Rewrite entire codebase to split into client and server

src/core/bootstrap.rs

@@ -0,0 +1,177 @@
+use std::{fs, path::PathBuf};
+
+use anyhow::Context;
+use nix::libc::{exit, EXIT_SUCCESS};
+use std::os::unix::net::UnixStream as StdUnixStream;
+use tokio::net::UnixStream as TokioUnixStream;
+
+use crate::{
+    core::{
+        bootstrap::authenticated_unix_socket::client_authenticate,
+        common::{UnixUser, DEFAULT_CONFIG_PATH, DEFAULT_SOCKET_PATH},
+    },
+    server::{config::read_config_form_path, server_loop::handle_requests_for_single_session},
+};
+
+pub mod authenticated_unix_socket;
+
+// TODO: this function is security critical, it should be integration tested
+//       in isolation.
+/// Drop privileges to the real user and group of the process.
+/// If the process is not running with elevated privileges, this function
+/// is a no-op.
+pub fn drop_privs() -> anyhow::Result<()> {
+    log::debug!("Dropping privileges");
+    let real_uid = nix::unistd::getuid();
+    let real_gid = nix::unistd::getgid();
+
+    nix::unistd::setuid(real_uid).context("Failed to drop privileges")?;
+    nix::unistd::setgid(real_gid).context("Failed to drop privileges")?;
+
+    debug_assert_eq!(nix::unistd::getuid(), real_uid);
+    debug_assert_eq!(nix::unistd::getgid(), real_gid);
+
+    log::debug!("Privileges dropped successfully");
+    Ok(())
+}
+
+/// This function is used to bootstrap the connection to the server.
+/// This can happen in two ways:
+/// 1. If a socket path is provided, or exists in the default location,
+///    the function will connect to the socket and authenticate with the
+///    server to ensure that the server knows the uid of the client.
+/// 2. If a config path is provided, or exists in the default location,
+///    and the config is readable, the function will assume it is either
+///    setuid or setgid, and will fork a child process to run the server
+///    with the provided config. The server will exit silently by itself
+///    when it is done, and this function will only return for the client
+///    with the socket for the server.
+/// If neither of these options are available, the function will fail.
+pub fn bootstrap_server_connection_and_drop_privileges(
+    server_socket_path: Option<PathBuf>,
+    config_path: Option<PathBuf>,
+) -> anyhow::Result<StdUnixStream> {
+    if server_socket_path.is_some() && config_path.is_some() {
+        anyhow::bail!("Cannot provide both a socket path and a config path");
+    }
+
+    log::debug!("Starting the server connection bootstrap process");
+
+    let (socket, do_authenticate) = bootstrap_server_connection(server_socket_path, config_path)?;
+
+    drop_privs()?;
+
+    let result: anyhow::Result<StdUnixStream> = if do_authenticate {
+        tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+            .block_on(async {
+                let mut socket = TokioUnixStream::from_std(socket)?;
+                client_authenticate(&mut socket, None).await?;
+                Ok(socket.into_std()?)
+            })
+    } else {
+        Ok(socket)
+    };
+
+    result
+}
+
+/// Inner function for [`bootstrap_server_connection_and_drop_privileges`].
+/// See that function for more information.
+fn bootstrap_server_connection(
+    socket_path: Option<PathBuf>,
+    config_path: Option<PathBuf>,
+) -> anyhow::Result<(StdUnixStream, bool)> {
+    // TODO: ensure this is both readable and writable
+    if let Some(socket_path) = socket_path {
+        log::debug!("Connecting to socket at {:?}", socket_path);
+        return match StdUnixStream::connect(socket_path) {
+            Ok(socket) => Ok((socket, true)),
+            Err(e) => match e.kind() {
+                std::io::ErrorKind::NotFound => Err(anyhow::anyhow!("Socket not found")),
+                std::io::ErrorKind::PermissionDenied => Err(anyhow::anyhow!("Permission denied")),
+                _ => Err(anyhow::anyhow!("Failed to connect to socket: {}", e)),
+            },
+        };
+    }
+    if let Some(config_path) = config_path {
+        // ensure config exists and is readable
+        if fs::metadata(&config_path).is_err() {
+            return Err(anyhow::anyhow!("Config file not found or not readable"));
+        }
+
+        log::debug!("Starting server with config at {:?}", config_path);
+        return invoke_server_with_config(config_path).map(|socket| (socket, false));
+    }
+
+    if fs::metadata(DEFAULT_SOCKET_PATH).is_ok() {
+        return match StdUnixStream::connect(DEFAULT_SOCKET_PATH) {
+            Ok(socket) => Ok((socket, true)),
+            Err(e) => match e.kind() {
+                std::io::ErrorKind::NotFound => Err(anyhow::anyhow!("Socket not found")),
+                std::io::ErrorKind::PermissionDenied => Err(anyhow::anyhow!("Permission denied")),
+                _ => Err(anyhow::anyhow!("Failed to connect to socket: {}", e)),
+            },
+        };
+    }
+
+    let config_path = PathBuf::from(DEFAULT_CONFIG_PATH);
+    if fs::metadata(&config_path).is_ok() {
+        return invoke_server_with_config(config_path).map(|socket| (socket, false));
+    }
+
+    anyhow::bail!("No socket path or config path provided, and no default socket or config found");
+}
+
+// TODO: we should somehow ensure that the forked process is killed on completion,
+//       just in case the client does not behave properly.
+/// Fork a child process to run the server with the provided config.
+/// The server will exit silently by itself when it is done, and this function
+/// will only return for the client with the socket for the server.
+fn invoke_server_with_config(config_path: PathBuf) -> anyhow::Result<StdUnixStream> {
+    let (server_socket, client_socket) = StdUnixStream::pair()?;
+    let unix_user = UnixUser::from_uid(nix::unistd::getuid().as_raw())?;
+
+    match (unsafe { nix::unistd::fork() }).context("Failed to fork")? {
+        nix::unistd::ForkResult::Parent { child } => {
+            log::debug!("Forked child process with PID {}", child);
+            Ok(client_socket)
+        }
+        nix::unistd::ForkResult::Child => {
+            log::debug!("Running server in child process");
+
+            match run_forked_server(config_path, server_socket, unix_user) {
+                Err(e) => Err(e),
+                Ok(_) => unreachable!(),
+            }
+        }
+    }
+}
+
+/// Run the server in the forked child process.
+/// This function will not return, but will exit the process with a success code.
+fn run_forked_server(
+    config_path: PathBuf,
+    server_socket: StdUnixStream,
+    unix_user: UnixUser,
+) -> anyhow::Result<()> {
+    let config = read_config_form_path(Some(config_path))?;
+
+    let result: anyhow::Result<()> = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap()
+        .block_on(async {
+            let socket = TokioUnixStream::from_std(server_socket)?;
+            handle_requests_for_single_session(socket, &unix_user, &config).await?;
+            Ok(())
+        });
+
+    result?;
+
+    unsafe {
+        exit(EXIT_SUCCESS);
+    }
+}