Add --mount flag for 9p host directory sharing

Add support for sharing host directories into the VM via QEMU 9p
(virtio-9p).  The --mount flag can be repeated; each host directory
appears at the same absolute path inside the VM.

Design:
- QEMU: each --mount gets a short 9p tag (slim0, slim1, …) via
  -virtfs local,path=…,mount_tag=slimN,security_model=mapped-xattr.
  Tags are kept short because 9p mount_tag has a ~31-byte limit.
- Kernel cmdline: the full destination path is passed as
  slim.mount=<tag>:<base64(path)> so the init script knows where to
  mount each tag.  Base64 avoids issues with spaces/special chars.
- slim-init.sh: after networking, parse slim.mount= entries, mkdir -p
  the destination, and mount -t 9p <tag> <dest> -o trans=virtio,version=9p2000.L

Tests verify: 9p share detection via sysfs mount_tag, 9p entry in
mount output, file content accessible at the expected path, and clean
VM exit.

example/test.sh

@@ -103,6 +103,55 @@ EOF
     rm -rf "$work"
 }
 
+test_mount() {
+    echo "=== Testing --mount (9p shares) ==="
+
+    work="$(mktemp -d)"
+    img="slim-test-mount"
+    share_dir="$work/share"
+
+    mkdir -p "$share_dir"
+    echo "hello from host" > "$share_dir/testfile.txt"
+
+    cat > "$work/Containerfile" <<'EOF'
+FROM alpine:latest
+CMD ["/bin/sh", "-c", "poweroff -f"]
+EOF
+
+    echo "-- Building container image..."
+    if ! podman build --network=none -t "$img" -f "$work/Containerfile" >/dev/null 2>&1; then
+        report fail "--mount (podman build failed)"
+        rm -rf "$work"
+        return
+    fi
+
+    echo "-- Building slim VM..."
+    if ! "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1; then
+        report fail "--mount (slim build failed)"
+        cleanup "$img" "$img"
+        rm -rf "$work"
+        return
+    fi
+
+    # The 9p share appears at the same path inside the VM as on the host.
+    # Use --cmd to run the verification snippet with the canonical path.
+    share_canon=$(readlink -f "$share_dir")
+
+    echo "-- Booting VM with --mount $share_dir ..."
+    output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" \
+        --mount "$share_dir" \
+        --cmd "echo Available 9p shares:; for f in /sys/bus/virtio/drivers/9pnet_virtio/virtio*/mount_tag; do [ -f \"\$f\" ] && echo \"\$f: \$(tr -d '\\0' < \"\$f\")\"; done; echo Mounts:; mount -v; cat \"${share_canon}/testfile.txt\" 2>/dev/null || echo NO_FILE; echo MOUNT_VERIFY_DONE; poweroff -f" \
+        2>&1 || true)
+
+    check_output "$output" "mount_tag: slim0" "--mount 9p share detected"
+    check_output "$output" "type 9p" "--mount 9p filesystem in mount list"
+    check_output "$output" "hello from host" "--mount file accessible in VM"
+    check_output "$output" "MOUNT_VERIFY_DONE" "--mount VM ran to completion"
+
+    cleanup "$img" "$img"
+    rm -rf "$work"
+}
+
 test_service() {
     echo "=== Testing nextcloud service ==="
 
@@ -171,6 +220,8 @@ cargo build 2>&1
 test_distro "alpine" "alpine:latest" ""
 test_distro "archlinux" "archlinux:latest" "RUN pacman -Sy --noconfirm iproute2 wget; pacman -Sc --noconfirm"
 
