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[John Reiser]

On 05/22/2013 12:04 PM, Clemens Eisserer wrote:
> As UPX requires executeables to be decompressed during startup (and
> furthermore does not support ELF shared libraries), 

An executable main program which has been compressed by UPX
*can* use ELF shared libraries, and nearly all programs on Linux
that are compressed by UPX *do* use shared libraries.
The execution-time difference between a compressed and a never-
compressed program is that the Linux kernel loads the PT_INTERP for
execve() of a regular program, while the UPX runtime stub loads the PT_INTERP
for a compressed program.  With respect to byte values, there is essentially
no difference in the memory image of the process.

On a few platforms (including Linux i386 and x86_64) UPX also can
compress a shared library itself.  You don't get any actual sharing
of execution-time pages, but storage space in the filesystem is less.
Compression is not as tight as with a main program, because in a shared
library only instructions can be compressed: not symbol tables, not .data,
etc, because these are needed by ld-linux to perform relocation
which happens before UPX can gain control (which it does via DT_INIT.)

> its not a sane
> option for a distribution.

It depends on the desired goals.  On distribution media, .rpm packages
are compressed using xz (lzma2), and the space saved by xz compression
often is similar to that saved by UPX.  Once installed, the UPX compression
saves storage space in the filesystem at the cost of no sharing:
not among multiple simultaneous processes, nor demand paged between
the filesystem and process memory space.


[I am the maintainer of UPX for ELF.]

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