virtiofs: add --mount-mode {unshare,direct}

Adds a boot flag choosing how virtiofsd is launched per --mount:

- unshare (default, previous behavior): virtiofsd runs inside
  'podman unshare' with --sandbox chroot, mapping guest UIDs into
  the invoking user's subuid range.

- direct: virtiofsd runs as the invoking user with --sandbox none,
  mapping guest UIDs 1:1 to host UIDs.

The direct mode is needed when the shared directory itself lives on
a virtiofs/fuse mount that rejects subuid-range UIDs (e.g. nested
boco): guest writes through the subuid mapping then fail with
EINVAL, breaking anything that writes to shares as a non-root guest
user (databases, agent workspaces).

Direct mode also cannot use the chroot sandbox (unprivileged
chroot() is not permitted), so it falls back to --sandbox none;
served trees are then not symlink-escape-protected.

Verified both modes boot and serve mounts (alpine guest).

src/qemu.rs

@@ -4,10 +4,12 @@
 //! 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).
+//! virtio-fs: one virtiofsd daemon per `--mount`, plus a `-chardev socket`
+//! + `-device vhost-user-fs-pci` pair on the QEMU side. virtiofsd runs
+//! under `podman unshare` (rootless UID mapping) by default, or directly
+//! as the invoking user with `--mount-mode direct` (1:1 UID mapping).
+//! The guest mounts each share with `mount -t virtiofs <tag> <dest>`
+//! (see `boco.mount=` cmdline).
 
 use anyhow::{Context, Result, bail};
 use base64::Engine;
@@ -17,7 +19,7 @@ use std::{fmt::Write as _, process::Command};
 use crate::{
     forward::PortForward,
     registry::{registry_base_dir, registry_dir},
-    virtiofs::{self, VirtiofsShare},
+    virtiofs::{self, MountMode, VirtiofsShare},
 };
 
 #[derive(Args, Debug)]
@@ -44,6 +46,13 @@ pub struct BootCmd {
     /// relative to the image's WORKINGDIR. Can be repeated.
     #[clap(long)]
     mount: Vec<String>,
+
+    /// How virtiofsd is launched: `unshare` runs it inside
+    /// `podman unshare` (rootless UID mapping, guest UIDs map into the
+    /// subuid range); `direct` runs it as the invoking user (guest UIDs
+    /// map 1:1 to host UIDs — needed for nested virtiofs shares).
+    #[clap(long, value_enum, default_value = "unshare")]
+    mount_mode: MountMode,
 }
 
 pub(crate) fn boot(
@@ -53,6 +62,7 @@ pub(crate) fn boot(
         forward,
         cmd,
         mount,
+        mount_mode,
     }: BootCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
@@ -108,7 +118,7 @@ pub(crate) fn boot(
             Some((h, g)) => (h, g.to_string()),
             None => (spec.as_str(), spec.to_string()),
         };
-        let share = virtiofs::spawn_share(i, host_path, guest_path)?;
+        let share = virtiofs::spawn_share(i, host_path, guest_path, mount_mode)?;
         let tag = share.tag.clone();
         let chardev = format!("char{i}");
         let socket = share

src/virtiofs.rs

@@ -6,13 +6,21 @@
 //! hands the socket path to QEMU via `-chardev socket,path=...`, and
 //! cleans up when the VM exits.
 //!
-//! To serve files owned by arbitrary guest UIDs (e.g. when an image
-//! uses the `USER` directive) without running as root, virtiofsd is
-//! launched under `podman unshare` in a user namespace with
-//! `--sandbox chroot`.  This reuses rootless podman's existing
-//! subuid/subgid mapping: guest UID 0 maps to the invoking user, and
-//! other guest UIDs map into the subuid range, so each guest UID
-//! becomes a distinct host UID.
+//! Launch modes ([`MountMode`]):
+//!
+//! * `unshare` (default): virtiofsd runs under `podman unshare` in a
+//!   user namespace with `--sandbox chroot`.  This serves files owned
+//!   by arbitrary guest UIDs (e.g. when an image uses the `USER`
+//!   directive) without running as root, reusing rootless podman's
+//!   subuid/subgid mapping: guest UID 0 maps to the invoking user, and
+//!   other guest UIDs map into the subuid range, so each guest UID
+//!   becomes a distinct host UID.
+//!
+//! * `direct`: virtiofsd runs as the invoking user with `--sandbox
+//!   none`.  Guest UIDs map 1:1 to host UIDs.  Use this when the
+//!   shared directory lives on a filesystem that rejects subuid-range
+//!   UIDs (e.g. nested virtiofs), or when the guest should act as the
+//!   invoking user rather than the subuid range.
 //!
 //! Lifecycle: boco spawns all virtiofsd children before launching QEMU,
 //! waits for each listening socket to appear, then drops the
@@ -28,10 +36,26 @@ use std::thread;
 use std::time::{Duration, Instant};
 
