shook

chook (short for Child Hook) is a Rust crate that allows you to register a fn<ArgT: Deserialize, RetT: Serialize>(arg: ArgT) -> RetT an inject it into a child process via LD_PRELOAD.

But why tho?

This allows you to run code in the address space of your child process, which lets you do some things that you can't do otherwise. If you don't control the implementation of the child process, this can let you do such things despite not being able to put any custom code directly into the child process.

In particular, this is used by shpool to update environment variables in child shells.

Under the hood

Your hook routine will get packaged up in a .so file with an _init routine that listens for a few magic env vars that tell it

1. Where to listen on a unix domain socket for incoming calls
2. The PID that ought to be allowed to make calls (this is restricted for security purposes).

It will then create a unix domain socket and start listening for RPC calls, which it will then use to call your registered hook.

When you create a subprocess, you'll be able to register a chook with the process before launch, and subsequently place calls to the hook via the same handle that you used for registration.