Header
---------------
Immediatly following the file header is the IMAGE_OPTIONAL_HEADER
(which, in spite of the name, is always there). It contains
information about how to treat the PE-file exactly. We'll also have the
members from top to bottom.
The first 16-bit-word is 'Magic' and has, as far as I looked into
PE-files, always the value 0x010b.
The next 2 bytes are the version of the linker ('MajorLinkerVersion' and
'MinorLinkerVersion') that produced the file. These values, again, are
unreliable and do not always reflect the linker version properly.
(Several linkers simply don't set this field.)
And, coming to think about it, what good is the version if you have got
no idea *which* linker was used?
The next 3 longwords (32 bit each) are intended to be the size of the
executable code ('SizeOfCode'), the size of the initialized data
('SizeOfInitializedData', the so-called "data segment"), and the size of
the uninitialized data ('SizeOfUninitializedData', the so-called "bss
segment"). These values are, again, unreliable (e.g. the data segment
may actually be split into several segments by the compiler or linker),
and you get better sizes by inspecting the 'sections' that follow the
optional header.
Next is a 32-bit-value that is a RVA. This RVA is the offset to the
codes's entry point ('AddressOfEntryPoint'). Execution starts here
it
is e.g. the address of a DLL's LibMain() or a program's startup code
(which will in turn call main()) or a driver's DriverEntry().
If you dare to load the image "by hand", you call this address to start
the process after you have done all the fixups and relocations.
The next 2 32-bit-values are the offsets to the executable code
('BaseOfCode') and the initialized data ('BaseOfData'), both of them
RVAs again, and both of them being of little interest because you get
more reliable information by inspecting the 'sections' that follow the
headers.
There is no offset to the uninitialized data because, being
uninitialized, there is little point in providing this data in the
image.
The next entry is a 32-bit-value that is a RVA giving the preferred load
address ('ImageBase'). This is the address the file has been relocated
to by the linker
if the binary can in fact be loaded to that address,
the loader doesn't need to relocate the file again, which is a big win
in loading time.
The preferred load address can not be used if another image has already
been loaded to that address (an "address clash", which happens quite
often if you load several DLLs that are all relocated to the linker's
default), or the memory in question has been used for other purposes
(stack, malloc(), uninitialized data, whatever). In these cases, the
image must be loaded to some other address and it needs to be relocated
(see 'relocation directory' below). This has further consequences if the
image is a DLL, because then the "bound imports" are no longer valid,
and fixups have to be made to the binary that uses the DLL - see 'import
directory' below.
The next 2 32-bit-values are the alignments of the PE-file's sections in
RAM ('SectionAlignment', when the image has been loaded) and in the file
('FileAlignment'). Usually both values are 32, or FileAlignment is 512
and SectionAlignment is 4096. Sections will be discussed later.
The next 2 16-bit-words are the expected operating system version
('MajorOperatingSystemVersion' and 'MinorOperatingSystemVersion' [they
_do_ like self-documenting names at MS]). This version information is
intended to be the operating system's (e.g. NT or Win95) version, as
opposed to the subsystem's version (e.g. Win32)
it is often not
supplied, or wrong supplied. The loader doesn't use it, apparently.
The next 2 16-bit-words are the binary's version, ('MajorImageVersion' and
'MinorImageVersion'). Many linkers don't set this information correctly
and many programmers don't bother to supply it, so it is better to rely
on the version-resource if one exists.
The next 2 16-bit-words are the expected subsystem version
('MajorSubsystemVersion' and 'MinorSubsystemVersion'). This should be
the Win32 version or the POSIX version, because 16-bit-programs or
OS/2-programs won't be in PE-format, obviously.
This subsystem version should be supplied correctly, because it *is*
checked and used:
If the application is a Win32-GUI-application and runs on NT, and the
subsystem version is *not* 4.0, the dialogs won't be 3D-style and
certain other features will also work "old-style" because the
application expects to run on NT 3.51, which had the program manager
instead of explorer and so on, and NT 4.0 will mimic that behaviour as
faithfully as possible.
Then we have a 'Win32VersionValue' of 32 bits. I don't know what it is
good for. It has been 0 in all the PE files that I inspected.
Next is a 32-bits-value giving the amount of memory the image will need,
in bytes ('SizeOfImage'). It is the sum of all sections' lengths. It is
a hint to the loader how many pages it will need in order to load the image.
The next thing is a 32-bit-value giving the total length of all headers
including the data directories and the section headers
('SizeOfHeaders'). It is at the same time the offset from the beginning
of the file to the first section's raw data.
Then we have got a 32-bit-checksum ('CheckSum'). This checksum is, for
current versions of NT, only checked if the image is a NT-driver (the
driver will fail to load if the checksum isn't correct).
The algorithm to compute the checksum is property of Microsoft, and they
won't tell you. However, several tools of the Win32 SDK will compute a
valid checksum, and the function CheckSumMappedFile() in the
imagehelp.dll will do so too.
The checksum is supposed to prevent loading of damaged binaries that
would crash anyway - and a crashing driver would result in a BSOD, so
it is better not to load it at all.
Then there is a 16-bit-word 'Subsystem' that tells in which of the
NT-subsystems the image runs:
IMAGE_SUBSYSTEM_NATIVE (1)
The binary doesn't need a subsystem. This is used for drivers.
IMAGE_SUBSYSTEM_WINDOWS_GUI (2)
The image is a Win32 graphical binary. (It can still open a
console with AllocConsole() but won't get one automatically at
startup.)
IMAGE_SUBSYSTEM_WINDOWS_CUI (3)
The binary is a Win32 console binary. (It will get a console
per default at startup.)
IMAGE_SUBSYSTEM_OS2_CUI (5)
The binary is a OS/2 console binary. (OS/2 binaries will be in
OS/2 format, so this value will seldom be used in a PE
file.)
IMAGE_SUBSYSTEM_POSIX_CUI (7)
The binary uses the POSIX console subsystem.
Windows 95 binaries will always use the Win32 subsystem, so the only
legal values for these binaries are 2 and 3
I don't know if "native"
binaries on windows 95 are possible.
The next thing is a 16-bit-value that tells, if the image is a DLL, when
to call the DLL's entry point ('DllCharacteristics'). This seems not to
be used
apparently, the DLL is always notified about everything.
If bit 0 is set, the DLL is notified about process attachment (i.e.
DLL load).
If bit 1 is set, the DLL is notified about thread detachments (i.e.
thread terminations).
If bit 2 is set, the DLL is notified about thread attachments (i.e.
thread creations).
If bit 3 is set, the DLL is notified about process detachment (i.e.
DLL unload).
The next 4 32-bit-values are the size of reserved stack
('SizeOfStackReserve'), the size of initially committed stack
('SizeOfStackCommit'), the size of the reserved heap
('SizeOfHeapReserve') and the size of the committed heap
('SizeOfHeapCommit').
The 'reserved' amounts are address space (not real RAM) that is reserved
for the specific purpose
at program startup, the 'committed' amount is
actually allocated in RAM. The 'committed' value is also the amount by
which the committed stack or heap grows if necessary.
So, as an example, if a program has a reserved heap of 1 MB and a
committed heap of 64 KB, the heap will start out at 64 KB and is
guaranteed to be enlargeable up to 1 MB. The heap will grow in
64-KB-chunks.
The 'heap' in this context is the primary (default) heap.
DLLs don't have a stack or heap of their own, so the values are ignored
for their images.
After these stack- and heap-descriptions, we find 32 bits of
'LoaderFlags', which I didn't find a useful description of. I only found
that you may set bits that automatically invoke a breakpoint or a
debugger after loading the image
however, this doesn't seem to work.
