Decompressing ELF Directly to Memory on Linux/x86
        Copyright (C) 2000-2017 John F. Reiser  jreiser@BitWagon.com

References:
  <elf.h>   definitions for the ELF file format
  /usr/src/linux/fs/binfmt_elf.c   what Linux execve() does with ELF
  objdump --private-headers a.elf  dump the Elf32_Phdr
  http://www.cygnus.com/pubs/gnupro/5_ut/b_Usingld/ldLinker_scripts.html
     how to construct unusual ELF using /bin/ld

There is exactly one immovable object:  In all of the Linux kernel,
only the execve() system call sets the initial value of "the brk(0)",
the value that is manipulated by system call 45 (__NR_brk in
/usr/include/asm/unistd.h).  For "direct to memory" decompression,
there will be no execve() except for the execve() of the decompressor
program itself.  So, the decompressor program (which contains the
compressed version of the original executable) must have the same
brk() as the original executable.  So, the second PT_LOAD
ELF "segment" of the compressed program is used only to set the brk(0).
See src/p_lx_elf.cpp, function PackLinuxElf32::generateElfHdr.
All of the decompressor's code, and all of the compressed image
of the original executable, reside in the first PT_LOAD of the
decompressor program.

The decompressor program stub is just under 2K bytes when linked.
After linking, the decompressor code is converted to an initialized
array, and #included into the compilation of the compressor;
see src/stub/l_le_n2b.h.  To make self-contained compressed
executables even smaller, the compressor also compresses all but the
startup and decompression subroutine of the decompressor itself,
saving a few hundred bytes.  The startup code first decompresses the
rest of the decompressor, then jumps to it.  A nonstandard linker
script src/stub/l_lx_elf86.lds places both the .text and .data
of the decompressor into the same PT_LOAD at 0x00401000.  The
compressor includes the compressed bytes of the original executable
at the end of this first PT_LOAD.

At runtime, the decompressed stub lives at 0x00400000.  In order for the
decompressed stub to work properly at an address that is different
from its link-time address, the compiled code must contain no absolute
addresses.  So, the data items in l_lx_elf.c must be only parameters
and automatic (on-stack) local variables; no global data, no static data,
and no string constants.  Use "size l_le_n2b.o l_6e_n2b.o" to check
that both data and bss have length zero.  Also, the '&' operator
may not be used to take the address of a function.

The address  0x00400000 was chosen to be out of the way of the usual
load address 0x08048000, and to minimize fragmentation in kernel
page tables; one page of page tables covers 4 MiB. The address
0x00401000 was chosen as 1 page up from a 64 KiB boundary, to
make the startup code and its constants smaller.

Decompression of the executable begins by decompressing the Elf32_Ehdr
and Elf32_Phdr, and then uses the Ehdr and Phdrs to control decompression
of the PT_LOAD segments.
Subroutine do_xmap() of src/stub/l_lx_elf.c performs the
"virtual execve()" using the compressed data as source, and stores
the decompressed bytes directly into the appropriate virtual addresses.

Before transferring control to the PT_INTERP "program interpreter",
minor tricks are required to setup the Elf32_auxv_t entries,
clear the free portion of the stack (to compensate for ld-linux.so.2
assuming that its automatic stack variables are initialized to zero),
and remove (all but 4 bytes of) the decompression program (and
compressed executable) from the address space.

As of upx-3.05, by default on Linux, upon decompression then one page
of the compressed executable remains mapped into the address space
of the process.  If all of the pages of the compressed executable are
unmapped, then the Linux kernel erases the symlink /proc/self/exe,
and this can cause trouble for the runtime shared library loader
expanding $ORIGIN in -rpath, or for application code that relies on
/proc/self/exe.  Use the compress-time command-line option
--unmap-all-pages to achieve that effect at run time.  Upx-3.04
and previous versions did this by default with no option.  However,
too much other software erroneously assumes that /proc/self/exe
always exists.

On arm*-linux-elf there is no good address at which to retain one
page of the compressed executable.  Pages below the usual .p_vaddr
0x8000 (32KiB) are rejected by the kernel.  Using a page above the
original uncompressed brk(0) would require placing the entire initial
compressed program above uncompressed brk(0), which would significantly
increase the running brk(0); but too many programs break if brk(0)
moves.  Thus on arm*-linux-elf the compressed executable begins
with 0x8000==.p_vaddr, all pages mapped by execve() that are also
occupied by decompressed bytes are removed before overwriting, and
/proc/self/exe becomes a "(deleted)" symlink.  It might be possible
to preserve /proc/self/exe if the original uncompressed executable
were created with 0x9000==.p_vaddr (one page higher than the usual
0x8000) so that the compressed page mapped at 0x8000 would linger.
[This has not been tested.]

Linux stores the pathname argument that was specified to execve()
immediately after the '\0' which terminates the character string of the
last environment variable [as of execve()].  This is true for at least
all Linux 2.6, 2.4, and 2.2 kernels.  Linux kernel 2.6.29 and later
records a pointer to that character string in Elf32_auxv[AT_EXECFN].
The pathname is not "bound" to the file as strongly as /proc/self/exe
(the file may be changed without affecting the pathname), but the
pathname does provide some information.  The pathname may be relative
to the working directory, so look before any chdir().

The Elf formats for Linux add an environment variable named "   " [three
spaces] which saves the results of readlink("/proc/self/exe",,) before
the runtime stub unmaps all its pages.  As of 2006-10-03 this works
for linux/elf386 and linux/ElfAMD.