vcontainer: add tiny initramfs image infrastructure

Add proper Yocto image recipes for the tiny initramfs used by
vdkr/vpdmn in the switch_root boot flow:

- vcontainer-tiny-initramfs-image.inc: Shared image configuration
- vcontainer-preinit_1.0.bb: Preinit script package (shared)
- vdkr-tiny-initramfs-image.bb: Tiny initramfs for vdkr
- vpdmn-tiny-initramfs-image.bb: Tiny initramfs for vpdmn

The tiny initramfs contains only busybox and a preinit script that:
1. Mounts devtmpfs, proc, sysfs
2. Mounts the squashfs rootfs.img from /dev/vda
3. Creates tmpfs overlay for writes
4. Performs switch_root to the real rootfs

This replaces ad-hoc file extraction with proper image-based builds,
improving reproducibility and maintainability.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

5 files changed, 142 insertions, 3 deletions

diff --git a/recipes-containers/vcontainer/files/vdkr-preinit.sh b/recipes-containers/vcontainer/files/vcontainer-preinit.sh
index 08738022..6f1b7e09 100644
--- a/recipes-containers/vcontainer/files/vdkr-preinit.sh
+++ b/recipes-containers/vcontainer/files/vcontainer-preinit.sh
@@ -3,7 +3,7 @@
 #
 # SPDX-License-Identifier: GPL-2.0-only
 #
-# vdkr-preinit.sh
+# vcontainer-preinit.sh
 # Minimal init for initramfs - mounts rootfs and does switch_root
 #
 # This script runs from the initramfs and:
@@ -12,7 +12,7 @@
 # 3. Creates overlayfs with tmpfs for writes
 # 4. Executes switch_root to the overlay root filesystem
 #
-# The real init (/init or /sbin/init on rootfs) then runs vdkr-init.sh logic
+# The real init (/init or /sbin/init on rootfs) then continues boot
 
 # Mount essential filesystems first (needed to check cmdline)
 mount -t proc proc /proc
@@ -31,7 +31,7 @@ log() {
     [ "$QUIET" = "0" ] && echo "$@"
 }
 
-log "=== vdkr preinit (squashfs) ==="
+log "=== vcontainer preinit (squashfs) ==="
 