Then we find 32 bits of 'NumberOfRvaAndSizes', which is the number of
valid entries in the directories that follow immediatly. I've found this
value to be unreliable
you might wish use the constant
IMAGE_NUMBEROF_DIRECTORY_ENTRIES instead, or the lesser of both.
After the 'NumberOfRvaAndSizes' there is an array of
IMAGE_NUMBEROF_DIRECTORY_ENTRIES (16) IMAGE_DATA_DIRECTORYs.
Each of these directories describes the location (32 bits RVA called
'VirtualAddress') and size (also 32 bit, called 'Size') of a particular
piece of information, which is located in one of the sections that
follow the directory entries.
For example, the security directory is found at the RVA and has the size
that are given at index 4.
The directories that I know the structure of will be discussed later.
Defined directory indexes are:
IMAGE_DIRECTORY_ENTRY_EXPORT (0)
The directory of exported symbols
mostly used for DLLs.
Described below.
IMAGE_DIRECTORY_ENTRY_IMPORT (1)
The directory of imported symbols
see below.
IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
Directory of resources. Described below.
IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
Exception directory - structure and purpose unknown.
IMAGE_DIRECTORY_ENTRY_SECURITY (4)
Security directory - structure and purpose unknown.
IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
Base relocation table - see below.
IMAGE_DIRECTORY_ENTRY_DEBUG (6)
Debug directory - contents is compiler dependent. Moreover, many
compilers stuff the debug information into the code section and
don't create a separate section for it.
IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
Description string - some arbitrary copyright note or the like.
IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
Machine Value (MIPS GP) - structure and purpose unknown.
IMAGE_DIRECTORY_ENTRY_TLS (9)
Thread local storage directory - structure unknown
contains
variables that are declared "declspec(thread)".
IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
Load configuration directory - structure and purpose unknown.
IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
Bound import directory - see description of import directory.
IMAGE_DIRECTORY_ENTRY_IAT (12)
Import Address Table - structure and purpose unknown
As an example, if we find at index 7 the 2 longwords 0x12000 and 33, and
the load address is 0x10000, we know that the copyright data is at
address 0x10000+0x12000 (in whatever section there may be), and the
copyright note is 33 bytes long.
If a directory of a particular type is not used in a binary, the Size
and VirtualAddress are both 0.
Section directories
-------------------
The sections consist of two major parts: first, a section description
(of type IMAGE_SECTION_HEADER) and then the raw section data. So after
the data directories we find an array of 'NumberOfSections' section
headers.
A section header is actually a very *small* struct. It contains:
An array of IMAGE_SIZEOF_SHORT_NAME (8) bytes that make up the name
(ASCII) of the section. If all of the 8 bytes are used there is no 0-
terminator for the string! The name is typically something like ".data"
or ".text" or ".bss". There need not be a leading '.', the names may
also be "CODE" or "IAT" or the like.
Please note that the names are not at all related to the contents of the
section. A section named ".code" may or may not contain the executable
code
it may just as well contain the import address table
it may also
contain the code *and* the address table *and* the initialized data.
To find information in the sections, you will have to look it up via the
data directories of the optional header. Do not rely on the names, and
do not assume that the section's raw data starts at the beginning of a
section.
The next member of the IMAGE_SECTION_HEADER is a 32-bit-union of
'PhysicalAddress' and 'VirtualSize'. In an object file, this is the
address the contents is relocated to
in an executable, it is the size of
the contents. In fact, the field seems to be unused
There are linkers
that enter the size, and there are linkers that enter the address, and
I've also found a linker that enters a 0, and all the executables run
like the gentle wind.
The next member is 'VirtualAddress', a 32-bit-value holding the RVA to
the section's data when it is loaded in RAM.
Then we have got 32 bits of 'SizeOfRawData', which is the size of the
secion's data rounded up to the next multiple of 'FileAlignment'.
Next is 'PointerToRawData' (32 bits), which is incredibly usefull
because it is the offset from the file's beginning to the section's data.
If it is 0, the section's data are not contained in the file and will be
arbitrary.
Then we have got 'PointerToRelocations' (32 bits) and
'PointerToLinenumbers' (also 32 bits), 'NumberOfRelocations' (16 bits)
and 'NumberOfLinenumbers' (also 16 bits). All of these are information
that's only used for object files. Executables have a special base
relocation directory, and the line number information, if present at
all, is usually contained in a special purpose debugging segment or
elsewhere.
The last member of a section header is the 32 bits 'Characteristics',
which is a bunch of flags describing how the section's memory should be
treated:
If bit 5 (IMAGE_SCN_CNT_CODE) is set, the section contains
executable code.
If bit 6 (IMAGE_SCN_CNT_INITIALIZED_DATA) is set, the section
contains data that gets a defined value before execution starts. In
other words: the section's data in the file is meaningful.
If bit 7 (IMAGE_SCN_CNT_UNINITIALIZED_DATA) is set, this section's
data is uninitialized data and will be initialized to all-0-bytes
before execution starts. This is normally the BSS.
If bit 9 (IMAGE_SCN_LNK_INFO) is set, the section doesn't contain
image data but comments, description or other documentation.
If bit 11 (IMAGE_SCN_LNK_REMOVE) is set, the data is part of an
object file's section that is supposed to be left out when the
executable file is linked.
If bit 12 (IMAGE_SCN_LNK_COMDAT) is set, the section contains
"common block data", which are packaged functions of some sort.
If bit 15 (IMAGE_SCN_MEM_FARDATA) is set, we have far data -
whatever that means. This bit's meaning is unsure.
If bit 17 (IMAGE_SCN_MEM_PURGEABLE) is set, the section's data
is purgeable - but I don't think that this is the same as
"discardable", which has a bit of its own, see below.
The same bit is apparently used to indicate 16-bit-information as
there is also a define IMAGE_SCN_MEM_16BIT for it.
This bit's meaning is unsure.
If bit 18 (IMAGE_SCN_MEM_LOCKED) is set, the section should not be
moved in memory? This bit's meaning is unsure.
If bit 19 (IMAGE_SCN_MEM_PRELOAD) is set, the section should be
paged in before execution starts? This bit's meaning is unsure.
Bits 20 to 23 specify an alignment that I have no information
about. There are #defines IMAGE_SCN_ALIGN_16BYTES and the like. The
only value I've ever seen used is 0, for the default 16-byte-
alignment. I suspect that this is the alignment of objects in a
library file or the like.
If bit 24 (IMAGE_SCN_LNK_NRELOC_OVFL) is set, the section contains
some extended relocations that I don't know about.
If bit 25 (IMAGE_SCN_MEM_DISCARDABLE) is set, the section's data is
not needed after the process has started. This is the case,
for example, with the relocation information. I've seen it also for
startup routines of drivers and services that are only executed
once.
If bit 26 (IMAGE_SCN_MEM_NOT_CACHED) is set, the section's data
should not be cached. Don't ask my why not. Does this mean to switch
off the 2nd-level-cache?
If bit 27 (IMAGE_SCN_MEM_NOT_PAGED) is set, the section's data
should not be paged out. This may be interesting for drivers.
If bit 28 (IMAGE_SCN_MEM_SHARED) is set, the section's data is
shared among all running instances of the image. If it is e.g. the
initialized data of a DLL, all running instances of the DLL will at
any time have the same variable contents.
Note that only the first instance's section is initialized.
Sections containing code are always shared.
If bit 29 (IMAGE_SCN_MEM_EXECUTE) is set, the process gets
'execute'-access to the section's memory.
If bit 30 (IMAGE_SCN_MEM_READ) is set, the process gets
'read'-access to the section's memory.
