[v15] Add the fdpass-teleport utility (#42726)

* Add the fdpass-teleport utility

* license and comments

---------

Co-authored-by: Edoardo Spadolini <edoardo.spadolini@goteleport.com>

tool/fdpass-teleport/.gitattributes

@@ -0,0 +1 @@
+/Cargo.lock -merge

tool/fdpass-teleport/.gitignore

@@ -0,0 +1 @@
+/target

tool/fdpass-teleport/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "fdpass-teleport"
+version = "0.1.0"
+edition = "2021"
+
+[workspace]
+
+[dependencies]
+nix = { version = "0.29.0", features = ["socket", "uio", "fs"] }
+simple-eyre = "0.3.1"
+
+[profile.release]
+codegen-units = 1
+lto = "fat"
+panic = "abort"
+strip = "symbols"

tool/fdpass-teleport/src/main.rs

@@ -0,0 +1,109 @@
+// Teleport
+// Copyright (C) 2024 Gravitational, Inc.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+use nix::{
+    errno::Errno,
+    fcntl::{self, OFlag},
+    libc,
+    sys::socket::{self, ControlMessage, MsgFlags},
+};
+use simple_eyre::eyre::{self, OptionExt, Result, WrapErr};
+use std::{
+    env,
+    io::{IoSlice, Write},
+    os::{
+        fd::AsRawFd,
+        unix::{ffi::OsStrExt, net::UnixStream},
+    },
+    process,
+};
+
+fn main() -> Result<()> {
+    simple_eyre::install()?;
+
+    let mut args = env::args_os();
+
+    if args.len() != 3 {
+        let pkg_name = env!("CARGO_PKG_NAME");
+        let version = option_env!("VERSION").unwrap_or("unknown");
+
+        let argv0 = args.next();
+        let argv0 = match &argv0 {
+            Some(o) => o.to_string_lossy(),
+            None => pkg_name.into(),
+        };
+
+        eprint!(
+            "{pkg_name} (version {version})\n\nusage: {argv0} <path to mux socket> <connection target>\n",
+        );
+        process::exit(libc::EXIT_FAILURE);
+    }
+
+    let mux_path = args.nth(1).ok_or_eyre("missing mux path")?;
+    let mut target = args.next().ok_or_eyre("missing connection target")?;
+    target.push("\0");
+
+    // in OpenSSH ProxyCommand+ProxyUseFdPass the program is executed with a
+    // unix domain socket as stdout, which we can check with a getsockname();
+    // we'll get a ENOTSOCK if stdout is not a socket, or EINVAL if the address
+    // returned by getsockname() is not AF_UNIX (i.e. stdout is a socket but not
+    // a unix domain socket)
+    socket::getsockname::<socket::UnixAddr>(libc::STDOUT_FILENO)
+        .wrap_err("stdout is not a unix socket")?;
+
+    // to not have to bother with poll() for the later sendmsg() we have to
+    // confirm that the socket is set to blocking mode, or we might end up
+    // busylooping with EAGAIN; OpenSSH at the time of writing (9.7) will give
+    // the ProxyCommand a blocking socketpair() half, so we should be good
+    let fl = fcntl::fcntl(libc::STDOUT_FILENO, fcntl::F_GETFL)
+        .wrap_err("could not check stdout for blocking mode")?;
+    let fl = OFlag::from_bits_retain(fl);
+    if fl.contains(OFlag::O_NONBLOCK) {
+        let mut fl = fl;
+        fl.set(OFlag::O_NONBLOCK, false);
+        fcntl::fcntl(libc::STDOUT_FILENO, fcntl::F_SETFL(fl))
+            .wrap_err("could not set stdout to blocking mode")?;
+    }
+
+    let mut mux_conn = UnixStream::connect(mux_path).wrap_err("could not connect to mux")?;
+    // we added a final NUL to target above
+    mux_conn
+        .write_all(target.as_bytes())
+        .wrap_err("could not send connection target to mux")?;
+
+    // we can now pass the connection to OpenSSH (or whoever launched us) over
+    // stdout, sending a byte of actual data together with the connection's file
+    // descriptor; logic lifted from OpenBSD's netcat fdpass code
+    // (https://github.com/openbsd/src/blob/master/usr.bin/nc/netcat.c)
+    loop {
+        match socket::sendmsg::<()>(
+            libc::STDOUT_FILENO,
+            &[IoSlice::new(&[0])],
+            &[ControlMessage::ScmRights(&[mux_conn.as_raw_fd()])],
+            MsgFlags::empty(),
+            None,
+        ) {
+            Ok(1) => break,
+            Ok(s) => eyre::bail!("unexpected sendmsg return value {s}"),
+            Err(Errno::EAGAIN | Errno::EINTR) => continue,
+            Err(e) => Err(e).wrap_err("could not pass connection to stdout")?,
+        };
+    }
+
+    // returning would close and deallocate things, and there's just no need for
+    // that
+    process::exit(libc::EXIT_SUCCESS);
+}