upx - compress or expand executable files
upx [ command ] [ options ] filename...
The Ultimate Packer for eXecutables
Copyright (c) 1996-2024 Markus Oberhumer, Laszlo Molnar & John Reiser
https://upx.github.ioUPX 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, because of in-place decompression.
UPX comes with ABSOLUTELY NO WARRANTY; for details see the file COPYING.
Please report all problems or suggestions to the authors. Thanks.
IMPORTANT NOTE: UPX inherits the security context of any files it handles.
This means that packing, unpacking, or even testing or listing a file requires the same security considerations as actually executing the file.
Use UPX on trusted files only!
UPX is a versatile executable packer with the following features:
- secure: as UPX is documented Open Source since many years any relevant
Security/Antivirus software is able to peek inside UPX compressed
apps to verify them
- excellent compression ratio: typically compresses better than Zip,
use UPX to decrease the size of your distribution !
- very fast decompression: more than 500 MB/sec on any reasonably modern
machine
- no memory overhead for your compressed executables for most of the
supported formats because of in-place decompression
- 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, including
Windows programs and DLLs, macOS apps and Linux executables
- 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 is distributed with full source code under the GNU General
Public License v2+, with special exceptions granting the free usage
for commercial programsYou 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
--exact: when compressing, require to be able to get a byte-identical file after decompression with option -d. [NOTE: this is work in progress and is not supported for all formats yet. If you do care, as a workaround you can compress and then decompress your program a first time - any further compress-decompress steps should then yield byte-identical results as compared to the first decompressed version.]
-k: keep backup files
-o file: write output to file
[ ...more docs need 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 KiB, and -7 otherwise.
Compression levels 1, 2 and 3 are pretty fast.
Compression levels 4, 5 and 6 achieve a good time/ratio performance.
Compression levels 7, 8 and 9 favor compression ratio over speed.
Compression level --best may take a long time.
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.
Quick info for achieving the best compression ratio:
Try upx --brute --no-lzma myfile.exe or even upx --ultra-brute --no-lzma myfile.exe.
The option --lzma enables LZMA compression, which compresses better but is *significantly slower* at decompression. You probably do not want to use it for large files.
(Note that --lzma is automatically enabled by --all-methods and --brute, use --no-lzma to override.)
Try if --overlay=strip works.
For win32/pe programs there's --strip-relocs=0. See notes below.
Info: An "overlay" means auxiliary data attached after the logical end of an executable, and it often contains application specific data (this is a common practice to avoid an extra data file, though it would be better to use resource sections).
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, depending on how an application actually accesses this overlaid data.
--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 explicitly use the --no-env option to ignore the environment variable.
This is the executable format used by the Atari ST/TT, a Motorola 68000 based personal computer which was popular in the late '80s. Support of this format is only because of nostalgic feelings of one of the authors and serves no practical purpose :-). See https://freemint.github.io 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:
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.Same as vmlinuz/i386.
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.
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.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.
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.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.
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.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 KiB alignment 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.
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.Introduction
Linux/386 support in UPX consists of 3 different executable formats,
one optimized for ELF executables ("linux/elf386"), one optimized
for shell scripts ("linux/sh386"), and one generic format
("linux/386").
We will start with a general discussion first, but please
also read the relevant docs for each of the individual formats.
Also, there is special support for bootable kernels - see the
description of the vmlinuz/386 format.General user's overview
Running a compressed executable program trades less space on a
``permanent'' storage medium (such as a hard disk, floppy disk,
CD-ROM, flash memory, EPROM, etc.) for more space in one or more
``temporary'' storage media (such as RAM, swap space, /tmp, etc.).
Running a compressed executable also requires some additional CPU
cycles to generate the compressed executable in the first place,
and to decompress it at each invocation.
How much space is traded? It depends on the executable, but many
programs save 30% to 50% of permanent disk space. How much CPU
overhead is there? Again, it depends on the executable, but
decompression speed generally is at least many megabytes per second,
and frequently is limited by the speed of the underlying disk
or network I/O.
Depending on the statistics of usage and access, and the relative
speeds of CPU, RAM, swap space, /tmp, and file system storage, then
invoking and running a compressed executable can be faster than
directly running the corresponding uncompressed program.
The operating system might perform fewer expensive I/O operations
to invoke the compressed program. Paging to or from swap space
or /tmp might be faster than paging from the general file system.
``Medium-sized'' programs which access about 1/3 to 1/2 of their
stored program bytes can do particularly well with compression.
Small programs tend not to benefit as much because the absolute
savings is less. Big programs tend not to benefit proportionally
because each invocation may use only a small fraction of the program,
yet UPX decompresses the entire program before invoking it.
But in environments where disk or flash memory storage is limited,
then compression may win anyway.
Currently, executables compressed by UPX do not share RAM at runtime
in the way that executables mapped from a file system do. As a
result, if the same program is run simultaneously by more than one
process, then using the compressed version will require more RAM and/or
swap space. So, shell programs (bash, csh, etc.) and ``make''
might not be good candidates for compression.