If bit 31 (IMAGE_SCN_MEM_WRITE) is set, the process gets
'write'-access to the section's memory.
After the section headers we find the sections themselves. They are, in
the file, aligned to 'FileAlignment' bytes (that is, after the optional
header and after each section's data there will be padding bytes). When
loaded (in RAM), the sections are aligned to 'SectionAlignment' bytes.
As an example, if the optional header ends at file offset 981 and
'FileAlignment' is 512, the first section will start at byte 1024. Note
that you can find the sections via the 'PointerToRawData' or the
'VirtualAddress', so there is hardly any need to actually fuss around
with the alignments.
I will try to make an image of it all:
+-------------------+
| DOS-stub |
+-------------------+
| file-header |
+-------------------+
| optional header |
|- - - - - - - - - -|
| |----------------+
| data directories | |
| | |
|(RVAs to direc- |-------------+ |
|tories in sections)| | |
| |---------+ | |
| | | | |
+-------------------+ | | |
| |-----+ | | |
| section headers | | | | |
| (RVAs to section |--+ | | | |
| borders) | | | | | |
+-------------------+<-+ | | | |
| | | <-+ | |
| section data 1 | | | |
| | | <-----+ |
+-------------------+<----+ |
| | |
| section data 2 | |
| | <--------------+
+-------------------+
There is one section header for each section, and each data directory
will point to one of the sections (several data directories may point to
the same section, and there may be sections without data directory
pointing to them).
Sections' raw data
------------------
general
-------
All sections are aligned to 'SectionAlignment' when loaded in RAM, and
'FileAlignment' in the file. The sections are described by entries in
the section headers: You find the sections in the file via
'PointerToRawData' and in memory via 'VirtualAddress'
the length is in
'SizeOfRawData'.
There are several kinds of sections, depending on what's contained in
them. In most cases (but not in all) there will be at least one
data directory in a section, with a pointer to it in the optional
header's data directory array.
code section
------------
First, I will mention the code section. The section will have, at least,
the bits 'IMAGE_SCN_CNT_CODE', 'IMAGE_SCN_MEM_EXECUTE' and
'IMAGE_SCN_MEM_READ' set, and 'AddressOfEntryPoint' will point somewhere
into the section, to the start of the function that the developer wants
to execute first.
'BaseOfCode' will normally point to the start of this section, but may
point to somewhere later in the section if some non-code-bytes are
placed before the code in the section.
Normally, there will be nothing but executable code in this section, and
there will be only one code section, but don't rely on this.
Typical section names are ".text", ".code", "AUTO" and the like.
data section
-------------
The next thing we'll discuss is the initialized variables
this section
contains initialized static variables (like "static int i = 5;"). It will
have, at least, the bits 'IMAGE_SCN_CNT_INITIALIZED_DATA',
'IMAGE_SCN_MEM_READ' and 'IMAGE_SCN_MEM_WRITE' set. Some linkers may
place constant data into a section of their own that doesn't have the
writeable-bit. If part of the data is shareable, or there are other
peculiarities, there may be more sections with the apropriate section-
bits set.
The section, or sections, will be in the range 'BaseOfData' up to
'BaseOfData'+'SizeOfInitializedData'.
Typical section names are '.data', '.idata', 'DATA' and so on.
bss section
-----------
Then there is the uninitialized data (for static variables like "static
int k;")
this section is quite like the initialized data, but will have
a file offset ('PointerToRawData') of 0 to indicate its contents is not
stored in the file, and 'IMAGE_SCN_CNT_UNINITIALIZED_DATA' is set
instead of 'IMAGE_SCN_CNT_INITIALIZED_DATA' to indicate that the
contents should be set to 0-bytes at load-time.
The length will be 'SizeOfUninitializedData'.
Typical names are '.bss', 'BSS' and the like.
These were the section data that are *not* pointed to by data
directories. Their contents and structure is supplied by the compiler,
not by the linker.
(The stack-segment and heap-segment are not sections in the binary but
created by the loader from the stacksize- and heapsize-entries in the
optional header.)
copyright
---------
To begin with a simple directory-section, let's look at the data
directory 'IMAGE_DIRECTORY_ENTRY_COPYRIGHT'. The contents is a
copyright- or description string in ASCII (not 0-terminated), like
"Gonkulator control application, copyright (c) 1848 Hugendubel &
Cie".
This string is, normally, supplied to the linker with the command line.
This string is not needed at runtime and may be discarded. It is not
writeable
in fact, the application doesn't need access at all.
So the linker will find out if there is a discardable non-writeable
section already and if not, create one (named '.descr' or the like). It
will then stuff the string into the section and let the
copyright-directory-pointer point to the string. The
'IMAGE_SCN_CNT_INITIALIZED_DATA' bit should be set.
exported symbols
----------------
The next-simplest thing is the export directory,
'IMAGE_DIRECTORY_ENTRY_EXPORT'. This is a directory typically found
in DLLs
it contains the entry points of exported functions (and the
addresses of exported objects etc.). Executables may of course also have
exported symbols but usually they don't.
The containing section should be "initialized data" and "readable". It
can not be "discardable" because the process might call "GetProcAddress()"
to find a function's entry point at runtime.
The section is normally called '.edata' if it is a separate thing
often
enough, it is merged into some other section like "initialized data".
The structure of the export table ('IMAGE_EXPORT_DIRECTORY') comprises a
header and the export data, that is: the symbol names, their ordinals
and the offsets to their entry points.
First, we have 32 bits of 'Characteristics' that are unused and normally
0. Then there is a 32-bit-'TimeDateStamp', which presumably should give
the time the table was created in the time_t-format
alas, it is not
always valid (some linkers set it to 0). Then we have 2 16-bit-words of
version-info ('MajorVersion' and 'MinorVersion'), and these, too, are
often enough set to 0.
The next thing is 32 bits of 'Name'
this is an RVA to the DLL name as a
0-terminated ASCII string. (The name is necessary in case the DLL file is
renamed - see "binding" at the import directory.)
Then, we have got a 32-bit-'Base'. We'll come to that in a moment.
Now we have got sort of a problem, because the next two 32-bit-values are
the number of exported functions ('NumberOfFunctions') and the number of
exported names ('NumberOfNames'). These values are *not* always equal,
and weird things happen if you try to decipher the export directory past
the minimum of both. In *most* export-directories the values are equal
and we're fine. It seems to be always safe to decipher up to the minimum
of both values.
I've got a suspicion that it is somehow possible do export symbols
without a name (just by ordinal), but I didn't find anything about this
topic.
The next 3 32-bit-values are RVAs to 3 arrays. The arrays run parallel:
the array of entry points 'AddressOfFunctions' (given as 32-bit-RVAs),
the array of 32-bit-RVAs to symbol names 'AddressOfNames', and the array
of 16-bit-ordinals 'AddressOfNameOrdinals'.
To get information about the i-th exported symbol, find the export
directory, follow the 3 RVAs, skip to index i in the 3 arrays, and
you have got a) the RVA of the function's entry point
b) the RVA to the
function's name (a 0-terminated ASCII-string) and c) the function's
ordinal (you have to add the 'Base' to the value to get the real
ordinal).
For functions, the entry-point-RVA will normally point into the code
section
for objects, it will almost always point into the data- or
bss-section.
imported symbols
----------------
This is one for the tough.
When the compiler finds a call to a function that is in a different
executable (mostly in a DLL), it will, in the most simplistic case, not
know anything about the circumstances and simply output a normal
call-instruction to that symbol, the address of which the linker will
have to fix. The linker uses an import library to supply the addresses,
and that library has stubs for all the exported symbols, each of which
consists of a jump-instruction
the stubs are the actual call-targets.
