upx - compress or expand executable files
upx [ command ] [ options ] filename...
The Ultimate Packer for eXecutables
Copyright (c) 1996-2004 Markus Oberhumer & Laszlo Molnar
http://upx.sourceforge.net
UPX is a portable, extendable, high-performance executable packer for several different executable formats. It achieves an excellent compression ratio and offers *very* fast decompression. Your executables suffer no memory overhead or other drawbacks for most of the formats supported.
While you may use UPX freely for both non-commercial and commercial executables (for details see the file LICENSE), we would highly appreciate if you credit UPX and ourselves in the documentation, possibly including a reference to the UPX home page. Thanks.
[ Using UPX in non-OpenSource applications without proper credits is considered not politically correct ;-) ]
UPX comes with ABSOLUTELY NO WARRANTY; for details see the file LICENSE.
Having said that, we think that UPX is quite stable now. Indeed we have compressed lots of files without any problems. Also, the current version has undergone several months of beta testing - actually it's more than 4 years since our first public beta back in 1998.
This is the first production quality release, and we plan that future 1.xx releases will be backward compatible with this version.
Please report all problems or suggestions to the authors. Thanks.
UPX is a versatile executable packer with the following features:
- excellent compression ratio: compresses better than zip/gzip,
use UPX to decrease the size of your distribution !
- very fast decompression: about 10 MB/sec even on my old Pentium 133
- no memory overhead for your compressed executables for most of the
supported formats
- safe: you can list, test and unpack your executables
Also, a checksum of both the compressed and uncompressed file is
maintained internally.
- universal: UPX can pack a number of executable formats:
* atari/tos
* dos/exe
* dos/sys
* dos/com
* djgpp2/coff
* linux/386
* rtm32/pe
* tmt/adam
* watcom/le (supporting DOS4G, PMODE/W, DOS32a and CauseWay)
* win32/pe
- portable: UPX is written in portable endian-neutral C++
- extendable: because of the class layout it's very easy to support
new executable formats or add new compression algorithms
- free: UPX can be distributed and used freely. And from version 0.99
the full source code of UPX is released under the GNU General Public
License (GPL) !
You probably understand now why we call UPX the ``ultimate'' executable packer.
This is the default operation, eg. upx yourfile.exe will compress the file specified on the command line.
All UPX supported file formats can be unpacked using the -d switch, eg. upx -d yourfile.exe will uncompress the file you've just compressed.
The -t command tests the integrity of the compressed and uncompressed data, eg. upx -t yourfile.exe check whether your file can be safely decompressed. Note, that this command doesn't check the whole file, only the part that will be uncompressed during program execution. This means that you should not use this command instead of a virus checker.
The -l command prints out some information about the compressed files specified on the command line as parameters, eg upx -l yourfile.exe shows the compressed / uncompressed size and the compression ratio of yourfile.exe.
-q: be quiet, suppress warnings
-q -q (or -qq): be very quiet, suppress errors
-q -q -q (or -qqq): produce no output at all
--help: prints the help
--version: print the version of UPX
[ ...to be written... - type `upx --help' for now ]
UPX offers ten different compression levels from -1 to -9, and --best. The default compression level is -8 for files smaller than 512 kB, and -7 otherwise.
Note that compression level --best can be somewhat slow for large files, but you definitely should use it when releasing a final version of your program.
Since UPX 0.70 there is also an extra compression level --best which squeezes out even some more compression ratio. While it is usually fine to use this option with your favorite .com file it may take a long time to compress a multi-megabyte program. You have been warned.
Quick start for achieving the best compression ratio:
Try upx --best --crp-ms=999999 --nrv2b -o result_2b.exe and upx --best --crp-ms=999999 --nrv2d -o result_2d.exe, and keep the smaller of the two resulting executables.
Details for achieving the best compression ratio:
NUMBER must be a decimal value from 10000 to 999999, inclusive. The default value is 10000 (ten thousand).
Manually compress the program multiple times, thereby trying all available compression methods and keep the best version. Currently available compression methods options are --nrv2b and --nrv2d. Try if --overlay=strip works. For win32/pe programs there's --strip-relocs=0. See notes below.
UPX handles overlays like many other executable packers do: it simply copies the overlay after the compressed image. This works with some files, but doesn't work with others.
Since version 0.90 UPX defaults to --overlay=copy for all executable formats.
--overlay=copy Copy any extra data attached to the file. [DEFAULT]
--overlay=strip Strip any overlay from the program instead of
copying it. Be warned, this may make the compressed
program crash or otherwise unusable.