UPX recognizes three executable formats for Linux: Linux/elf386,
Linux/sh386, and Linux/386. Linux/386 is the most generic format;
it accommodates any file that can be executed. At runtime, the UPX
decompression stub re-creates in /tmp a copy of the original file,
and then the copy is (re-)executed with the same arguments.
ELF binary executables prefer the Linux/elf386 format by default,
because UPX decompresses them directly into RAM, uses only one
exec, does not use space in /tmp, and does not use /proc.
Shell scripts where the underlying shell accepts a ``-c'' argument
can use the Linux/sh386 format. UPX decompresses the shell script
into low memory, then maps the shell and passes the entire text of the
script as an argument with a leading ``-c''.General 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.
- For the same reason compressed executables should run under
FreeBSD and other systems which can run Linux binaries.
[ Please send feedback on this topic ]General drawbacks:
- It is not advisable to compress programs which usually have many
instances running (like `sh' or `make') because the common segments of
compressed programs won't be shared any longer between different
processes.
- `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. The file patches/patch-elfcode.h has a
patch to fix this bug in `size' and other programs which use GNU BFD.General notes:
- As UPX leaves your original program untouched it is advantageous
to strip it before compression.
- 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.
Running `strace -o strace.log compressed_file' will tell you more.Please read the general Linux description first.
The linux/elf386 format decompresses directly into RAM, uses only one exec, does not use space in /tmp, and does not use /proc.
Linux/elf386 is automatically selected for Linux ELF executables.
Packed programs will be byte-identical to the original after uncompression.
How it works:
For ELF executables, UPX decompresses directly to memory, simulating
the mapping that the operating system kernel uses during exec(),
including the PT_INTERP program interpreter (if any).
The brk() is set by a special PT_LOAD segment in the compressed
executable itself. UPX then wipes the stack clean except for
arguments, environment variables, and Elf_auxv entries (this is
required by bugs in the startup code of /lib/ld-linux.so as of
May 2000), and transfers control to the program interpreter or
the e_entry address of the original executable.
The UPX stub is about 1700 bytes long, partly written in assembler
and only uses kernel syscalls. It is not linked against any libc.Specific drawbacks:
- For linux/elf386 and linux/sh386 formats, you will be relying on
RAM and swap space to hold all of the decompressed program during
the lifetime of the process. If you already use most of your swap
space, then you may run out. A system that is "out of memory"
can become fragile. Many programs do not react gracefully when
malloc() returns 0. With newer Linux kernels, the kernel
may decide to kill some processes to regain memory, and you
may not like the kernel's choice of which to kill. Running
/usr/bin/top is one way to check on the usage of swap space.Extra options available for this executable format:
(none)Please read the general Linux description first.
Shell scripts where the underling shell accepts a ``-c'' argument can use the Linux/sh386 format. UPX decompresses the shell script into low memory, then maps the shell and passes the entire text of the script as an argument with a leading ``-c''. It does not use space in /tmp, and does not use /proc.
Linux/sh386 is automatically selected for shell scripts that use a known shell.
Packed programs will be byte-identical to the original after uncompression.
How it works:
For shell script executables (files beginning with "#!/" or "#! /")
where the shell is known to accept "-c <command>", UPX decompresses
the file into low memory, then maps the shell (and its PT_INTERP),
and passes control to the shell with the entire decompressed file
as the argument after "-c". Known shells are sh, ash, bash, bsh, csh,
ksh, tcsh, pdksh. Restriction: UPX cannot use this method
for shell scripts which use the one optional string argument after
the shell name in the script (example: "#! /bin/sh option3\n".)
The UPX stub is about 1700 bytes long, partly written in assembler
and only uses kernel syscalls. It is not linked against any libc.Specific drawbacks:
- For linux/elf386 and linux/sh386 formats, you will be relying on
RAM and swap space to hold all of the decompressed program during
the lifetime of the process. If you already use most of your swap
space, then you may run out. A system that is "out of memory"
can become fragile. Many programs do not react gracefully when
malloc() returns 0. With newer Linux kernels, the kernel
may decide to kill some processes to regain memory, and you
may not like the kernel's choice of which to kill. Running
/usr/bin/top is one way to check on the usage of swap space.Extra options available for this executable format:
(none)Please read the general Linux description first.
The generic linux/386 format decompresses to /tmp and needs /proc file system support. It starts the decompressed program via the execve() syscall.
Linux/386 is only selected if the specialized linux/elf386 and linux/sh386 won't recognize a file.
Packed programs will be byte-identical to the original after uncompression.
How it works:
For files which are not ELF and not a script for a known "-c" shell,
UPX uses kernel execve(), which first requires decompressing to a
temporary file in the file system. Interestingly -
because of the good memory management of the Linux kernel - this
often does not introduce a noticeable 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 file system 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.Specific 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 file system 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.
- 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).