These jump-instructions will actually jump to an address that's fetched
from the import address table we're discussing here.
In more sophisticated applications (when "declspec(dllimport)" is used),
the compiler knows the function is imported, and outputs a call to the
address that's in the import address table, bypassing the jump.
Hm. So far for that matter. You are kindly referred to [1] and [2].
However, the address of the function in the DLL is always necessary and
will be supplied by the loader when the application is loaded. The
loader knows which symbols in what libraries have to be looked up and
their addresses fixed by searching the import directory.
I will better give you an example. The calls with or without
__declspec(dllimport) look like this:
source:
int symbol(char *);
__declspec(dllimport) int symbol2(char*);
void foo(void)
{
int i=symbol("bar");
int j=symbol2("baz");
}
assembly:
...
call _symbol
without declspec(dllimport)
...
call ptr cs:0xdeadbeef
with declspec(dllimport)
...
the stub provided by the import library (not used with
__declspec(dllimport)) is located in the code section
and is the above mentioned jump-instruction:
_symbol: jmp dword ptr [0xdeadbeef]
So however you do it, there is a call or a jump to a location the
address of which is at 0xdeadbeef (in our example). This 0xdeadbeef is
right in the middle of the import directory
it is in the entry for the
function "symbol" in the DLL "foo.dll".
Now we'll look how an import directory is made up.
The import directory should reside in a section that's "initialized
data" and "readable".
The import directory is an array of IMAGE_IMPORT_DESCRIPTORs, one for
each used DLL. The list is terminated by a IMAGE_IMPORT_DESCRIPTOR
that's entirely filled with 0-bytes.
An IMAGE_IMPORT_DESCRIPTOR is a struct with these members:
OriginalFirstThunk
An RVA (32 bit) to a 0-terminated array of RVAs to
IMAGE_THUNK_DATAs, each describing one imported function. The
array will never change.
TimeDateStamp
A 32-bit-timestamp that has several purposes. Let's pretent that
the timestamp is 0, and handle the advanced cases later.
ForwarderChain
The 32-bit-index of the first forwarder in the list of imported
functions. Forwarders are also advanced stuff.
Name
A 32-bit-RVA to the Name (a 0-terminated ASCII string) of the
DLL.
FirstThunk
An RVA (32 bit) to a 0-terminated array of RVAs to
IMAGE_THUNK_DATAs, each describing one imported function. The
array will change.
Each IMAGE_IMPORT_DESCRIPTOR in the array gives you the name of the
exporting DLL and, apart from the forwarder and timestamp, it gives you
2 RVAs to arrays of IMAGE_THUNK_DATAs, using 32 bits. (The last member
of each array is entirely filled with 0-bytes.)
Each IMAGE_THUNK_DATA is, for now, an RVA to a IMAGE_IMPORT_BY_NAME
which describes the imported function.
The interesting point is now, the arrays run parallel, i.e.: they point
to the same IMAGE_IMPORT_BY_NAMEs.
No need to be desparate, I will draw another picture. This is the
essential contents of one IMAGE_IMPORT_DESCRIPTOR:
OriginalFirstThunk FirstThunk
| |
| |
| |
V V
0--> func1 <--0
1--> func2 <--1
2--> func3 <--2
3--> foo <--3
4--> mumpitz <--4
5--> knuff <--5
6-->0 0<--6 /* the last RVA is 0! */
where the names in the center are the yet to discuss
IMAGE_IMPORT_BY_NAMEs. Each of them is a 16-bit-number (the ordinal of
the imported symbol) followed by an unspecified amount of bytes, being
the 0-terminated ASCII name of the imported symbol.
Mind you, the ordinal may be wrong. It is only a hint where to start
looking for the symbol name in the export table of the exporting DLL;
you may not compare the ordinals and take the matching entry, but you
must find the matching symbol name.
To summarize, if you want to look up information about the imported
function "foo" from DLL "knurr", you first find the entry
IMAGE_DIRECTORY_ENTRY_IMPORT in the data directories, get an RVA, find
that address in the raw section data and now have an array of
IMAGE_IMPORT_DESCRIPTORs. Get the member of this array that relates to
the DLL "knurr" by inspecting the strings pointed to by the 'Name's.
When you have found the right IMAGE_IMPORT_DESCRIPTOR, follow its
'OriginalFirstThunk' and get hold of the pointed-to array of
IMAGE_THUNK_DATAs
inspect the RVAs and find the function "foo".
This was the basic structure, for simple cases. Now we'll learn about
tweaks in the import directories.
First, the bit IMAGE_ORDINAL_FLAG (that is: the MSB) of the
IMAGE_THUNK_DATA in the arrays can be set, in which case there is no
symbol-name-information in the list and the symbol is imported purely by
ordinal. You get the ordinal by inspecting the lower word of the
IMAGE_THUNK_DATA.
Obviously, in this case the warning about unreliable ordinals doesn't
apply because all you have *is* the ordinal.
Ok, now, why do we have *two* lists of pointers to the
IMAGE_IMPORT_BY_NAMEs? Because at runtime the application doesn't need
the imported functions' names but the addresses. The loader will look up
each imported symbol in the export-directory of the DLL in question and
replace the IMAGE_THUNK_DATA in the 'FirstThunk'-list with the linear
address of the DLL's entry point. Remember the stubs "jmp dword ptr
[0xdeadbeef]"
this address 0xdeadbeef is exactly the address of the
IMAGE_THUNK_DATA in the 'FirstThunk'-list.
The 'OriginalFirstThunk' remains untouched, so you can always look up
the original list of imported names via the 'OriginalFirstThunk'-list.
Now we get the so-called "bound imports".
Think about the loader's task: when a binary that it wants to execute
needs a function from a DLL, the loader loads the DLL, finds its export
directory, looks up the function's RVA and calculates the function's
entry point. Then it patches the so-found address into the 'FirstThunk'-
list.
Given that the programmer was clever and supplied unique preferred load
addresses for the DLLs that don't clash, we can assume that the
functions' entry points will always be the same. They can be computed
and patched into the 'FirstThunk'-list at link-time, and that's what
happens with the "bound imports". (The utility "bind" does this.)
Of course, one must be cautious: The user's DLL may have a different
version, or it may be necessary to relocate the DLL, thus invalidating
the pre-patched 'FirstThunk'-list
in this case, the loader will still
be able to walk the 'OriginalFirstThunk'-list, find the imported symbols
and re-patch the 'FirstThunk'-list. The loader knows that this is
necessary if a) the versions of the exporting DLL don't match or b) the
exporting DLL had to be relocated.
To decide whether there were relocations is no problem for the loader,
but how to find out if the versions differ? This is where the
'TimeDateStamp' of the IMAGE_IMPORT_DESCRIPTOR comes in. If it is 0, the
import-list has not been bound, and the loader must fix the entry points
always. Otherwise, the imports are bound, and 'TimeDateStamp' must match
the 'TimeDateStamp' of the exporting DLL's 'FileHeader'
if it doesn't
match, the loader assumes that the binary is bound to a "wrong" DLL and
will re-patch the import list.
This was the so-called "old-style" binding.
There is an additional quirk about "forwarders" in the import-list. A DLL
can export a symbol that's not defined in the DLL but imported from
another DLL
such a symbol is said to be forwarded. Now, obviously you
can't tell if the symbol's entry point is valid by looking into the
timestamp of a DLL that doesn't actually contain the entry point. So the
forwarded symbols' entry points must always be fixed up, for safety
reasons. In the import list of a binary, imports of forwarded symbols
need to be found so the loader can patch them.
This is done via the 'ForwarderChain'. It is an index in the thunk-
lists
the import at the indexed position is a forwarded export, and the
contents of the 'FirstThunk'-list at this position is the index of the
*next* forwarded import, and so on, until the index is "-1" which
indicates there are no more forwards.
