sendmsg fails with EINVAL when sending ScmRights over unconnected socket

[swsnr]

I can send a file descriptor over a connected unix socket just fine with the following code:

let target = UnixDatagram::unbound().unwrap();
let fd = write_and_seal("rustix_send_fd_connected_socket");

target.connect(JOURNALD_PATH).unwrap();
let fds = &[fd.as_fd()];
let scm_rights = rustix::net::SendAncillaryMessage::ScmRights(fds);
let mut buffer = vec![0; scm_rights.size()];
let mut buffer = SendAncillaryBuffer::new(&mut buffer);
assert!(buffer.push(scm_rights), "Failed to push ScmRights message");
sendmsg(&target, &[], &mut buffer, SendFlags::NOSIGNAL).unwrap();

I can also send an FD over an unconnected socket with libc, see https://github.com/swsnr/rustix-sendfd/blob/86540a6d37ee8670c77f467ddde3d7430d22ccb8/src/lib.rs#L74 (too long to paste here).

However, doing that in rustix with the following code fails with called Result::unwrap() on an Err value: Os { code: 22, kind: InvalidInput, message: "Invalid argument" } at sendmsg_unix:

let target = UnixDatagram::unbound().unwrap();
let fd = write_and_seal("rustix_send_fd_unconnected_socket");

let fds = &[fd.as_fd()];
let scm_rights = rustix::net::SendAncillaryMessage::ScmRights(fds);
let mut buffer = vec![0; scm_rights.size()];
let mut buffer = SendAncillaryBuffer::new(&mut buffer);
assert!(buffer.push(scm_rights), "Failed to push ScmRights message");
let sockaddr = SocketAddrUnix::new(JOURNALD_PATH).unwrap();
sendmsg_unix(&target, &sockaddr, &[], &mut buffer, SendFlags::NOSIGNAL).unwrap();

What did I do wrong here?

See https://github.com/swsnr/rustix-sendfd for a repo with three tests which reproduce this, and the corresponding workflow for the actual failure at https://github.com/swsnr/rustix-sendfd/actions/runs/6572703065/job/17854300669#step:4:33

[sunfishcode]

Thanks for the report! This is a rustix bug; I've now filed #885 with a fix.

[swsnr]

Thank you very much 🙏

[sunfishcode]

This is now released in rustix 0.38.20.