Support mounting rootfs from virtio-blk via initrd

Due to the 1 GiB kernel space limit on RV32-Linux, embedding large root
filesystems in the kernel image can be impractical for certain testing
scenarios.

This commit introduces a script to generate a mountable ext4 rootfs image and
modifies the init script to support switching the root filesystem to a
virtio-blk device.

During boot, the init script checks whether /dev/vda contains a valid rootfs.
If so, it switches the root to the virtio-blk device. Otherwise, it falls back
to the default initramfs.

Author: shengwen-tw

scripts/build-image.sh

@@ -24,6 +24,7 @@ function do_buildroot
     ASSERT git clone https://github.com/buildroot/buildroot -b 2024.11.1 --depth=1
     cp -f configs/buildroot.config buildroot/.config
     cp -f configs/busybox.config buildroot/busybox.config
+    cp -f target/init buildroot/fs/cpio/init
     # Otherwise, the error below raises:
     #   You seem to have the current working directory in your
     #   LD_LIBRARY_PATH environment variable. This doesn't work.

scripts/rootfs_ext4.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/bash
+
+ROOTFS_CPIO="rootfs_full.cpio"
+IMG="rootfs.ext4.img"
+IMG_SIZE=$((1024 * 1024 * 1024)) # 1GB
+IMG_SIZE_BLOCKS=$((${IMG_SIZE} / 4096)) # IMG_SIZE / 4k
+
+DIR=rootfs
+
+echo "[*] Remove old rootfs directory ..."
+rm -rf $DIR
+mkdir -p $DIR
+
+echo "[*] Extract CPIO"
+pushd $DIR
+cpio -idmv < ../$ROOTFS_CPIO 
+popd
+
+echo "[*] Create empty image"
+dd if=/dev/zero of=${IMG} bs=4k count=${IMG_SIZE_BLOCKS}
+
+echo "[*] Create ext4 rootfs image"
+fakeroot mkfs.ext4 -F ${IMG} -d $DIR
+
+# Show image size
+du -h ${IMG}

target/init

@@ -0,0 +1,41 @@
+#!/bin/sh
+export PATH=/bin:/sbin:/usr/bin:/usr/sbin
+
+ROOTFS_DEV=/dev/vda
+
+mount -t devtmpfs devtmpfs /dev
+mount -t proc none /proc
+mount -t sysfs none /sys
+
+# use the /dev/console device node from devtmpfs if possible to not
+# confuse glibc's ttyname_r().
+# This may fail (e.g., booted with console=), and errors from exec will
+# terminate the shell, so use a subshell for the test
+if (exec 0</dev/console) 2>/dev/null; then
+    exec 0</dev/console
+    exec 1>/dev/console
+    exec 2>/dev/console
+fi
+
+echo "[*] Attempting to mount $ROOTFS_DEV"
+mkdir -p /mnt
+
+mount -t ext4 $ROOTFS_DEV /mnt
+if [ $? -ne 0 ]; then
+    echo "[!] Failed to mount $ROOTFS_DEV. Using initramfs."
+    exec /sbin/init "$@"
+fi
+
+echo "[*] $ROOTFS_DEV mounted successfully. Checking for root filesystem..."
+if [ -x /mnt/sbin/init ]; then
+    echo "[*] Valid root filesystem found. Switching root to $ROOTFS_DEV"
+    #cd /mnt
+    mount --move /sys  /mnt/sys
+    mount --move /proc /mnt/proc
+    mount --move /dev  /mnt/dev
+    exec switch_root -c /dev/console /mnt /sbin/init "$@"
+else
+    echo "[!] No valid root filesystem found on $ROOTFS_DEV. Using initramfs."
+    umount /mnt
+    exec /sbin/init "$@"
+fi