ephemeral: Fix boot for images without virtiofs dracut module

Some images don't have the dracut virtiofs module enabled. It's
tiny, and they probably *should*, but...on the other hand
since we already inject other content into the initramfs here,
we can just change things so instead of injecting kargs, we just
inject a trivial unit into the initramfs - no dracut involved
or needed.

Assisted-by: OpenCode (Claude Sonnet 4.6)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/kit/src/cpio.rs

@@ -29,7 +29,7 @@ fn write_file(writer: &mut impl Write, path: &str, content: &[u8]) -> io::Result
     Ok(())
 }
 
-/// CPIO entry: either a directory or a file with content.
+/// CPIO entry: either a directory or a regular file (0644).
 enum Entry {
     Dir(&'static str),
     File(&'static str, &'static [u8]),
@@ -41,6 +41,7 @@ pub fn create_initramfs_units_cpio() -> io::Result<Vec<u8>> {
 
     const UNIT_DIR: &str = "usr/lib/systemd/system";
     const DROPIN_DIR: &str = "usr/lib/systemd/system/initrd-fs.target.d";
+    const ROOT_FS_DROPIN_DIR: &str = "usr/lib/systemd/system/initrd-root-fs.target.d";
 
     let entries: &[Entry] = &[
         // Directory hierarchy
@@ -49,6 +50,17 @@ pub fn create_initramfs_units_cpio() -> io::Result<Vec<u8>> {
         Dir("usr/lib/systemd"),
         Dir(UNIT_DIR),
         Dir(DROPIN_DIR),
+        Dir(ROOT_FS_DROPIN_DIR),
+        // sysroot.mount — mounts the virtiofs "rootfs" tag read-only at
+        // /sysroot.  bcvk does not set root= on the kernel cmdline, so
+        // systemd-fstab-generator never generates a competing sysroot.mount,
+        // and dracut sets rootok=1 via its UNSET branch (no root= arg → trust
+        // systemd generators).  The bcvk-sysroot.conf drop-in below wires
+        // this unit into initrd-root-fs.target.
+        File(
+            "usr/lib/systemd/system/sysroot.mount",
+            include_bytes!("units/sysroot.mount"),
+        ),
         // Service units
         File(
             "usr/lib/systemd/system/bcvk-etc-overlay.service",
@@ -66,6 +78,15 @@ pub fn create_initramfs_units_cpio() -> io::Result<Vec<u8>> {
             "usr/lib/systemd/system/bcvk-journal-stream.service",
             include_bytes!("units/bcvk-journal-stream.service"),
         ),
+        // Drop-in to pull sysroot.mount into initrd-root-fs.target.  Without
+        // this, nothing in the dependency graph actually requests the mount;
+        // dracut-rootfs-generator normally creates an
+        // initrd-root-fs.target.requires/sysroot.mount symlink for block-device
+        // roots, but for virtiofs (not a block device) it skips that step.
+        File(
+            "usr/lib/systemd/system/initrd-root-fs.target.d/bcvk-sysroot.conf",
+            b"[Unit]\nRequires=sysroot.mount\nAfter=sysroot.mount\n",
+        ),
         // Drop-in configs to pull units into initrd-fs.target
         File(
             "usr/lib/systemd/system/initrd-fs.target.d/bcvk-etc-overlay.conf",
@@ -104,6 +125,7 @@ mod tests {
 
         let mut entries = Vec::new();
         let mut etc_overlay_content = None;
+        let mut sysroot_mount_content = None;
 
         loop {
             let mut reader = cpio::NewcReader::new(cursor).expect("failed to read CPIO entry");
@@ -115,13 +137,20 @@ mod tests {
             let size = reader.entry().file_size() as usize;
             let mode = reader.entry().mode();
 
-            // Read file content for verification
-            if name == "usr/lib/systemd/system/bcvk-etc-overlay.service" {
-                let mut content = vec![0u8; size];
-                reader
-                    .read_exact(&mut content)
-                    .expect("failed to read file content");
-                etc_overlay_content = Some(String::from_utf8(content).expect("invalid UTF-8"));
+            let mut content_buf = vec![0u8; size];
+            reader
+                .read_exact(&mut content_buf)
+                .expect("failed to read file content");
+            let content_str = String::from_utf8(content_buf).ok();
+
+            match name.as_str() {
+                "usr/lib/systemd/system/bcvk-etc-overlay.service" => {
+                    etc_overlay_content = content_str.clone()
+                }
+                "usr/lib/systemd/system/sysroot.mount" => {
+                    sysroot_mount_content = content_str.clone()
+                }
+                _ => {}
             }
 
             entries.push((name, size, mode));
@@ -130,37 +159,64 @@ mod tests {
 
         let names: Vec<_> = entries.iter().map(|(n, _, _)| n.as_str()).collect();
 
-        // Verify directories
+        // Verify directory hierarchy
         assert!(names.contains(&"usr"));
         assert!(names.contains(&"usr/lib"));
         assert!(names.contains(&"usr/lib/systemd"));
         assert!(names.contains(&"usr/lib/systemd/system"));
         assert!(names.contains(&"usr/lib/systemd/system/initrd-fs.target.d"));
+        assert!(names.contains(&"usr/lib/systemd/system/initrd-root-fs.target.d"));
 