--overlay=skip Refuse to compress any program which has an overlay.
The environment variable UPX can hold a set of default options for UPX. These options are interpreted first and can be overwritten by explicit command line parameters. For example:
for DOS/Windows: set UPX=-9 --compress-icons#0
for sh/ksh/zsh: UPX="-9 --compress-icons=0"; export UPX
for csh/tcsh: setenv UPX "-9 --compress-icons=0"
Under DOS/Windows you must use '#' instead of '=' when setting the environment variable because of a COMMAND.COM limitation.
Not all of the options are valid in the environment variable - UPX will tell you.
You can use the --no-env option to turn this support off.
This is the executable format used by the Atari ST/TT, a 68000 based personal computer which was popular in the late '80s. See http://www.freemint.de for more info.
Packed programs will be byte-identical to the original after uncompression. All debug information will be stripped, though.
Extra options available for this executable format:
(none)
Obviously UPX won't work with executables that want to read data from themselves (like some commandline utilities that ship with Win95/98/ME).
Compressed programs only work on a 286+.
Packed programs will be byte-identical to the original after uncompression.
Maximum uncompressed size: ~65100 bytes.
Extra options available for this executable format:
--8086 Create an executable that works on any 8086 CPU.
dos/exe stands for all ``normal'' 16-bit DOS executables.
Obviously UPX won't work with executables that want to read data from themselves (like some command line utilities that ship with Win95/98/ME).
Compressed programs only work on a 286+.
Extra options available for this executable format:
--8086 Create an executable that works on any 8086 CPU.
--no-reloc Use no relocation records in the exe header.
You can only compress plain sys files, sys/exe (two in one) combos are not supported.
Compressed programs only work on a 286+.
Packed programs will be byte-identical to the original after uncompression.
Maximum uncompressed size: ~65350 bytes.
Extra options available for this executable format:
--8086 Create an executable that works on any 8086 CPU.
First of all, it is recommended to use UPX *instead* of strip. strip has the very bad habit of replacing your stub with its own (outdated) version. Additionally UPX corrects a bug/feature in strip v2.8.x: it will fix the 4 KByte aligment of the stub.
UPX includes the full functionality of stubify. This means it will automatically stubify your COFF files. Use the option --coff to disable this functionality (see below).
UPX automatically handles Allegro packfiles.
The DLM format (a rather exotic shared library extension) is not supported.
Packed programs will be byte-identical to the original after uncompression. All debug information and trailing garbage will be stripped, though.
Extra options available for this executable format:
--coff Produce COFF output instead of EXE. By default
UPX keeps your current stub.
How it works:
Because Linux is a real operating system, the in-place in-memory decompression scheme used in the other executable formats doesn't work here.
Instead we must use temporary decompression to disk. Interestingly - because of the good memory management of the Linux kernel - this often does not introduce a noticable delay, and in fact there will be no disk access at all if you have enough free memory as the entire process takes places within the filesystem buffers.
A compressed executable consists of the UPX stub and an overlay which contains the original program in a compressed form.
The UPX stub is a statically linked ELF executable and does the following at program startup:
1) decompress the overlay to a temporary location in /tmp
2) open the temporary file for reading
3) try to delete the temporary file and start (execve)
the uncompressed program in /tmp using /proc/<pid>/fd/X as
attained by step 2)
4) if that fails, fork off a subprocess to clean up and
start the program in /tmp in the meantime
The UPX stub is about 1700 bytes long, partly written in assembler and only uses kernel syscalls. It is not linked against any libc.
Benefits:
- UPX can compress all executables, be it AOUT, ELF, libc4, libc5,
libc6, Shell/Perl/Python/... scripts, standalone Java .class
binaries, or whatever...
All scripts and programs will work just as before.
- Compressed programs are completely self-contained. No need for
any external program.
- UPX keeps your original program untouched. This means that
after decompression you will have a byte-identical version,
and you can use UPX as a file compressor just like gzip.
[ Note that UPX maintains a checksum of the file internally,
so it is indeed a reliable alternative. ]
- As the stub only uses syscalls and isn't linked against libc it
should run under any Linux configuration that can run ELF
binaries and has working /proc support.
- For the same reason compressed executables should run under
FreeBSD and other systems which can run Linux binaries.
[ Please send feedback on this topic ]
Drawbacks:
- You need additional free disk space for the uncompressed program
in your /tmp directory. This program is deleted immediately after
decompression, but you still need it for the full execution time
of the program.
- You must have /proc filesystem support as the stub wants to open
/proc/<pid>/exe and needs /proc/<pid>/fd/X. This also means that you
cannot compress programs that are used during the boot sequence
before /proc is mounted.
