Add --guestfwd arg to boco boot

init/src/main.rs

@@ -245,13 +245,7 @@ fn setup_virtiofs_shares(workdir: &str) {
             };
 
             try_io(&format!("mkdir {dest}"), || fs::create_dir_all(&dest));
-            try_mount(
-                Some(tag),
-                &dest,
-                Some("virtiofs"),
-                MsFlags::empty(),
-                None,
-            );
+            try_mount(Some(tag), &dest, Some("virtiofs"), MsFlags::empty(), None);
         }
     }
 }
@@ -359,6 +353,9 @@ fn exec_command(argv: &[String], user: Option<UserSpec>) -> ! {
     let c_prog = cstrings[0].clone();
     let c_refs: Vec<&CString> = cstrings.iter().collect();
 
+    // If UserSpec is 0:0, don't fork.
+    let user = user.filter(|u| !u.uid.is_root() || u.gid != Gid::from_raw(0));
+
     match user {
         None => {
             // No privilege drop: exec directly, PID 1 becomes the command.
@@ -368,6 +365,7 @@ fn exec_command(argv: &[String], user: Option<UserSpec>) -> ! {
             std::process::exit(1);
         }
         Some(user) => {
+            warn(&format!("dropping privileges to {}:{}", user.uid, user.gid));
             // Drop privileges in a child so PID 1 (root) can poweroff afterwards.
             match unsafe { fork() } {
                 Ok(ForkResult::Child) => {

scripts/setup.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -ex
+
+cd "$(dirname "$0")/.."
+
+mkdir -p "$HOME/.local/share/boco"
+cargo build --release --target x86_64-unknown-linux-musl -p boco-init
+
+target_dir="${CARGO_TARGET_DIR:-target}"
+cp "$target_dir/x86_64-unknown-linux-musl/release/boco-init" "$HOME/.local/share/boco/"
+

src/qemu.rs

@@ -1,13 +1,4 @@
-//! QEMU: boot a registry entry with qemu-system-x86_64 using direct kernel
-//! boot (-kernel/-initrd or -drive). The kernel is shared across all VMs
-//! at $XDG_DATA_HOME/boco/registry/vmlinuz. A universal /boco/init script (injected
-//! at build time) handles VM bootstrap; /boco/exec runs the container's
-//! CMD/ENTRYPOINT. Runtime overrides are delivered as `boco.cmd=<base64>`
-//! on the kernel cmdline. Host directories are shared into the guest via
-//! virtio-fs: one virtiofsd daemon (under `podman unshare` for rootless
-//! UID mapping) per `--mount`, plus a `-chardev socket` + `-device
-//! vhost-user-fs-pci` pair on the QEMU side. The guest mounts each share
-//! with `mount -t virtiofs <tag> <dest>` (see `boco.mount=` cmdline).
+//! See [`boot`]
 
 use anyhow::{Context, Result, bail};
 use base64::Engine;
@@ -16,7 +7,7 @@ use std::{fmt::Write as _, process::Command};
 
 use crate::{
     forward::PortForward,
-    registry::{registry_base_dir, registry_dir},
+    registry::{registry_dir, vmlinuz_path},
     virtiofs::{self, VirtiofsShare},
 };
 
@@ -33,6 +24,10 @@ pub struct BootCmd {
     #[clap(long)]
     forward: Vec<PortForward>,
 
+    /// Pass a `guestfwd=...` argument to qemu `-nic` argument.
+    #[clap(long)]
+    guestfwd: Vec<String>,
+
     /// Override the command to exec in the VM (base64-encoded on the kernel
     /// cmdline as `boco.cmd=<b64>`). Overrides the CMD inferred at build time.
     #[clap(long)]
@@ -44,25 +39,34 @@ pub struct BootCmd {
     /// relative to the image's WORKINGDIR. Can be repeated.
     #[clap(long)]
     mount: Vec<String>,
+
+    /// Extra arguments to qemu
+    extra: Vec<String>,
 }
 
+/// Invoke `qemu` to boot a boco image. See [`BootCmd`] for options
+///
+/// This uses qemu's direct-kernel-boot with `-kernel`, so it does not require a bootloader.
+/// The boco image is just an ext4 rootfs.
 pub(crate) fn boot(
     BootCmd {
         name,
         memory,
         forward,
+        guestfwd,
         cmd,
         mount,
+        extra,
     }: BootCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
-    let vmlinuz_path = registry_base_dir()?.join("vmlinuz");
+    let vmlinuz_path = vmlinuz_path()?;
     let initrd_path = reg_dir.join("initrd");
     let qcow2_path = reg_dir.join("image.qcow2");
     if !vmlinuz_path.exists() {
         // TODO: guide user in how to set up a kernel
         bail!(
-            "No kernel available. Place a vmlinuz at {}",
+            "No kernel available. Expected vmlinuz at {}",
             vmlinuz_path.display()
         );
     }
@@ -154,6 +158,9 @@ pub(crate) fn boot(
     for forward in &forward {
         _ = write!(&mut network, ",hostfwd={forward}");
     }
+    for guestfwd in &guestfwd {
+        _ = write!(&mut network, ",guestfwd={guestfwd}");
+    }
 
     let mut command = Command::new("qemu-system-x86_64");
 
@@ -170,7 +177,8 @@ pub(crate) fn boot(
         .args(["-no-reboot"])
         .args(["-append", &cmdline])
         .args(["-nographic"])
-        .args(["-nic", &network]);
+        .args(["-nic", &network])
+        .args(&extra);
 
     println!("{command:?}");
 

src/registry.rs

@@ -26,6 +26,12 @@ pub(crate) fn validate_image_name(name: &str) -> Result<()> {
     Ok(())
 }
 
+pub(crate) fn vmlinuz_path() -> Result<PathBuf> {
+    let base = xdg::BaseDirectories::with_prefix("boco");
+    base.place_data_file("vmlinuz")
+        .context("Failed to create XDG_DATA_HOME subdirectory")
+}
+
 pub(crate) fn registry_base_dir() -> Result<PathBuf> {
     let base = xdg::BaseDirectories::with_prefix("boco");
     base.create_data_directory("registry")