+test_mount
+
 test_service
 
 echo ""

src/qemu.rs

@@ -8,7 +8,7 @@
 use anyhow::{Context, Result, bail};
 use base64::Engine;
 use clap::Args;
-use std::{fmt::Write as _, process::Command};
+use std::{fmt::Write as _, path::PathBuf, process::Command};
 
 use crate::{
     forward::PortForward,
@@ -32,6 +32,11 @@ pub struct RunCmd {
     /// cmdline as slim.cmd=<b64>). Overrides the CMD inferred at build time.
     #[clap(long)]
     cmd: Option<String>,
+
+    /// Share a host directory into the VM via 9p. The directory appears at
+    /// the same path inside the VM. Can be repeated for multiple mounts.
+    #[clap(long)]
+    mount: Vec<PathBuf>,
 }
 
 pub(crate) fn run(
@@ -40,6 +45,7 @@ pub(crate) fn run(
         memory,
         forward,
         cmd,
+        mount,
     }: RunCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
@@ -81,6 +87,27 @@ pub(crate) fn run(
         cmdline.push(format!("slim.cmd={encoded}"));
     }
 
+    // Build 9p shares for each --mount.  Tags are short (slim0, slim1, …)
+    // because 9p mount_tag has a ~31-byte limit.  The full destination path
+    // is passed on the kernel cmdline as slim.mount=<tag>:<base64(path)>.
+    let mut virtfs_args: Vec<String> = Vec::new();
+    for (i, host_path) in mount.iter().enumerate() {
+        let canonical = host_path
+            .canonicalize()
+            .with_context(|| format!("Cannot resolve mount path '{}'", host_path.display()))?;
+        let path_str = canonical
+            .to_str()
+            .context("Mount path is not valid UTF-8")?;
+        let tag = format!("slim{i}");
+        let dest_b64 = base64::engine::general_purpose::STANDARD.encode(path_str.as_bytes());
+
+        virtfs_args.push("-virtfs".into());
+        virtfs_args.push(format!(
+            "local,path={path_str},mount_tag={tag},security_model=mapped-xattr"
+        ));
+        cmdline.push(format!("slim.mount={tag}:{dest_b64}"));
+    }
+
     println!("Booting {name} from registry: {}", reg_dir.display());
 
     let cmdline = cmdline.join(" ");
@@ -101,6 +128,7 @@ pub(crate) fn run(
         .arg("-kernel")
         .arg(vmlinuz_path.as_os_str())
         .args(&fs_args)
+        .args(&virtfs_args)
         .args(["-snapshot"])
         .args(["-no-reboot"])
         .args(["-append", &cmdline])

src/scripts/slim-init.sh

@@ -34,6 +34,30 @@ ip addr add 10.0.2.15/24 dev eth0 2>/dev/null || echo "ip addr add eth0 failed"
 ip route add default via 10.0.2.2 2>/dev/null || echo "ip route add default failed"
 echo nameserver 10.0.2.3 > /etc/resolv.conf
 
+# === 9p shares (host directories shared via --mount) ===
+# Each share is passed as slim.mount=<tag>:<base64(dest_path)> on the
+# kernel cmdline.  The 9p tag is short (e.g. slim0) because mount_tag
+# has a ~31-byte limit; the full destination path is base64-encoded.
+# shellcheck disable=SC2013 # word-splitting is intentional: cmdline tokens are space-separated
+for tok in $(cat /proc/cmdline 2>/dev/null); do
+    case "$tok" in
+        slim.mount=*)
+            v=${tok#slim.mount=}
+            tag=${v%%:*}
+            dest_b64=${v#*:}
+            dest=$(printf '%s' "$dest_b64" | base64 -d 2>/dev/null) \
+                || dest=$(printf '%s' "$dest_b64" | openssl base64 -d 2>/dev/null) \
+                || echo "base64 decode failed"
+            if [ -n "$tag" ] && [ -n "$dest" ]; then
+                mkdir -p "$dest"
+                mount -t 9p "$tag" "$dest" -o trans=virtio,version=9p2000.L 2>/dev/null \
+                    && echo "mounted 9p '$tag' -> '$dest'" \
+                    || echo "failed to mount 9p '$tag' -> '$dest'"
+            fi
+            ;;
+    esac
+done
+
 # === Execute the configured command ===
 # If slim.cmd=<base64> is on the kernel cmdline, decode and exec it.
 # Otherwise, exec /slim/exec (generated from the image's CMD/ENTRYPOINT).