 use anyhow::{Context, Result, anyhow, bail};
+use clap::ValueEnum;
 use tempfile::TempDir;
 
 use crate::command::cmd;
 
+/// How virtiofsd is launched for a share.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, ValueEnum)]
+pub enum MountMode {
+    /// Run virtiofsd inside `podman unshare` (rootless UID mapping:
+    /// guest UID 0 maps to the invoking user, other guest UIDs map
+    /// into the subuid range).
+    Unshare,
+    /// Run virtiofsd directly as the invoking user. Guest UIDs map 1:1
+    /// to host UIDs. Needed when the shared directory itself lives on
+    /// a virtiofs/fuse mount that rejects subuid-range UIDs (e.g.
+    /// nested boco), where writes through the subuid mapping fail with
+    /// EINVAL.
+    Direct,
+}
+
 /// Locations searched for the virtiofsd binary, in order.
 const VIRTIOFSD_CANDIDATES: &[&str] = &[
     "/usr/libexec/virtiofsd",
@@ -114,7 +138,12 @@ fn wait_for_socket(socket: &Path) -> Result<()> {
 ///
 /// `index` becomes the suffix of the tag (`boco{index}`) and is used to
 /// derive a unique socket filename.
-pub fn spawn_share(index: usize, host_path: &str, guest_path: String) -> Result<VirtiofsShare> {
+pub fn spawn_share(
+    index: usize,
+    host_path: &str,
+    guest_path: String,
+    mode: MountMode,
+) -> Result<VirtiofsShare> {
     let virtiofsd = find_virtiofsd()?;
     let canonical = resolve_host_dir(host_path)?;
     let host_str = canonical
@@ -128,17 +157,29 @@ pub fn spawn_share(index: usize, host_path: &str, guest_path: String) -> Result<
     let tag = format!("boco{index}"); // short tag to fit on cmdline
     let socket = dir.path().join(format!("{tag}.sock"));
 
-    // TODO: Make rootless mode optional
-
-    // Run virtiofsd under `podman unshare` so it can serve files owned
-    // by arbitrary guest UIDs without root, mapping them into the host
-    // user's subuid range.  `--sandbox chroot` is required because the
-    // outer `podman unshare` already entered a user namespace (nested
-    // user namespaces are not allowed); chroot still prevents symlink
-    // escapes from the shared dir.
-    let mut command = Command::new("podman");
+    // In `unshare` mode, run virtiofsd under `podman unshare` so it can
+    // serve files owned by arbitrary guest UIDs without root, mapping
+    // them into the host user's subuid range.  `--sandbox chroot` is
+    // required because the outer `podman unshare` already entered a user
+    // namespace (nested user namespaces are not allowed); chroot still
+    // prevents symlink escapes from the shared dir.
+    //
+    // In `direct` mode, run virtiofsd as the invoking user with 1:1 UID
+    // mapping.  This cannot use the chroot sandbox (unprivileged
+    // chroot is not permitted), so it falls back to `--sandbox none`.
+    let mut command = match mode {
+        MountMode::Unshare => {
+            let mut command = Command::new("podman");
+            command.args(["unshare", "--", virtiofsd_str, "--socket-path"]);
+            command
+        }
+        MountMode::Direct => {
+            let mut command = Command::new(virtiofsd_str);
+            command.arg("--socket-path");
+            command
+        }
+    };
     command
-        .args(["unshare", "--", virtiofsd_str, "--socket-path"])
         .arg(&socket)
         .args([
             "--shared-dir",
@@ -146,7 +187,10 @@ pub fn spawn_share(index: usize, host_path: &str, guest_path: String) -> Result<
             "--tag",
             &tag,
             "--sandbox",
-            "chroot",
+            match mode {
+                MountMode::Unshare => "chroot",
+                MountMode::Direct => "none",
+            },
             "--cache",
             "auto",
         ])