This code emulates an exec() system call. That is, it reads an ELF format file, and loads it into memory at the correct address. It 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.

This code works with 64-bit Linux ELF files, compiled with GCC and linked against glibc. Other C runtimes (Musl libc, for example) cause errors mysterious to me.

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

./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.