upx/doc/loader.txt

This documentation is written for those brave souls who want to
understand and/or modify the UPX assembly stubs - the small snippets
that do the runtime decompression when a compressed program is started.

If you look at the C++ source files, you can find code fragments like
this:

    addLoader("PEMAIN20",
              ih.entry ? "PEDOJUMP" : "PERETURN",
              "IDENTSTR""UPX1HEAD",
              NULL
             );

    linker->defineSymbol("original_entry", ih.entry);

and in the assembly files fragments like this:

    section         PEISDLL1
                    cmpb    [esp + 8], 1
                    jnz     reloc_end_jmp

    section         PEMAIN21
    reloc_end_jmp:

    section         PERETURN
                    xor     eax, eax
                    inc     eax
                    ret     0x0C
    section         PEDOJUMP
                    jmp    original_entry

Everything works as you would expect. If you want to add the code
fragment which is in `section PERETURN' to the runtime stub, then
simply use `addLoader("PERETURN")' in the C++ source.

That's nice, you could say, but how cross section jumps and calls are
handled? Well, that is the nicest part of this stuff - they are handled
automatically. All you have to do is to add the required sections to the
loader using `addLoader()' and the rest is done by upx. It will resolve
every conditional or unconditional jumps or subroutine calls for you.

You can also use (undefined) symbols in the assembly for values that
can only be computed during compression time (like `original_entry').
These symbols can be defined later in C++ using

        linker->defineSymbol("xx", yy)

This functionality (we could say it's a simple linker) is achieved by
compiling the assembly into an ELF object file which a little C++
module (src/linker.cpp) can interpret and work with.