Switch over to a reverse connection scheme (bridge to a guest bus)

README.md

@@ -14,9 +14,8 @@ A cross-domain smart D-Bus proxying system that makes (some) [XDG Desktop Portal
 
 - `sidebus-broker` host process:
   - to be launched alongside the VMM
-  - hosts D-Bus servers in-process, based on [busd](https://github.com/dbus2/busd):
-    - a "private" bus for VM-instance-specific daemons such as permission-store and document-portal
-    - a "VM" bus, the one actually exposed to the guest over vsock
+  - hosts a "private" bus for VM-instance-specific daemons such as permission-store and document-portal
+  - listens on vsock (or on a unix socket that muvm would proxy as vsock) and connects to the VM bus as a client when the agent connects
   - orchestrates the lifecycle of the aforementioned daemons + virtiofsd
     - (we are sharing the directory *provided by* the document-portal FUSE filesystem!)
   - provides portal front-end interfaces like `org.freedesktop.portal.FileChooser` on the VM bus
@@ -24,11 +23,10 @@ A cross-domain smart D-Bus proxying system that makes (some) [XDG Desktop Portal
       - (not talking directly to impls: don't want to reimplement per-DE portal selection; also 1:1 mapping is nicer to code)
     - but with extra hooks like exposing files to the guest using our private (per-VM) document-portal!
 - `sidebus-agent` guest process:
-  - listens on a guest unix socket, proxies D-Bus messages to a vsock
-  - spawned on-demand by systemd via socket activation
+  - connects to the broker over vsock and splices the connection into the VM (session) bus
+  - can be spawned spawned on-demand by D-Bus
   - uses systemd credentials for config args like vsock port
     - (very convenient to pass via the VMM, e.g. qemu: `-smbios type=11,value=io.systemd.credential:sidebus.port=1337`)
-  - guest NixOS configuration exposed via the flake
 
 ## Development Notes
 

sidebus-agent/src/main.rs

@@ -1,5 +1,3 @@
-use eyre::OptionExt;
-use tokio::net::UnixListener;
 use tracing::info;
 
 #[tokio::main]
@@ -10,38 +8,23 @@ async fn main() -> eyre::Result<()> {
     let vsock_port = std::fs::read_to_string(creds_dir.join("sidebus.port"))?
         .trim()
         .parse::<u32>()?;
+    let vsock_addr = zbus::Address::from(zbus::address::Transport::Vsock(
+        zbus::address::transport::Vsock::new(2, vsock_port),
+    ));
 
-    let unix_listener = UnixListener::from_std(
-        listenfd::ListenFd::from_env()
-            .take_unix_listener(0)?
-            .ok_or_eyre("no unix listener provided")?,
-    )?;
-    info!("listening for unix clients");
+    info!("connecting to session bus");
+    let session_conn = zbus::connection::Builder::session()?.p2p().build().await?;
+
+    let vsock_conn = zbus::connection::Builder::address(vsock_addr)?
+        .server(session_conn.server_guid())
+        .unwrap()
+        .p2p()
+        .auth_mechanism(zbus::AuthMechanism::Anonymous)
+        .build()
+        .await
+        .unwrap();
+    info!(guid = %vsock_conn.server_guid(), "connected to vsock bus");
+    sidebus_common::raw::splice_conns(vsock_conn, session_conn).await;
 
-    while let Ok((unix_client, client_addr)) = unix_listener.accept().await {
-        info!(?client_addr, "new unix client");
-        tokio::spawn(async move {
-            let vsock_addr = zbus::Address::from(zbus::address::Transport::Vsock(
-                zbus::address::transport::Vsock::new(2, vsock_port),
-            ));
-            let vsock_conn = zbus::connection::Builder::address(vsock_addr)
-                .unwrap()
-                .p2p()
-                .auth_mechanism(zbus::AuthMechanism::Anonymous)
-                .build()
-                .await
-                .unwrap();
-            info!(guid = %vsock_conn.server_guid(), "connected to vsock bus");
-            let client_conn = zbus::connection::Builder::unix_stream(unix_client)
-                .server(vsock_conn.server_guid())
-                .unwrap()
-                .p2p()
-                .auth_mechanism(zbus::AuthMechanism::External)
-                .build()
-                .await
-                .unwrap();
-            sidebus_common::raw::splice_conns(client_conn, vsock_conn).await;
-        });
-    }
     Ok(())
 }

sidebus-broker/src/main.rs

@@ -5,7 +5,7 @@ mod vsock;
 use bus::SharedHostedBus;
 use clap::Parser;
 use futures::{TryFutureExt, stream::FuturesUnordered};
-use std::{path::PathBuf, sync::Arc};
+use std::{path::PathBuf, sync::Arc, time::Duration};
 use tokio::{net::UnixListener, process::Command, sync::Mutex};
 use tokio_stream::StreamExt as _;
 use tracing::{Instrument, debug, error, info_span};
@@ -71,7 +71,6 @@ async fn main() -> eyre::Result<()> {
 
     let cli = BrokerCli::parse();
 
-    let (vm_bus, vm_bus_guid, _) = new_hosted_bus().await?;
     let (priv_bus, _, mut priv_lst) = new_hosted_bus().await?;
 
     let mut server_tasks = tokio::task::JoinSet::new();
@@ -88,12 +87,6 @@ async fn main() -> eyre::Result<()> {
                 .clone()
                 .run_unix_listener(priv_dbg_listener, zbus::AuthMechanism::External),
         );
-        let vm_dbg_listener = UnixListener::bind(dir_path.join("vm.sock"))?;
-        server_tasks.spawn(
-            vm_bus
-                .clone()
-                .run_unix_listener(vm_dbg_listener, zbus::AuthMechanism::External),
-        );
         // TODO: unlink sockets on exit
     }
 