- `ldd' and `size' won't show anything useful because all they
see is the statically linked stub (since version 0.82 the section
headers are stripped from the UPX stub and `size' doesn't even
recognize the file format any longer - looks like a binutils bug).
- Utilities like `top' will display numerical values in the process
name field. This is because Linux computes the process name from
the first argument of the last execve syscall (which is typically
something like /proc/<pid>/fd/3).
- To reduce memory requirements during uncompression UPX splits the
original file into blocks, so the compression ratio is a little bit
worse than with the other executable formats (but still quite nice).
[ Advise from kernel experts which can tell me more about the
execve memory semantics is welcome. Maybe this shortcoming
could be removed. ]
- Because of temporary decompression to disk the decompression speed
is not as fast as with the other executable formats. Still, I can see
no noticable delay when starting programs like my ~3 MB emacs (which
is less than 1 MB when compressed :-).
Notes:
- As UPX leaves your original program untouched it is advantageous
to strip it before compression.
- It is not advisable to compress programs which usually have many
instances running (like `make') because the common segments of
compressed programs won't be shared any longer between different
processes.
- If you compress a script you will lose platform independence -
this could be a problem if you are using NFS mounted disks.
- Compression of suid, guid and sticky-bit programs is rejected
because of possible security implications.
- For the same reason there is no sense in making any compressed
program suid.
- Obviously UPX won't work with executables that want to read data
from themselves. E.g., this might be a problem for Perl scripts
which access their __DATA__ lines.
- In case of internal errors the stub will abort with exitcode 127.
Typical reasons for this to happen are that the program has somehow
been modified after compression, you have run out of disk space
or your /proc filesystem is not yet mounted.
Running `strace -o strace.log compressed_exe' will tell you more.
Extra options available for this executable format:
(none)
Same as win32/pe.
This format is used by the TMT Pascal compiler - see http://www.tmt.com/ .
Extra options available for this executable format:
(none)
UPX has been successfully tested with the following extenders: DOS4G, DOS4GW, PMODE/W, DOS32a, CauseWay. The WDOS/X extender is partly supported (for details see the file bugs BUGS).
DLLs and the LX format are not supported.
Extra options available for this executable format:
--le Produce an unbound LE output instead of
keeping the current stub.
The PE support in UPX is quite stable now, but definitely there are still some incompabilities with some files.
Because of the way UPX (and other packers for this format) works, you can see increased memory usage of your compressed files. If you start several instances of huge compressed programs you're wasting memory because the common segements of the program won't get shared across the instances. On the other hand if you're compressing only smaller programs, or running only one instance of larger programs, then this penalty is smaller, but it's still there.
If you're running executables from network, then compressed programs will load faster, and require less bandwidth during execution.
DLLs are supported.
Screensavers are supported, with the restriction that the filename must end with ``.scr'' (as screensavers are handled slightly different than normal exe files).
Extra options available for this executable format:
--compress-exports=0 Don't compress the export section.
Use this if you plan to run the compressed
program under Wine.
--compress-exports=1 Compress the export section. [DEFAULT]
Compression of the export section can improve the
compression ratio quite a bit but may not work
with all programs (like winword.exe).
UPX never compresses the export section of a DLL
regardless of this option.
--compress-icons=0 Don't compress any icons.
--compress-icons=1 Compress all but the first icon.
--compress-icons=2 Compress all icons which are not in the
first icon directory. [DEFAULT]
--compress-resources=0 Don't compress any resources at all.
--force Force compression even when there is an
unexpected value in a header field.
Use with care.
--strip-relocs=0 Don't strip relocation records.
--strip-relocs=1 Strip relocation records. [DEFAULT]
This option only works on executables with base
address greater or equal to 0x400000. Usually the
compressed files becomes smaller, but some files
may become larger. Note that the resulting file will
not work under Win32s.
UPX never strips relocations from a DLL
regardless of this option.
Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
UPX's diagnostics are intended to be self-explanatory.
Please report all bugs immediately to the authors.
Markus F.X.J. Oberhumer <markus@oberhumer.com> http://www.oberhumer.com
Laszlo Molnar <ml1050@users.sourceforge.net>
Copyright (C) 1996-2004 Markus Franz Xaver Johannes Oberhumer
Copyright (C) 1996-2004 Laszlo Molnar
This program may be used freely, and you are welcome to redistribute it under certain conditions.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the UPX License Agreement for more details.
You should have received a copy of the UPX License Agreement along with this program; see the file LICENSE. If not, visit the UPX home page.