UPX

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UPX, the Ultimate Packer for eXecutables, is a free and open source executable packer supporting a number of file formats from different operating systems.

[edit] Compression

UPX uses a compression algorithm called UCL, which is an open source implementation of portions of the proprietary NRV - Not Really Vanished - algorithm.

UCL has been designed to be simple enough that a decompressor can be implemented in just a few hundred bytes of code. UCL requires no additional memory to be allocated for decompression, a considerable advantage that means that a UPX packed executable usually requires no additional memory.

UPX (since 2.90 beta) can use LZMA on most platforms; however, this is disabled by default for 16-bit due to slow decompression speed on older computers (use—lzma to force it on).

[edit] Decompression

UPX supports two mechanisms for decompression - an in-place technique and extraction to temporary file.

The in-place technique, which decompresses the executable into memory, is not possible on all supported platforms. The rest use extraction to temporary file. This procedure involves additional overhead and other disadvantages; however, it allows any executable file format to be packed. The executable is extracted to a temporary location, and then open() is used to obtain a file descriptor.

Once a file descriptor is obtained, the temporary file can be unlink()ed, the stub then uses execve() on the file handle (via /proc) to overwrite the stub with the executable image of the temporary file.

The extraction to temporary file method has several disadvantages:

• special permissions are ignored, such as suid.
• argv[0] will not be meaningful.
• applications will be unable to share common segments.

Unmodified UPX packing is often detected and unpacked by anti-virus scanners. UPX also has a built-in feature for unpacking unmodified executables packed with itself. The default license for the existing stubs explicitly forbids modification that prevent manual unpacking / repacking with newer UPX versions.

[edit] Supported formats

• ARM/pe
• atari/tos
• *BSD/i386
• djgpp2/coff
• dos/com
• dos/exe
• dos/sys
• linux/i386 a.out
• linux/ELF on i386, x86-64, ARM, powerpc
• linux/kernel on i386, x86-64 and ARM
• mach-o/ppc32, mach-o/i386
• rtm32/pe
• tmt/adam
• ps1/exe
• watcom/le
• win32/pe (excluding files built using the .NET Framework)

[edit] External links

Free software portal

• UPX: the Ultimate Packer for eXecutables
• UPX - a powerful executable packer at SourceForge.net
• UPX at Freshmeat
• Automatic UPX unpacker