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Decomp is a decompiler for the x86 architecture.
Copyright @copyright{} 2001 Jonathan duSaint @email{dusaint@@earthlink.net}
Permission is granted to make and distribute verbatim copies of this
document provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified version of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified version,
except that this permission notice may be stated in a translation
approved by the author.
@end ifinfo
@c title page cruft
@titlepage
@title Decomp
@author Jonathan duSaint @email{dusaint@@earthlink.net}
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 2001 Jonathan duSaint @email{dusaint@@earthlink.net}
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified version of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified version,
except that this permission notice may be stated in a translation
approved by the author.
@end titlepage
@node Top, Overview, , (dir)
@top
Decomp is a decompiler which aims to be able to disassemble any
executable file in any format on any machine into an assembly language
file. It's current state falls somewhat short of that goal, but
the sky's the limit.
@menu
* Overview:: Decomp for the impatient
* Options:: Controlling Decomp
* Extending:: Adding new architectures and object file formats
* Copying:: Your rights
* Concept Index:: Key concepts
@end menu
@node Overview
@chapter Overview
Decomp is a decompiler. It can be used to convert an object file into a
assembly language source file. The assembly language source file can
then, in turn, be recompiled using an assembler and linked with any
appropriate libraries to recreate the original executable (or something
functionally identical).
To use Decomp, you need merely invoke it with the name of an object
file. For example, assuming that @file{foo} is the name of an object
file, execute the command @kbd{decomp foo}. The decompilation will be
located in the file @file{foo_decomp.S}. A quick perusal of the
decompilation will show that not only have the executable sections of
the object file been disassembled, but also the data sections have been
decompiled. The data may not be exactly like the original source data,
but they should be functionally equivalent.
@node Options
@chapter Options
There are a number of options to control the decompilation process and
the output.
@menu
* Getting Help:: Help and version information.
* Debugging:: Options which print debugging information.
* Extra Information:: Printing extra information in the output file.
* Run-time parameters:: Controlling how Decomp executes.
@end menu
@node Getting Help
@section Getting Help
There are two command line options which tell Decomp to print
information and exit.
@subsection @samp{-h, --help}
Invoking Decomp with this option will cause it to print a short message
explaining basic usage of the program and then exit.
@subsection @samp{-v, --version}
This option causes Decomp to print version and copyright information and
exit.
@node Debugging
@section Debugging
The following two commands tell Decomp to print out different types of
debugging information. Beware, though, as a lot of information can be
printed.
@subsection @samp{-d, --debug}
This option causes Decomp to print a lot of debugging information from
all stages of decompilation. To explain the output, here is an example.
This first block is a listing of the sections in the object
file. The first field is the name, the next is the index in the file,
and the last field is the offset in the file.
@example
read , idx 0, offset 0
read .text, idx 0x1, offset 0x74
read .data, idx 0x2, offset 0xb4
read .sbss, idx 0x3, offset 0xbc
read .bss, idx 0x4, offset 0xbc
read .shstrtab, idx 0x5, offset 0xbc
read .symtab, idx 0x6, offset 0x230
read .strtab, idx 0x7, offset 0x320
@end example
The next block describes which sections are being used by Decomp and
what they are being used for. Any sections under @samp{data:} will be
decompiled as regular data. Any sections under @samp{code:} will be
disassembled. Sections under @samp{symtab:} are used as the symbol
table, and the names of the symbols in the symbol table are found in the
sections listed under @samp{strtab:}. The fields may be interpreted as
follows: The first field is the section name and its index. The next
field is the section type as defined in @file{decomp.h}. The following
field is the offset of the section in the object file, and the field
after that is the size of the section in the object file. The last
field is the virtual address of the start of the section at run time.
@example
data:
.data (0x2), 1, 0xb4, 0x7, 0x80490b4
code:
.text (0x1), 0, 0x74, 0x3e, 0x8048074
symtab:
.symtab (0x6), 4, 0x230, 0xf0, 0
strtab:
.strtab (0x7), 3, 0x320, 0x36, 0
@end example
The last block of lines is the output from parsing the symbol table.
The first field is the virtual address of the symbol at run time
followed by the name of the symbol. The size of whatever the symbol
refers to follows. If the size is specified as 0 (as it is here), that
does not necessarily mean that the object has no size. It merely means
that when the file was originally assembled, the object size was not
specified. The @samp{info} and @samp{other} fields are the ELF32
symbol information and other information fields, and the values are
defined by the ELF32 standard. The last field is the index of the
section where this symbol is located.
@example
0x80490b4:msg: size 0, info 00, other 00, idx 0x2
0x7:len: size 0, info 00, other 00, idx 0xfff1
0x804809b:print_message: size 0, info 00, other 00, idx 0x1
0x8048074:_start: size 0, info 0x10, other 00, idx 0x1
0x80490bb:__bss_start: size 0, info 0x11, other 00, idx 0xfff1
0x80490bb:_edata: size 0, info 0x11, other 00, idx 0xfff1
@end example
@subsection @samp{-t, --trace}
This option causes the memory allocation routines to print out
information on every invocation. At the end of this information, a
summary of memory usage is printed. An in-depth explanation of the
memory allocation routine is beyond the scope of this manual. Those
wishing more information are free to peruse the source file
@file{memory.c}, which includes a lengthy discussion of the mechanism
used and justification for its use.
@node Extra Information
@section Extra Information
It is possible to include extra information in the output file in the
form of comments after the assembly language statements. This
information is of three types.
@subsection @samp{-a, --address}
This option causes the virtual address of each instruction to be printed
as a commment on the same line as the instruction.
@subsection @samp{-c, --opcode}
This option causes the opcode of the instruction to be printed as a
comment on the same line as the instruction, after the virtual address,
if it is present.
@subsection @samp{-p, --prefix}
If an opcode prefix is used for this instruction, then this option will
cause Decomp to print a comment on the line before the instruction
explaining what the prefix does.
@node Run-time parameters
@section Run-time parameters
A few run-time parameters may be adjusted to assist Decomp in
decompilation.
@subsection @samp{-q, --queue-size <size>}
During the disassembly process, Decomp buffers lines which are to be
output. This option controlls the size of the FIFO where they are
held. The reason for this buffering is that when a local label is
assembled, it is not preserved in the object file. When jump and call
instructions are disassembled, a temporary label is created for them and
inserted into the symbol table. However, if a jump or a call is to an
earlier location in the object file, the label will never be output,
causing a dangling jump. To prevent (or at least reduce) this, the
output is buffered and the line with any label is output after
@samp{<size>} lines have been disassembled. The default value is 512.
This value should work under most circumstances. However, if a label is
found to be missing, then this value should be increased until the label
is output. The disadvantage of using a large value is that more memory
is needed to hold the instruction and its corresponding data structure.
@node Extending
@chapter Extending
It is entirely possible to add computer architectures and object file
formats. However, at the moment it is about as easy as pulling teeth
(your own). Hopefully soon, this will be alleviated.
@menu
* Adding Machines:: Adding new machine types to Decomp.
* Adding File Formats:: Adding new executable file formats to Decomp.
@end menu
@node Adding Machines
@section Adding Machines
To add a machine type, create a separate file with the following function:
@code{MACHINE_get_next_instruction (unsigned char *text,
unsigned long *pos,
unsigned long flags,
unsigned long vaddr,
hash_t symtab)}
@node Adding File Formats
@section Adding File Formats
To add a file format, create a separate file with the following function:
@node Copying
@chapter Copying
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@node Concept Index
@unnumbered Concept Index
@printindex cp
@contents
@bye