At this point, we should sum up what we have had so far
Ok, I will assume you have found the IMAGE_DIRECTORY_ENTRY_IMPORT and you have
followed it to find the import-directory, which will be in one of the
sections. Now you're at the beginning of an array of
IMAGE_IMPORT_DESCRIPTORs the last of which will be entirely 0-bytes-
filled.
To decipher one of the IMAGE_IMPORT_DESCRIPTORs, you first look into the
'Name'-field, follow the RVA and thusly find the name of the exporting
DLL. Next you decide whether the imports are bound or not;
'TimeDateStamp' will be non-zero if the imports are bound. If they are
bound, now is a good time to check if the DLL version matches yours by
comparing the 'TimeDateStamp's.
Now you follow the 'OriginalFirstThunk'-RVA to go to the
IMAGE_THUNK_DATA-array
walk down this array (it is be 0-terminated),
and each member will be the RVA of a IMAGE_IMPORT_BY_NAME (unless the
hi-bit is set in which case you don't have a name but are left with a
mere ordinal). Follow the RVA, and skip 2 bytes (the ordinal), and now
you have got a pointer to a 0-terminated ASCII-string that's the name of
the imported function.
To find the supplied entry point addresses in case it is a bound import,
follow the 'FirstThunk' and walk it parallel to the
'OriginalFirstThunk'-array
the array-members are the linear addresses
of the entry points.
There is one thing I didn't mention until now: Apparently there are
linkers that exhibit a bug when they build the import directory (I've
found this bug being in use by a Borland C linker). These linkers set
the 'OriginalFirstThunk' in the IMAGE_IMPORT_DESCRIPTOR to 0 and create
only the 'FirstThunk'-array. Obviously, such import directories cannot
be bound (else the necessary information to re-fix the imports were
lost). In this case, you will have to follow the 'FirstThunk'-array to get
the imported symbol names, and you will never have pre-patched entry point
addresses.
The last tweak about the import directories is the so-called "new style"
binding (it is described in [3]), which can also be done with the
"bind"-utility. When this is used, the 'TimeDateStamp' is set to
all-bits-1 and there is no forwarderchain
all imported symbols get their
address patched, whether they are forwarded or not. Still, you need to
know the DLLs' version, and you need to distinguish forwarded symbols
from ordinary ones. For this purpose, the
IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT directory is created. This will, as
far as I could find out, *not* be in a section but in the header, after
the section headers and before the first section. (Hey, I didn't invent
this, I'm only describing it!)
This directory tells you, for each used DLL, from which other DLLs there
are forwarded exports.
The structure is an IMAGE_BOUND_IMPORT_DESCRIPTOR, comprising (in this
order):
A 32-bit number, giving you the 'TimeDateStamp' of the DLL;
a 16-bit-number 'OffsetModuleName', being the offset from the beginning
of the directory to the 0-terminated name of the forwarded-to DLL;
a 16-bit-number 'NumberOfModuleForwarderRefs' giving you the number of
DLLs that this DLL uses for its forwarders.
Immediatly following this struct you find 'NumberOfModuleForwarderRefs'
structs that tell you the names and versions of the DLLs that this DLL
forwards from. These structs are 'IMAGE_BOUND_FORWARDER_REF's:
A 32-bit-number 'TimeDateStamp';
a 16-bit-number 'OffsetModuleName', being the offset from the beginning
of the directory to the 0-terminated name of the forwarded-from DLL;
16 unused bits.
Following the 'IMAGE_BOUND_FORWARDER_REF's is the next
'IMAGE_BOUND_IMPORT_DESCRIPTOR' and so on
the list is terminated by an
all-0-bits-IMAGE_BOUND_IMPORT_DESCRIPTOR.
Sorry for the inconvenience, but that's what it looks like
Now, if you have a new-bound import directory, you load all the DLLs,
use the directory pointer IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT to find the
IMAGE_BOUND_IMPORT_DESCRIPTOR, scan through it and check if the
'TimeDateStamp's of the loaded DLLs match the ones given in this
directory. If not, fix them in the 'FirstThunk'-array of the import
directory.
It is easy if you know how it works
resources
---------
The resources, like dialog boxes, menus, icons and so on, are stored in
the data directory pointed to by IMAGE_DIRECTORY_ENTRY_RESOURCE. It is in
a section that has, at least, the bits 'IMAGE_SCN_CNT_INITIALIZED_DATA'
and 'IMAGE_SCN_MEM_READ' set.
A resource base is a 'IMAGE_RESOURCE_DIRECTORY's
it contains several
'IMAGE_RESOURCE_DIRECTORY_ENTRY's which in turn may point to a
'IMAGE_RESOURCE_DIRECTORY'. This way, you get a tree of
'IMAGE_RESOURCE_DIRECTORY's with 'IMAGE_RESOURCE_DIRECTORY_ENTRY's as
leafs
these leafs point to the actual resource data.
In real life, the situation is somewhat relaxed. Normally you won't find
convoluted trees you can't possibly sort out.
The hierarchy is, normally, like this: one directory is the root. It
points to directories, one for each resource type. These directories
point to subdirectories, each of which will have a name or an ID and
point to a directory of the languages provided for this resource
for
each language you will find one resource entry, which will finally point
to the data.
The tree, without the pointer to the data, may look like this:
(root)
|
+----------------+------------------+
| | |
menu dialog icon
| | |
+-----+-----+ +-+----+ +-+----+----+
| | | | | | |
"main" "popup" 0x10 "maindlg" 0x100 0x110 0x120
| | | | | | |
+---+-+ | | | | | |
| | default english default def. def. def.
german english
A IMAGE_RESOURCE_DIRECTORY comprises:
32 bits of unused flags called 'Characteristics';
32 bits 'TimeDateStamp' (again in the common time_t representation),
giving you the time the resource was created (if the entry is set);
16 bits 'MajorVersion' and 16 bits 'MinorVersion', thusly allowing you
to maintain several versions of the resource;
16 bits 'NumberOfNamedEntries' and another 16 bits 'NumberOfIdEntries'.
Immediatly following such a structure are
'NumberOfNamedEntries'+'NumberOfIdEntries' structs which are of the
format 'IMAGE_RESOURCE_DIRECTORY_ENTRY', those with the names coming first.
They may point to further 'IMAGE_RESOURCE_DIRECTORY's or they point to
the actual resource data.
A IMAGE_RESOURCE_DIRECTORY_ENTRY consists of:
32 bits giving you the id of the resource or the directory
it describes;
32 bits offset to the data or offset to the next sub-directory.
The meaning of the id depends on the level in the tree
the id may be a
number (if the hi-bit is clear) or a name (if the hi-bit is set). If it
is a name, the lower 31 bits are the offset from the beginning of the
resource section's raw data to the name (the name consists of 16 bits
length and trailing wide characters, in unicode, not 0-terminated).
If you are in the root-directory, the id, if it is a number, is the
resource-type:
1: cursor
2: bitmap
3: icon
4: menu
5: dialog
6: string table
7: font directory
8: font
9: accelerators
10: unformatted resource data
11: message table
12: group cursor
14: group icon
16: version information
Any other number is user-defined. Any resource-type with a type-name is
always user-defined.
If you are one level deeper, the id is the resource-id (or resource-
name).
If you are another level deeper, the id must be a number, and it is the
language-id of the specific instance of the resource
for example, you
can have the same dialog in english, french and german localized forms,
and they all share the same resource-id. The system will choose the
dialog to load based on the process's locale, which in turn will usually
reflect the user's "regional setting".
