I did try hooking __libc_start_main, but had difficulty using dl.Sym() with it. I wasn't sure if I converted the function signature to Go incorrectly or if it's a limitation of dl.Sym().
This was the error I was hitting: https://github.com/rainycape/dl/blob/484344929c1867aec9517b5...
So I chose a method with a simpler function signature I found using ltrace, just to get it working as an experiment.
In C++, constructors on static global objects get run automatically (via this mechanism), so if Go has something similar, that would work. (Stable) Rust goes out of its way to make you not able to do this because it's an anti-pattern, but there's a stupid trick if you can assume GNU: https://github.com/geofft/redhook/blob/master/src/lib.rs#L34...
Alternatively, you can just write an __attribute__((constructor)) function in C that calls an exported Go function that in turn calls `go backdoor()`, and link them all together.