Add some documentation, some raionale.

README.md

@@ -1,4 +1,4 @@
-# userlandexec
+# userlandexec-arm
 ## userland exec for Linux ARMv7
 
 This code emulates an `exec()` system call. That is, it reads an ELF format file,
@@ -10,8 +10,16 @@ For starters, the userland exec unmaps the currently-executing ELF file,
 so as to be able to put the new ELF file's contents in the right place
 in memory.
 
+I was inspired to do this by The Grugq. I did start with his [userland exec](https://github.com/grugq/grugq.github.com/blob/master/docs/ul_exec.txt) to do the x86_64 version.
+
 This code works with 32-bit Rasberry Pi 2 Linux ELF files, compiled with
-GCC and linked against glibc.
+GCC and linked against glibc. Specifically, I used Rasberry Pi Arch Linux,
+kernel 4.4.41, GCC 6.2.1, GNU binutils 2.27.
+
+I started out with my own [userland exec for x86_64](https://github.com/bediger4000/userlandexec),
+v1.0 and modified for ARM. I found it easier to do assembly language `crt.o` and system calls
+in ARM assembly, rather than do the `asm()` GCC directives I used for x86_64 `userlandexec`.
+A better programmer than I could have just interwoven ARM and x86_64 into the same project.
 
 ### Building
 
@@ -30,6 +38,27 @@ Show the ELF auxiliary vectors:
 The first program uses glibc extension `getauxval(3)`, and has the dynamic linker
 dump out the auxiliary vectors, for your inspection.
 
+This project has a corresponding completely statically-linked program, `elfauxv`. It's easier
+to do a userland exec for statically linked programs.
+
+Try out `open(2)`, setting `errno` and `close(2)`:
+
+    $ make oc
+    $ ./oc somegibberish
+
+It's easy to get `open(2)` to fail in a variety of ways by trying to open non-existent
+files, removing read permissions, etc. The actual userland exec code has to be position
+independent, so setting `errno` in ARM assembly became harder.
+
+Find the environment variables:
+
+    $ make env_test
+    $ ./env_test
+
+The userland exec creates copies of `argv` and `envp`. This just prints addresses
+and environment variables to ensure that the "C run time" code in `libstatic/crt.s`
+does `argc`, `argv` and `envp` correctly.
+
 ### Fun
 
 `./example  ./ulexec.so  `./example  ./ulexec.so  `./example  ./ulexec.so  /usr/bin/cat /proc/self/maps`