(If the resource cannot be found for the process locale, the system will
first try to find a resource for the locale using a neutral sublanguage;
if it still can't be found, the instance with the smallest language id will
be used.)
To decipher the language id, split it into the primary language id and
the sublanguage id using the macros PRIMARYLANGID() and SUBLANGID(),
giving you the bits 0 to 9 or 10 to 15, respectivly. The values are
defined in the file "winresrc.h".
Language-resources are only supported for accelerators, dialogs, menus,
rcdata or stringtables
other resource-types should be
LANG_NEUTRAL/SUBLANG_NEUTRAL.
To find out whether the next level below a resource directory is another
directory, you inspect the hi-bit of the offset. If it is set, the
remaining 31 bits are the offset from the beginning of the resource
section's raw data to the next directory, again in the format
IMAGE_RESOURCE_DIRECTORY with trailing IMAGE_RESOURCE_DIRECTORY_ENTRYs.
If the bit is clear, the offset is the distance from the beginning of
the resource section's raw data to the resource's raw data description,
a IMAGE_RESOURCE_DATA_ENTRY. It consists of 32 bits 'OffsetToData' (the
offset to the raw data, counting from the beginning of the resource
section's raw data), 32 bits of 'Size' of the data, 32 bits 'CodePage'
and 32 unused bits.
(The use of codepages is discouraged, you should use the 'language'-
feature to support multiple locales.)
The raw data format depends on the resource type
descriptions can be
found in the MS SDK documentation. Note that any string in resources is
always in UNICODE.
relocations
-----------
The last data directory I will describe is the base relocation
directory. It is pointed to by the IMAGE_DIRECTORY_ENTRY_BASERELOC entry
in the data directories of the optional header. It is typically
contained in a section if its own, with a name like ".reloc" and the
bits IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE and
IMAGE_SCN_MEM_READ set.
The relocation data is needed by the loader if the image cannot be
loaded to the preferred load address 'ImageBase' mentioned in the
optional header. In this case, the fixed addresses supplied by the
linker are no longer valid, and the loader has to apply fixups for
absolute addresses used for locations of static variables, string
literals and so on.
The relocation directory is a sequence of chunks. Each chunk contains
the relocation information for 4 KB of the image. A chunk starts with a
'IMAGE_BASE_RELOCATION' struct. It consists of 32 bits 'VirtualAddress'
and 32 bits 'SizeOfBlock'. It is followed by the chunk's actual
relocation data, being 16 bits each.
The 'VirtualAddress' is the base RVA that the relocations of this chunk
need to be applied to
the 'SizeOfBlock' is the size of the entire chunk
in bytes.
The number of trailing relocations is
('SizeOfBlock'-sizeof(IMAGE_BASE_RELOCATION))/2
The relocation information ends when you encounter a
IMAGE_BASE_RELOCATION struct with a 'VirtualAddress' of 0.
Each 16-bit-relocation information consists of the actual relocation
position in the lower 12 bits and a relocation type in the high 4 bits.
To get the relocation RVA, you need to add the IMAGE_BASE_RELOCATION's
'VirtualAddress' to the 12-bit-position. The type is one of:
IMAGE_REL_BASED_ABSOLUTE (0)
This is a no-op
it is used to align the chunk to a 32-bits-
border.
IMAGE_REL_BASED_HIGH (1)
The relocation must be applied to the high 16 bits of the DWORD
in question.
IMAGE_REL_BASED_LOW (2)
The relocation must be applied to the low 16 bits of the DWORD
in question.
IMAGE_REL_BASED_HIGHLOW (3)
The relocation must be applied to the entire 32 bits in question.
IMAGE_REL_BASED_HIGHADJ (4)
Unknown
IMAGE_REL_BASED_MIPS_JMPADDR (5)
Unknown
IMAGE_REL_BASED_SECTION (6)
Unknown
IMAGE_REL_BASED_REL32 (7)
Unknown
As an example, if you find the relocation information to be
0x00004000 (32 bits, starting RVA)
0x00000010 (32 bits, size of chunk)
0x3012 (16 bits reloc data)
0x3080 (16 bits reloc data)
0x30f6 (16 bits reloc data)
0x0000 (16 bits reloc data)
0x00000000 (next chunk's RVA)
0xff341234
you know the first chunk describes relocations starting at RVA 0x4000 and
is 16 bytes long. Because the header uses 8 bytes and one relocation
uses 2 bytes, there are (16-8)/2=4 relocations in the chunk.
The first relocation is to be applied to the DWORD at 0x4012, the next
to the DWORD at 0x4080, and the third to the DWORD at 0x40f6. The last
relocation is a no-op.
The next chunk has a RVA of 0 and finishes the list.
Now, how do you do a relocation?
You know that the image *is* relocated to the preferred load address
'ImageBase' in the optional header
you also know the address you did
load the image to. If they match, you don't need to do anything.
If they don't match, you calculate the difference
actual_base-preferred_base
and add that value to the relocation positions, which you will find with
the method described above.
Acknowledgments
---------------
Thanks go to David Binette for his debugging and proof-reading.
(The remaining errors are entirely mine.)
Copyright
---------
This text is copyright 1998 by B. Luevelsmeyer. It is freeware, and you
may use it for any purpose but on you own risk. It contains errors and
it is incomplete. You have been warned.
Bug reports
-----------
Send any bug reports (or other comments) to
bernd.luevelsmeyer@iplan.heitec.net
Literature
----------
[1]
"Peering Inside the PE: A Tour of the Win32 Portable Executable File
Format" (M. Pietrek), in: Microsoft Systems Journal 3/1994
[2]
"Why to Use _declspec(dllimport) &
_declspec(dllexport) In Code", MS
Knowledge Base Q132044
[3]
"Windows Q&A" (M. Pietrek), in: Microsoft Systems Journal 8/1995
[4]
"Writing Multiple-Language Resources", MS Knowledge Base Q89866
[5]
"The Portable Executable File Format from Top to Bottom" (Randy Kath),
in: Microsoft Developer Network
Appendix: hello world
---------------------
In this appendix I will show how to make programs by hand. The example
will use Intel-assembly, because I don't speak DEC Alpha.
The program will be the equivalent of
#include <stdio.h>
int main(void)
{
puts(hello,world);
return 0;
}
First, I translate it to use Win32 functions instead of the C runtime:
#define STD_OUTPUT_HANDLE -11UL
#define hello "hello, world/n"
__declspec(dllimport) unsigned long __stdcall
GetStdHandle(unsigned long hdl);
__declspec(dllimport) unsigned long __stdcall
WriteConsoleA(unsigned long hConsoleOutput,
const void *buffer,
unsigned long chrs,
unsigned long *written,
unsigned long unused
);
static unsigned long written;
void startup(void)
{
WriteConsoleA(GetStdHandle(STD_OUTPUT_HANDLE),hello,sizeof(hello)-1,&written,0);
return;
}
Now I will fumble out the assembly:
startup:
parameters for WriteConsole(), backwards
6A 00 push 0x00000000
68 ?? ?? ?? ?? push offset _written
6A 0D push 0x0000000d
68 ?? ?? ?? ?? push offset hello
parameter for GetStdHandle()
6A F5 push 0xfffffff5
2E FF 15 ?? ?? ?? ?? call dword ptr cs:__imp__GetStdHandle@4
result is last parameter for WriteConsole()
50 push eax
2E FF 15 ?? ?? ?? ?? call dword ptr cs:__imp__WriteConsoleA@20
C3 ret
hello:
68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A "hello, world/n"
_written:
00 00 00 00
I need to find the functions WriteConsoleA() and GetStdHandle(). They
happen to be in "kernel32.dll".
