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.