Ability to add custom socket options

[code-ape]

I was looking to do the following code except using nix instead of libc:

use libc::{c_ushort, c_int, c_void, socklen_t, SOL_SOCKET, setsockopt};

const SO_ATTACH_FILTER: c_int = 26;

pub fn attach_filter(fd: RawFd, prog: Prog) -> Result<(), Error> {
    match unsafe {
              setsockopt(fd as c_int,
                         SOL_SOCKET,
                         SO_ATTACH_FILTER,
                         &prog as *const _ as *const c_void,
                         size_of_val(&prog) as socklen_t)
          } {
        0 => Ok(()),
        _ => Err(Error::last_os_error()),
    }
}

However nix currently doesn't support this, which is fine. However, nix doesn't expose an easy way to use it's internal macros to construct the SO_ATTACH_FILTER command. I wanted to file an issue to suggest exposing this. I'll also look into contributing my use case to nix so it's added t the library for future users.

Code source: https://github.com/jedisct1/rust-bpf/blob/0122d0a95e6012abab0475b14cef20dbfa481704/src/bpf_linux.rs#L58

[Susurrus]

I don't quite understand the issue here as I'm not familiar with setsockopt. Do you have a proposal for how we might integrate this code example that would fit in with the rest of nix? Would exposing sockopt_impl! be enough for you?

[mexus]

Sorry for bringing up an old issue, but it happened so I need the same feature :)

Basically the issue is that the macro sockopt_impl! is not exposed, which will help a lot.
But it's not the only thing.

For example, for SO_ATTACH_FILTER we need to pass an object of a special type, which obviously does not implement Set. But the Set trait is not exposed as well :)

So I guess exposing sockopt_impl! macro and Set and Get traits will help a lot.

[Susurrus]

Is there a reason we can't provide a sockopt_impl! within nix that exposes SO_ATTACH_FILTER? This has been the need for both you and @code-ape, so I'd suggest we start going down this route. We'd welcome a PR for it.

If we were to expose sockopt_impl! and those other traits, we'd need them all documented much more extensively. If you'd like to help with this effort, we welcome all PRs!

[mexus]

That's great, thanks! 👍

I'll try to make it happen by the beginning of next week.

[mexus]

(I went for the second option)