Now I can start to make the executable. Question marks will take the
place of yet-to-find-out values
they will be patched afterwards.
First the DOS-header, starting at 0x0 and being 0x40 bytes long:
00 | 4d 5a 00 00 00 00 00 00 00 00 00 00 00 00 00 00
10 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
20 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
30 | 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 00
As you can see, this isn't really a MS-DOS program. It's just the header
with the signature "MZ" at the beginning and the e_lfanew pointing
immediatly after the header, without any code. That's because it isn't
intended to run on MS-DOS.
Then the signature, starting at 0x40 and being 0x4 bytes long:
50 45 00 00
Now the file-header, which will start at byte 0x44 and is 0x14 bytes long:
Machine 4c 01
i386
NumberOfSections 02 00
code and data
TimeDateStamp 00 00 00 00
who cares?
PointerToSymbolTable 00 00 00 00
unused
NumberOfSymbols 00 00 00 00
unused
SizeOfOptionalHeader e0 00
constant
Characteristics 02 01
executable on 32-bit-machine
And the optional header, which will start at byte 0x58 and is 0x60 bytes long:
Magic 0b 01
constant
MajorLinkerVersion 00
I'm version 0.0
MinorLinkerVersion 00
SizeOfCode 20 00 00 00
32 bytes of code
SizeOfInitializedData ?? ?? ?? ??
yet to find out
SizeOfUninitializedData 00 00 00 00
we don't have a BSS
AddressOfEntryPoint ?? ?? ?? ??
yet to find out
BaseOfCode ?? ?? ?? ??
yet to find out
BaseOfData ?? ?? ?? ??
yet to find out
ImageBase 00 00 10 00
1 MB, chosen arbitrarily
SectionAlignment 20 00 00 00
32-bytes-alignment
FileAlignment 20 00 00 00
32-bytes-alignment
MajorOperatingSystemVersion 04 00
NT 4.0
MinorOperatingSystemVersion 00 00
MajorImageVersion 00 00
version 0.0
MinorImageVersion 00 00
MajorSubsystemVersion 04 00
Win32 4.0
MinorSubsystemVersion 00 00
Win32VersionValue 00 00 00 00
unused?
SizeOfImage ?? ?? ?? ??
yet to find out
SizeOfHeaders ?? ?? ?? ??
yet to find out
CheckSum 00 00 00 00
not used for non-drivers
Subsystem 03 00
Win32 console
DllCharacteristics 00 00
unused (not a DLL)
SizeOfStackReserve 00 00 10 00
1 MB stack
SizeOfStackCommit 00 10 00 00
4 KB to start with
SizeOfHeapReserve 00 00 10 00
1 MB heap
SizeOfHeapCommit 00 10 00 00
4 KB to start with
LoaderFlags 00 00 00 00
unknown
NumberOfRvaAndSizes 10 00 00 00
constant
As you can see, I plan to have only 2 sections, one for code and one for
all the rest (data, constants and import directory). There will be no
relocations and no other stuff like resources.
The section alignment is the same in the file and in RAM (32 bytes);
this helps to keep the task easy.
Now we set up the data directories, beginning at byte 0xb8 and being 0x80 bytes long:
Address Size
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_EXPORT (0)
?? ?? ?? ?? ?? ?? ?? ??
IMAGE_DIRECTORY_ENTRY_IMPORT (1)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_SECURITY (4)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_DEBUG (6)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_TLS (9)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_IAT (12)
00 00 00 00 00 00 00 00
13
00 00 00 00 00 00 00 00
14
00 00 00 00 00 00 00 00
15
Only the import directory is in use.
Next are the section headers. First we make the code section, which will
contain the above mentioned assembly. It is 32 bytes long, and so will
be the code section. The header begins at 0x138 and is 0x28 bytes long:
Name 2e 63 6f 64 65 00 00 00
".code"
VirtualSize 00 00 00 00
unused
VirtualAddress ?? ?? ?? ??
yet to find out
SizeOfRawData 20 00 00 00
size of code
PointerToRawData ?? ?? ?? ??
yet to find out
PointerToRelocations 00 00 00 00
unused
PointerToLinenumbers 00 00 00 00
unused
NumberOfRelocations 00 00
unused
NumberOfLinenumbers 00 00
unused
Characteristics 20 00 00 60
code, executable, readable
The second section will contain the data. The header begins at 0x160 and
is 0x28 bytes long:
Name 2e 64 61 74 61 00 00 00
".data"
VirtualSize 00 00 00 00
unused
VirtualAddress ?? ?? ?? ??
yet to find out
SizeOfRawData ?? ?? ?? ??
yet to find out
PointerToRawData ?? ?? ?? ??
yet to find out
PointerToRelocations 00 00 00 00
unused
PointerToLinenumbers 00 00 00 00
unused
NumberOfRelocations 00 00
unused
NumberOfLinenumbers 00 00
unused
Characteristics 40 00 00 c0
initialized, readable, writeable
The next byte is 0x188, but the sections need to be aligned to 32 bytes
(because I chose so), so we need padding bytes up to 0x1a0:
00 00 00 00 00 00
padding
00 00 00 00 00 00
00 00 00 00 00 00
00 00 00 00 00 00
Now the first section, being the code section with the above mentioned
assembly, *does* come. It begins at byte 0x1a0 and is 0x20 bytes long:
6A 00
push 0x00000000
68 ?? ?? ?? ??
push offset _written
6A 0D
push 0x0000000d
68 ?? ?? ?? ??
push offset hello_string
6A F5
push 0xfffffff5
2E FF 15 ?? ?? ?? ??
call dword ptr cs:__imp__GetStdHandle@4
50
push eax
2E FF 15 ?? ?? ?? ??
call dword ptr cs:__imp__WriteConsoleA@20
C3
ret
Because of the previous section's length we don't need any padding
before the next section (data), and here it comes, beginning at 0x1c0:
68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A
"hello, world/n"
00 00 00
padding to align _written
00 00 00 00
_written
Now all that's left is the import directory. It will import 2 functions
from "kernel32.dll", and it's immediatly following the variables in the
same section. First we will align it to 32 bytes:
00 00 00 00 00 00 00 00 00 00 00 00
padding
It begins at 0x1e0 with the IMAGE_IMPORT_DESCRIPTOR:
OriginalFirstThunk ?? ?? ?? ??
yet to find out
TimeDateStamp 00 00 00 00
unbound
ForwarderChain ff ff ff ff
no forwarders
Name ?? ?? ?? ??
yet to find out
FirstThunk ?? ?? ?? ??
yet to find out
We need to terminate the import-directory with a 0-bytes-entry (we are at 0x1f4):
OriginalFirstThunk 00 00 00 00
terminator
TimeDateStamp 00 00 00 00
ForwarderChain 00 00 00 00
Name 00 00 00 00
FirstThunk 00 00 00 00
Now there's the DLL name left, and the 2 thunks, and the thunk-data, and
the function names. But we will be finished real soon now!
The DLL name, 0-terminated, beginning at 0x208:
6b 65 72 6e 65 6c 33 32 2e 64 6c 6c 00
"kernel32.dll"
00 00 00
padding to 32-bit-boundary
The original first thunk, starting at 0x218:
AddressOfData ?? ?? ?? ??
RVA to function name "WriteConsoleA"
AddressOfData ?? ?? ?? ??
RVA to function name "GetStdHandle"
00 00 00 00
terminator
The first thunk is exactly the same list and starts at 0x224:
(__imp__WriteConsoleA@20, at 0x224)
AddressOfData ?? ?? ?? ??
RVA to function name "WriteConsoleA"
(__imp__GetStdHandle@4, at 0x228)
AddressOfData ?? ?? ?? ??
