Create README.md

Rough README

README.md

@@ -0,0 +1,24 @@
+# userlandexec
+## userland exec for Linux x86_64
+
+This code emulates an `exec()` system call. That is, it reads and ELF format file,
+and loads it into memory at the right place, and then starts the newly-loaded
+executable to running.
+
+All this is usually done by the Linux kernel, so some bizarre things go on.
+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.
+
+### Building
+
+Run `make` - that should compile `example` and `ulexec.so`. Once you've
+done that, you can try it out: 
+
+`./example  ./ulexec.so  /usr/bin/cat /proc/self/maps`
+
+### Fun
+
+`./example  ./ulexec.so  `./example  ./ulexec.so  `./example  ./ulexec.so  /usr/bin/cat /proc/self/maps`
+
+Yes! You can have it overlay itself with another copy of itself.