 # Wait for block devices to appear
 log "Waiting for block devices..."
diff --git a/recipes-containers/vcontainer/vcontainer-preinit_1.0.bb b/recipes-containers/vcontainer/vcontainer-preinit_1.0.bb
new file mode 100644
index 00000000..078a5617
--- /dev/null
+++ b/recipes-containers/vcontainer/vcontainer-preinit_1.0.bb
@@ -0,0 +1,43 @@
+# SPDX-FileCopyrightText: Copyright (C) 2025 Bruce Ashfield
+#
+# SPDX-License-Identifier: MIT
+#
+# vcontainer-preinit_1.0.bb
+# ===========================================================================
+# Package containing the preinit script for vcontainer tiny initramfs
+# ===========================================================================
+#
+# This package installs the preinit script as /init for use in a tiny
+# initramfs. The script:
+#   1. Mounts essential filesystems (/proc, /sys, /dev)
+#   2. Mounts the squashfs rootfs from /dev/vda with overlayfs
+#   3. Executes switch_root to the real root filesystem
+#
+# Used by: vdkr-tiny-initramfs-image.bb, vpdmn-tiny-initramfs-image.bb
+#
+
+SUMMARY = "Preinit script for vcontainer initramfs"
+DESCRIPTION = "Minimal init script that mounts squashfs rootfs with overlayfs \
+               and performs switch_root for vcontainer QEMU environment."
+HOMEPAGE = "https://git.yoctoproject.org/meta-virtualization/"
+LICENSE = "GPL-2.0-only"
+LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/GPL-2.0-only;md5=801f80980d171dd6425610833a22dbe6"
+
+SRC_URI = "file://vcontainer-preinit.sh"
+
+S = "${UNPACKDIR}"
+
+# This package only provides the init script - no dependencies
+# The initramfs image will pull in busybox separately
+RDEPENDS:${PN} = ""
+
+do_install() {
+    install -d ${D}
+    install -m 0755 ${S}/vcontainer-preinit.sh ${D}/init
+}
+
+# Package the /init script
+FILES:${PN} = "/init"
+
+# Prevent QA warnings about /init location
+INSANE_SKIP:${PN} += "file-rdeps"
diff --git a/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc b/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc
new file mode 100644
index 00000000..1fef7822
--- /dev/null
+++ b/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc
@@ -0,0 +1,70 @@
+# SPDX-FileCopyrightText: Copyright (C) 2025 Bruce Ashfield
+#
+# SPDX-License-Identifier: MIT
+#
+# vcontainer-tiny-initramfs-image.inc
+# ===========================================================================
+# Shared tiny initramfs image for vdkr/vpdmn switch_root boot flow
+# ===========================================================================
+#
+# This produces a minimal cpio.gz initramfs containing only:
+#   - busybox (for mount, switch_root, sh, etc.)
+#   - vcontainer-preinit (the /init script)
+#
+# Boot flow:
+#   QEMU boots kernel + this initramfs
+#   -> /init (vcontainer-preinit) mounts squashfs rootfs.img from /dev/vda
+#   -> switch_root into rootfs.img with overlayfs
+#   -> ${runtime}-init.sh runs with full container tools
+#
+
+SUMMARY = "Tiny initramfs for vcontainer switch_root"
+DESCRIPTION = "Minimal initramfs containing busybox and preinit script for \
+               mounting squashfs rootfs and performing switch_root."
+LICENSE = "MIT"
+LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
+
+inherit core-image
+
+# Minimal packages: just busybox and the preinit script
+# busybox provides: mount, switch_root, sh, mkdir, sleep, cat, ls, echo, reboot
+PACKAGE_INSTALL = "busybox vcontainer-preinit"
+
+# No extra features - keep it tiny
+IMAGE_FEATURES = ""
+
+# Don't include kernel in initramfs
+PACKAGE_EXCLUDE = "kernel-image-*"
+
+# Output as cpio.gz (standard initramfs format)
+IMAGE_FSTYPES = "${INITRAMFS_FSTYPES}"
+
+# Keep the image small
+IMAGE_ROOTFS_SIZE = "8192"
+IMAGE_ROOTFS_EXTRA_SPACE = "0"
+
+# No locales needed
+IMAGE_LINGUAS = ""
+
+# Suppress the suffix for cleaner naming
+IMAGE_NAME_SUFFIX = ""
+
+# Compatible with x86_64 and aarch64 targets
+COMPATIBLE_HOST = "(x86_64|aarch64).*-linux"
+
+# Create required directories that busybox/preinit expect
+ROOTFS_POSTPROCESS_COMMAND += "create_initramfs_dirs;"
+
+create_initramfs_dirs() {
+    # Create mount points used by preinit
+    install -d ${IMAGE_ROOTFS}/proc
+    install -d ${IMAGE_ROOTFS}/sys
+    install -d ${IMAGE_ROOTFS}/dev
+    install -d ${IMAGE_ROOTFS}/mnt/lower
+    install -d ${IMAGE_ROOTFS}/mnt/upper
+    install -d ${IMAGE_ROOTFS}/mnt/work
+    install -d ${IMAGE_ROOTFS}/mnt/root
+
+    # Create console device node (needed before /dev is mounted)
+    mknod -m 622 ${IMAGE_ROOTFS}/dev/console c 5 1 2>/dev/null || true
+}
diff --git a/recipes-containers/vcontainer/vdkr-tiny-initramfs-image.bb b/recipes-containers/vcontainer/vdkr-tiny-initramfs-image.bb
new file mode 100644
index 00000000..42aa8637
--- /dev/null
+++ b/recipes-containers/vcontainer/vdkr-tiny-initramfs-image.bb
@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: Copyright (C) 2025 Bruce Ashfield
+#
+# SPDX-License-Identifier: MIT
+#
+# vdkr-tiny-initramfs-image.bb - Tiny initramfs for vdkr
+#
+# Build with:
+#   bitbake mc:vruntime-aarch64:vdkr-tiny-initramfs-image
+#   bitbake mc:vruntime-x86-64:vdkr-tiny-initramfs-image
+#
+# Output: ${DEPLOY_DIR_IMAGE}/vdkr-tiny-initramfs-image-${MACHINE}.cpio.gz
+
+require vcontainer-tiny-initramfs-image.inc
diff --git a/recipes-containers/vcontainer/vpdmn-tiny-initramfs-image.bb b/recipes-containers/vcontainer/vpdmn-tiny-initramfs-image.bb
new file mode 100644
index 00000000..c407f013
--- /dev/null
+++ b/recipes-containers/vcontainer/vpdmn-tiny-initramfs-image.bb
@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: Copyright (C) 2025 Bruce Ashfield
+#
+# SPDX-License-Identifier: MIT
+#
+# vpdmn-tiny-initramfs-image.bb - Tiny initramfs for vpdmn
+#
+# Build with:
+#   bitbake mc:vruntime-aarch64:vpdmn-tiny-initramfs-image
+#   bitbake mc:vruntime-x86-64:vpdmn-tiny-initramfs-image
+#
+# Output: ${DEPLOY_DIR_IMAGE}/vpdmn-tiny-initramfs-image-${MACHINE}.cpio.gz
+
+require vcontainer-tiny-initramfs-image.inc