RVA to function name "GetStdHandle"
00 00 00 00
terminator
Now what's left is the two function names in the shape of an
IMAGE_IMPORT_BY_NAME. We are at byte 0x230.
01 00
ordinal, need not be correct
57 72 69 74 65 43 6f 6e 73 6f 6c 65 41 00
"WriteConsoleA"
02 00
ordinal, need not be correct
47 65 74 53 74 64 48 61 6e 64 6c 65 00
"GetStdHandle"
Ok, that's about all. The next byte, which we don't really need, is
0x24f. We need to fill the section with padding up to 0x260:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
padding
00
------------
We are done. Now that we know all the byte-offsets, we can apply fixups
to all those addresses and sizes that were indicated as "unknown" with
'??'-marks.
I won't force you to read that step-by-step (it's quite
straightforward), and simply present the result:
------------
DOS-header, starting at 0x0:
00 | 4d 5a 00 00 00 00 00 00 00 00 00 00 00 00 00 00
10 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
20 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
30 | 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 00
signature, starting at 0x40:
50 45 00 00
file-header, starting at 0x44:
Machine 4c 01
i386
NumberOfSections 02 00
code and data
TimeDateStamp 00 00 00 00
who cares?
PointerToSymbolTable 00 00 00 00
unused
NumberOfSymbols 00 00 00 00
unused
SizeOfOptionalHeader e0 00
constant
Characteristics 02 01
executable on 32-bit-machine
optional header, starting at 0x58:
Magic 0b 01
constant
MajorLinkerVersion 00
I'm version 0.0
MinorLinkerVersion 00
SizeOfCode 20 00 00 00
32 bytes of code
SizeOfInitializedData a0 00 00 00
data section size
SizeOfUninitializedData 00 00 00 00
we don't have a BSS
AddressOfEntryPoint a0 01 00 00
beginning of code section
BaseOfCode a0 01 00 00
RVA to code section
BaseOfData c0 01 00 00
RVA to data section
ImageBase 00 00 10 00
1 MB, chosen arbitrarily
SectionAlignment 20 00 00 00
32-bytes-alignment
FileAlignment 20 00 00 00
32-bytes-alignment
MajorOperatingSystemVersion 04 00
NT 4.0
MinorOperatingSystemVersion 00 00
MajorImageVersion 00 00
version 0.0
MinorImageVersion 00 00
MajorSubsystemVersion 04 00
Win32 4.0
MinorSubsystemVersion 00 00
Win32VersionValue 00 00 00 00
unused?
SizeOfImage c0 00 00 00
sum of all section sizes
SizeOfHeaders a0 01 00 00
offset to 1st section
CheckSum 00 00 00 00
not used for non-drivers
Subsystem 03 00
Win32 console
DllCharacteristics 00 00
unused (not a DLL)
SizeOfStackReserve 00 00 10 00
1 MB stack
SizeOfStackCommit 00 10 00 00
4 KB to start with
SizeOfHeapReserve 00 00 10 00
1 MB heap
SizeOfHeapCommit 00 10 00 00
4 KB to start with
LoaderFlags 00 00 00 00
unknown
NumberOfRvaAndSizes 10 00 00 00
constant
data directories, starting at 0xb8:
Address Size
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_EXPORT (0)
e0 01 00 00 6f 00 00 00
IMAGE_DIRECTORY_ENTRY_IMPORT (1)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_SECURITY (4)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_DEBUG (6)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_TLS (9)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
00 00 00 00 00 00 00 00
IMAGE_DIRECTORY_ENTRY_IAT (12)
00 00 00 00 00 00 00 00
13
00 00 00 00 00 00 00 00
14
00 00 00 00 00 00 00 00
15
section header (code), starting at 0x138:
Name 2e 63 6f 64 65 00 00 00
".code"
VirtualSize 00 00 00 00
unused
VirtualAddress a0 01 00 00
RVA to code section
SizeOfRawData 20 00 00 00
size of code
PointerToRawData a0 01 00 00
file offset to code section
PointerToRelocations 00 00 00 00
unused
PointerToLinenumbers 00 00 00 00
unused
NumberOfRelocations 00 00
unused
NumberOfLinenumbers 00 00
unused
Characteristics 20 00 00 60
code, executable, readable
section header (data), starting at 0x160:
Name 2e 64 61 74 61 00 00 00
".data"
VirtualSize 00 00 00 00
unused
VirtualAddress c0 01 00 00
RVA to data section
SizeOfRawData a0 00 00 00
size of data section
PointerToRawData c0 01 00 00
file offset to data section
PointerToRelocations 00 00 00 00
unused
PointerToLinenumbers 00 00 00 00
unused
NumberOfRelocations 00 00
unused
NumberOfLinenumbers 00 00
unused
Characteristics 40 00 00 c0
initialized, readable, writeable
(padding)
00 00 00 00 00 00
padding
00 00 00 00 00 00
00 00 00 00 00 00
00 00 00 00 00 00
code section, starting at 0x1a0:
6A 00
push 0x00000000
68 d0 01 10 00
push offset _written
6A 0D
push 0x0000000d
68 c0 01 10 00
push offset hello_string
6A F5
push 0xfffffff5
2E FF 15 28 02 10 00
call dword ptr cs:__imp__GetStdHandle@4
50
push eax
2E FF 15 24 02 10 00
call dword ptr cs:__imp__WriteConsoleA@20
C3
ret
data section, beginning at 0x1c0:
68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A
"hello, world/n"
00 00 00
padding to align _written
00 00 00 00
_written
padding:
00 00 00 00 00 00 00 00 00 00 00 00
padding
IMAGE_IMPORT_DESCRIPTOR, starting at 0x1e0:
OriginalFirstThunk 18 02 00 00
RVA to orig. 1st thunk
TimeDateStamp 00 00 00 00
unbound
ForwarderChain ff ff ff ff
no forwarders
Name 08 02 00 00
RVA to DLL name
FirstThunk 24 02 00 00
RVA to 1st thunk
terminator (0x1f4):
OriginalFirstThunk 00 00 00 00
terminator
TimeDateStamp 00 00 00 00
ForwarderChain 00 00 00 00
Name 00 00 00 00
FirstThunk 00 00 00 00
The DLL name, at 0x208:
6b 65 72 6e 65 6c 33 32 2e 64 6c 6c 00
"kernel32.dll"
00 00 00
padding to 32-bit-boundary
original first thunk, starting at 0x218:
AddressOfData 30 02 00 00
RVA to function name "WriteConsoleA"
AddressOfData 40 02 00 00
RVA to function name "GetStdHandle"
00 00 00 00
terminator
first thunk, starting at 0x224:
AddressOfData 30 02 00 00
RVA to function name "WriteConsoleA"
AddressOfData 40 02 00 00
RVA to function name "GetStdHandle"
00 00 00 00
terminator
IMAGE_IMPORT_BY_NAME, at byte 0x230:
01 00
ordinal, need not be correct
57 72 69 74 65 43 6f 6e 73 6f 6c 65 41 00
"WriteConsoleA"
IMAGE_IMPORT_BY_NAME, at byte 0x240:
02 00
ordinal, need not be correct
47 65 74 53 74 64 48 61 6e 64 6c 65 00
"GetStdHandle"
(padding)
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
padding
00
First unused byte: 0x260
--------------
Alas, this works on NT but didn't on windows 95. windows95 can't run
applications with a section alignment of 32 bytes, it needs an
alignment of 4 KB and, apparently, a file alignment of 512 bytes. So for
windows95 you'll have to insert a large number of 0-bytes (for padding)
and adjust the RVAs. Thanks go to D. Binette for testing on windows95.
-- end of text --
]