- Because of temporary decompression to disk the decompression speed
is not as fast as with the other executable formats. Still, I can see
no noticeable delay when starting programs like my ~3 MiB emacs (which
is less than 1 MiB when compressed :-).Extra options available for this executable format:
--force-execve Force the use of the generic linux/386 "execve"
format, i.e. do not try the linux/elf386 and
linux/sh386 formats.This is the executable format used by the Sony PlayStation (PSone), a MIPS R3000 based gaming console which is popular since the late '90s. Support of this format is very similar to the Atari one, because of nostalgic feelings of one of the authors.
Packed programs will be byte-identical to the original after uncompression, until further notice.
Maximum uncompressed size: ~1.89 / ~7.60 MiB.
Notes:
- UPX creates as default a suitable executable for CD-Mastering
and console transfer. For a CD-Master main executable you could also try
the special option "--boot-only" as described below.
It has been reported that upx packed executables are fully compatible with
the Sony PlayStation 2 (PS2, PStwo) and Sony PlayStation Portable (PSP) in
Sony PlayStation (PSone) emulation mode.
- Normally the packed files use the same memory areas like the uncompressed
versions, so they will not override other memory areas while unpacking.
If this isn't possible UPX will abort showing a 'packed data overlap'
error. With the "--force" option UPX will relocate the loading address
for the packed file, but this isn't a real problem if it is a single or
the main executable.Extra options available for this executable format:
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--8-bit Uses 8 bit size compression [default: 32 bit]
--8mib-ram PSone has 8 MiB ram available [default: 2 MiB]
--boot-only This format is for main exes and CD-Mastering only !
It may slightly improve the compression ratio,
decompression routines are faster than default ones.
But it cannot be used for console transfer !
--no-align This option disables CD mode 2 data sector format
alignment. May slightly improves the compression ratio,
but the compressed executable will not boot from a CD.
Use it for console transfer only !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:
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.The vmlinuz/386 and bvmlinuz/386 formats take a gzip-compressed bootable Linux kernel image ("vmlinuz", "zImage", "bzImage"), gzip-decompress it and re-compress it with the UPX compression method.
vmlinuz/386 is completely unrelated to the other Linux executable formats, and it does not share any of their drawbacks.
Notes:
- Be sure that "vmlinuz/386" or "bvmlinuz/386" is displayed
during compression - otherwise a wrong executable format
may have been used, and the kernel won't boot.Benefits:
- Better compression (but note that the kernel was already compressed,
so the improvement is not as large as with other formats).
Still, the bytes saved may be essential for special needs like
boot disks.
For example, this is what I get for my 2.2.16 kernel:
1589708 vmlinux
641073 bzImage [original]
560755 bzImage.upx [compressed by "upx -9"]
- Much faster decompression at kernel boot time (but kernel
decompression speed is not really an issue these days).Drawbacks:
(none)Extra options available for this executable format:
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.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 probably there are still some incompatibilities 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 because the whole program is loaded into memory at startup. If you start several instances of huge compressed programs you're wasting memory because the common segments 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. But UPX compressed DLLs can not share common data and code when they got used by multiple applications. So compressing msvcrt.dll is a waste of memory, but compressing the dll plugins of a particular application may be a better idea.
Screensavers are supported, with the restriction that the filename must end with ".scr" (as screensavers are handled slightly different than normal exe files).
UPX compressed PE files have some minor memory overhead (usually in the 10 - 30 KiB range) which can be seen by specifying the "-i" command line switch during compression.
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-icons=3 Compress all icons.
--compress-resources=0 Don't compress any resources at all.
--keep-resource=list Don't compress resources specified by the list.
The members of the list are separated by commas.
A list member has the following format: I<type[/name]>.
I<Type> is the type of the resource. Standard types
must be specified as decimal numbers, user types can be
specified by decimal IDs or strings. I<Name> is the
identifier of the resource. It can be a decimal number
or a string. For example:
--keep-resource=2/MYBITMAP,5,6/12345
UPX won't compress the named bitmap resource "MYBITMAP",
it leaves every dialog (5) resource uncompressed, and
it won't touch the string table resource with identifier
12345.
--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 Windows 3.x (Win32s).
UPX never strips relocations from a DLL
regardless of this option.
--all-methods Compress the program several times, using all
available compression methods. This may improve
the compression ratio in some cases, but usually
the default method gives the best results anyway.
--all-filters Compress the program several times, using all
available preprocessing filters. This may improve
the compression ratio in some cases, but usually
the default filter gives the best results anyway.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 <ezerotven+github@gmail.com>
John F. Reiser <jreiser@BitWagon.com>Copyright (C) 1996-2024 Markus Franz Xaver Johannes Oberhumer
Copyright (C) 1996-2024 Laszlo Molnar
Copyright (C) 2000-2024 John F. Reiser
UPX is distributed with full source code under the terms of the GNU General Public License v2+; either under the pure GPLv2+ (see the file COPYING), or (at your option) under the GPLv+2 with special exceptions and restrictions granting the free usage for all binaries including commercial programs (see the file LICENSE).
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.
You should have received a copy of the UPX License Agreements along with this program; see the files COPYING and LICENSE. If not, visit the UPX home page.