@@ -148,7 +141,6 @@ async fn main() -> eyre::Result<()> {
             .spawn()?,
     );
 
-    let vm_bus_conn = vm_bus.lock().await.connect_channel(false).await?;
     let priv_bus_conn = priv_bus.lock().await.connect_channel(false).await?;
     let host_session_conn = zbus::connection::Builder::session()?.build().await?;
     let file_chooser_imp = portal::file_chooser::FileChooser::new(
@@ -157,47 +149,49 @@ async fn main() -> eyre::Result<()> {
         cli.guest_mountpoint,
     )
     .await?;
-    vm_bus_conn
-        .request_name("org.freedesktop.portal.Desktop")
-        .await?;
-    let true = vm_bus_conn
-        .object_server()
-        .at("/org/freedesktop/portal/desktop", file_chooser_imp)
-        .await?
-    else {
-        unreachable!("our own fresh bus can't have interfaces already provided");
-    };
 
-    // NOTE: Every individual D-Bus client inside of the VM is a new client on the VM bus listeners!
+    async fn on_vm_bus_connected(
+        vm_bus_conn: zbus::Connection,
+        file_chooser: portal::file_chooser::FileChooser,
+    ) -> Result<(), eyre::Report> {
+        vm_bus_conn
+            .request_name("org.freedesktop.portal.Desktop")
+            .await?;
+        if !vm_bus_conn
+            .object_server()
+            .at("/org/freedesktop/portal/desktop", file_chooser)
+            .await?
+        {
+            error!("org.freedesktop.portal.Desktop already provided");
+        };
+        // XXX: no method for "wait until the conn dies"?
+        loop {
+            tokio::time::sleep(Duration::from_millis(5000)).await;
+        }
+    }
 
     if let Some(path) = cli.unix_path {
-        // XXX: going through the channel just to strip fds
         let vm_unix_listener = UnixListener::bind(path)?;
-        server_tasks.spawn(enclose! { (vm_bus, vm_bus_guid) async move {
+        server_tasks.spawn(enclose!((file_chooser_imp) async move {
             while let Ok((socket, remote_addr)) = vm_unix_listener.accept().await {
-                let f = enclose! { (vm_bus, vm_bus_guid) async move {
+                let f = enclose!((file_chooser_imp) async move {
                     let client_conn = zbus::connection::Builder::unix_stream(socket)
-                        .server(&vm_bus_guid)
-                        .unwrap()
-                        .p2p()
                         .auth_mechanism(zbus::AuthMechanism::Anonymous)
                         .build()
                         .await?;
-                    let vmbus_conn = vm_bus.lock().await.connect_channel(true).await?;
-                    sidebus_common::raw::splice_conns(client_conn, vmbus_conn).await;
-                    Ok::<(), eyre::Report>(())
-                } };
+                    on_vm_bus_connected(client_conn, file_chooser_imp).await
+                });
                 tokio::spawn(
                     async {
                         match f.await {
-                            Ok(()) => debug!("done with client"),
-                            Err(err) => error!(%err, "error dealing with client"),
+                            Ok(()) => debug!("done with server"),
+                            Err(err) => error!(%err, "error dealing with server"),
                         }
                     }
                     .instrument(info_span!("serve", ?remote_addr)),
                 );
             }
-        } });
+        }));
     }
 
     if let Some(port) = cli.vsock_port {
@@ -206,13 +200,11 @@ async fn main() -> eyre::Result<()> {
             vsock::ListenerBuilder::new(vsock::VsockAddr::new(vsock::VMADDR_CID_HOST, port))
                 .with_label("VM Bus")
                 .listen(move |client| {
-                    enclose! { (vm_bus, vm_bus_guid) async move {
+                    enclose!((file_chooser_imp) async move {
                         // TODO: Not necessary to go through the channel, add vsock support to the Peer too?
-                        let client_conn = client.build((&vm_bus_guid).into()).await?;
-                        let vmbus_conn = vm_bus.lock().await.connect_channel(true).await?;
-                        sidebus_common::raw::splice_conns(client_conn, vmbus_conn).await;
-                        Ok(())
-                    } }
+                        let client_conn = client.build().await?;
+                        on_vm_bus_connected(client_conn, file_chooser_imp).await
+                    })
                 })
                 .map_ok_or_else(
                     |e| {

sidebus-broker/src/portal/file_chooser.rs

@@ -8,6 +8,7 @@ use zbus::{Connection, ObjectServer, fdo::Result, zvariant};
 use super::documents::DocumentsProxy;
 use super::request::{RESPONSE_SUCCESS, ReqHandler, ResultTransformer};
 
+#[derive(Clone)]
 pub struct FileChooser {
     host: FileChooserProxy<'static>,
     docs: DocumentsProxy<'static>,

sidebus-broker/src/vsock.rs

@@ -13,11 +13,8 @@ impl ConnectionBuilder {
         &self.remote_addr
     }
 
-    pub async fn build<'a>(self, guid: zbus::Guid<'a>) -> eyre::Result<zbus::Connection> {
+    pub async fn build<'a>(self) -> eyre::Result<zbus::Connection> {
         zbus::connection::Builder::vsock_stream(self.socket)
-            .server(guid)
-            .unwrap()
-            .p2p()
             .auth_mechanism(zbus::AuthMechanism::Anonymous)
             .build()
             .await