-        // Verify service files
+        // sysroot.mount must be present and correct
+        assert!(
+            names.contains(&"usr/lib/systemd/system/sysroot.mount"),
+            "sysroot.mount must be injected"
+        );
+        let sysroot = sysroot_mount_content.expect("sysroot.mount content missing");
+        assert!(
+            sysroot.contains("Type=virtiofs"),
+            "sysroot.mount must use virtiofs"
+        );
+        assert!(
+            sysroot.contains("What=rootfs"),
+            "sysroot.mount must mount the 'rootfs' tag"
+        );
+        assert!(
+            sysroot.contains("Where=/sysroot"),
+            "sysroot.mount must target /sysroot"
+        );
+        assert!(
+            sysroot.contains("Options=ro"),
+            "sysroot.mount must be read-only"
+        );
+
+        // Service units
         assert!(names.contains(&"usr/lib/systemd/system/bcvk-etc-overlay.service"));
         assert!(names.contains(&"usr/lib/systemd/system/bcvk-var-ephemeral.service"));
         assert!(names.contains(&"usr/lib/systemd/system/bcvk-copy-units.service"));
         assert!(names.contains(&"usr/lib/systemd/system/bcvk-journal-stream.service"));
 
-        // Verify drop-in configs
+        // initrd-root-fs.target drop-in
+        assert!(names.contains(&"usr/lib/systemd/system/initrd-root-fs.target.d/bcvk-sysroot.conf"));
+
+        // Drop-in configs
         assert!(names.contains(&"usr/lib/systemd/system/initrd-fs.target.d/bcvk-etc-overlay.conf"));
         assert!(
             names.contains(&"usr/lib/systemd/system/initrd-fs.target.d/bcvk-var-ephemeral.conf")
         );
         assert!(names.contains(&"usr/lib/systemd/system/initrd-fs.target.d/bcvk-copy-units.conf"));
 
-        // Verify file modes
+        // Verify file modes: all entries are either regular files (0644) or directories
         for (name, _size, mode) in &entries {
             let file_type = *mode & 0o170000;
-            if name.ends_with(".service") || name.ends_with(".conf") {
-                assert_eq!(file_type, 0o100000, "{} should be regular file", name);
+            if name.ends_with(".service") || name.ends_with(".conf") || name.ends_with(".mount") {
+                assert_eq!(file_type, 0o100000, "{} should be a regular file", name);
             } else {
-                assert_eq!(file_type, 0o040000, "{} should be directory", name);
+                assert_eq!(file_type, 0o040000, "{} should be a directory", name);
             }
         }
 
-        // Verify file content is valid systemd unit
+        // bcvk-etc-overlay.service must be a valid systemd unit
         let content = etc_overlay_content.expect("bcvk-etc-overlay.service not found");
         assert!(content.contains("[Unit]"));
         assert!(content.contains("[Service]"));

crates/kit/src/run_ephemeral.rs

@@ -880,8 +880,6 @@ fn parse_service_exit_code(status_content: &str) -> Result<i32> {
     Ok(0)
 }
 
-/// Check for required binaries in the privileged container
-///
 /// These binaries must be present in the privileged container that runs bcvk,
 /// not the guest bootc image that gets booted inside the VM.
 fn check_required_container_binaries() -> Result<()> {
@@ -1365,15 +1363,16 @@ StandardOutput=file:/dev/virtio-ports/executestatus
         qemu_config.add_smbios_credential(credential);
     }
 
-    // Build kernel command line for direct boot
+    // Build kernel command line for direct boot.
+    //
+    // We deliberately omit root=, rootfstype=, and rootflags= from the
+    // cmdline.  When root= is absent dracut sets rootok=1 via its UNSET
+    // branch and defers entirely to systemd generators.  systemd-fstab-
+    // generator likewise produces nothing without a root= arg.  The
+    // virtiofs mount is handled solely by the sysroot.mount unit bcvk
+    // injects into every initramfs via the CPIO append, together with the
+    // initrd-root-fs.target.d/bcvk-sysroot.conf drop-in that wires it in.
     let mut kernel_cmdline = [
-        // At the core we boot from the mounted container's root,
-        "rootfstype=virtiofs",
-        "root=rootfs",
-        // But read-only. We set up /etc overlay and /var copyup via
-        // systemd credentials rather than systemd.volatile=overlay
-        // to have more control over individual directories.
-        "rootflags=ro",
         // This avoids having journald interact with the rootfs
         // at all, which lessens the I/O traffic for virtiofs
         "systemd.journald.storage=volatile",

crates/kit/src/units/sysroot.mount

@@ -0,0 +1,19 @@
+[Unit]
+Description=bcvk ephemeral virtiofs root mount
+Documentation=https://github.com/bootc-dev/bcvk
+DefaultDependencies=no
+# bcvk does not put root= on the kernel cmdline, so neither dracut nor
+# systemd-fstab-generator will generate a competing sysroot.mount unit.
+# This unit is the sole handler for the virtiofs root mount in all
+# initramfs types (dracut, bootc composefs initramfs, mkosi-initrd, …).
+#
+# The initrd-root-fs.target.d/bcvk-sysroot.conf drop-in wires this unit
+# into the dependency graph so initrd-root-fs.target waits for it.
+ConditionPathExists=/etc/initrd-release
+Before=initrd-root-fs.target
+
+[Mount]
+What=rootfs
+Where=/sysroot
+Type=virtiofs
+Options=ro