Make a simple PID 1 program.

.gitignore

@@ -0,0 +1,4 @@
+p.vim
+result
+bin/
+build/

Makefile

@@ -18,6 +18,13 @@ PCH_RELEASE := $(PCH_DIR_RELEASE)/pch.h.gch
 MRUBY_CFLAGS ?= -I./libs/mruby/build/host/include
 MRUBY_LIBS ?= -L./libs/mruby/build/host/lib -lmruby
 
+BUSYBOX ?= $(shell which busybox)
+
+INITRAMFS_DIR_DEBUG := $(OBJ_DIR)/initramfs-dbg
+INITRAMFS_ARCHIVE_DEBUG := $(BIN_DIR)/initramfs-dbg.cpio.gz
+INITRAMFS_DIR_RELEASE := $(OBJ_DIR)/initramfs
+INITRAMFS_ARCHIVE_RELEASE := $(BIN_DIR)/initramfs.cpio.gz
+
 CFLAGS_DEBUG :=\
 	-std=c++23 -Wall -Wextra -fno-rtti \
 	-Og -fno-inline -gsplit-dwarf -DDEBUG \
@@ -46,9 +53,31 @@ DEP_RELEASE := $(OBJ_RELEASE:.o=.d)
 
 all: release
 
-debug: $(TARGET_DEBUG)
+debug: $(INITRAMFS_ARCHIVE_DEBUG)
 
-release: $(TARGET_RELEASE)
+release: $(INITRAMFS_ARCHIVE_RELEASE)
+
+$(INITRAMFS_ARCHIVE_DEBUG): $(TARGET_DEBUG)
+	@set -eu; \
+	root="$(INITRAMFS_DIR_DEBUG)"; \
+	mkdir -p "$$root/bin" "$$root/dev"; \
+	install -m 0755 "$(TARGET_DEBUG)" "$$root/init"; \
+	install -m 0755 "$(BUSYBOX)" "$$root/bin/busybox"; \
+	ln -sfn busybox "$$root/bin/sh"; \
+	cd "$$root"; \
+	find . -print0 | cpio --null -o --format=newc | gzip -9 > "$(abspath $(INITRAMFS_ARCHIVE_DEBUG))"; \
+	echo "Initramfs generated: $(INITRAMFS_ARCHIVE_DEBUG)"
+
+$(INITRAMFS_ARCHIVE_RELEASE): $(TARGET_RELEASE)
+	@set -eu; \
+	root="$(INITRAMFS_DIR_RELEASE)"; \
+	mkdir -p "$$root/bin" "$$root/dev"; \
+	install -m 0755 "$(TARGET_RELEASE)" "$$root/init"; \
+	install -m 0755 "$(BUSYBOX)" "$$root/bin/busybox"; \
+	ln -sfn busybox "$$root/bin/sh"; \
+	cd "$$root"; \
+	find . -print0 | cpio --null -o --format=newc | gzip -9 > "$(abspath $(INITRAMFS_ARCHIVE_RELEASE))"; \
+	echo "Initramfs generated: $(INITRAMFS_ARCHIVE_RELEASE)"
 
 $(PCH_DEBUG): $(INCLUDE_DIR)/pch.h
 	@mkdir -p $(PCH_DIR_DEBUG)
@@ -67,7 +96,7 @@ $(TARGET_DEBUG): $(PCH_DEBUG) $(OBJ_DEBUG)
 
 $(TARGET_RELEASE): $(PCH_RELEASE) $(OBJ_RELEASE)
 	@mkdir -p $(BIN_DIR)
-	@$(CXX) $(CFLAGS_RELEASE) -o $@ $(OBJ_RELEASE)$(LIBS)
+	@$(CXX) $(CFLAGS_RELEASE) -o $@ $(OBJ_RELEASE) $(LIBS)
 	@strip $@
 	@echo "Release build complete: $@"
 

flake.nix

@@ -59,35 +59,64 @@
           gnumake
           findutils
           binutils
+          cpio
+          gzip
         ];
 
         buildInputs = [
+          pkgs.pkgsStatic.busybox
           mruby
         ];
 
         buildPhase = ''
           make \
             MRUBY_CFLAGS="-I${mruby}/include" \
-            MRUBY_LIBS="-L${mruby}/lib -lmruby"
+            MRUBY_LIBS="-L${mruby}/lib -lmruby" \
+            BUSYBOX="${pkgs.pkgsStatic.busybox}/bin/busybox"
         '';
 
         installPhase = ''
           mkdir -p $out/bin
           cp bin/blade $out/bin/blade
+          cp bin/initramfs.cpio.gz $out/bin/initramfs.cpio.gz
         '';
       };
+
+      run-blade = hostPkgs.writeShellScriptBin "run-blade" ''
+        set -euo pipefail
+        exec ${hostPkgs.qemu}/bin/qemu-system-x86_64 \
+          --enable-kvm \
+          -m 512M \
+          -smp 2 \
+          -kernel "${hostPkgs.linux}/bzImage" \
+          -initrd ${blade}/bin/initramfs.cpio.gz \
+          -append "console=ttyS0 rdinit=/init" \
+          -nographic
+      '';
     in
     {
       packages.${system} = {
         default = blade;
         blade = blade;
         mruby = mruby;
+        run-blade = run-blade;
       };
 
-      devShells.${system}.default = hostPkgs.mkShell {
+      apps.${system} = {
+        default = {
+          type = "app";
+          program = "${run-blade}/bin/run-blade";
+        };
+      };
+
+      devShells.${system}.default = pkgs.mkShell {
         inputsFrom = [ blade ];
 
         packages = with hostPkgs; [
+          linux
+          qemu
+          cpio
+          gzip
           clang-tools
           pkg-config
           ccache
@@ -105,6 +134,8 @@
 
           export MRUBY_CFLAGS=-I${mruby}/include
           export MRUBY_LIBS="-L${mruby}/lib -lmruby"
+          export BUSYBOX="${pkgs.pkgsStatic.busybox}/bin/busybox"
+          export LINUX="${hostPkgs.linux}/bzImage"
         '';
       };
     };

src/init/init.cc

@@ -1,3 +1,9 @@
+#include <sys/reboot.h>
+#include <linux/reboot.h>
+#include <sys/mount.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -5,38 +11,39 @@
 #include <cstdlib>
 #include <iostream>
 
-int main()
-{
-    // PID 1
+int main() {
+  // PID 1
 
-    int console = open("/dev/console", O_RDWR);
+  if (mount("devtmpfs", "/dev", "devtmpfs", 0, nullptr) < 0) {
+    perror("mount /dev");
+    return 1;
+  }
 
-    if (console < 0)
-    {
-        perror("open /dev/console");
-        return 1;
-    }
+  int console = open("/dev/console", O_RDWR);
 
-    dup2(console, STDIN_FILENO);
-    dup2(console, STDOUT_FILENO);
-    dup2(console, STDERR_FILENO);
+  if (console < 0) {
+    perror("open /dev/console");
+    return 1;
+  }
 
-    close(console);
+  dup2(console, STDIN_FILENO);
+  dup2(console, STDOUT_FILENO);
+  dup2(console, STDERR_FILENO);
 
-    while (true)
-    {
-        pid_t pid = fork();
+  close(console);
 
-        if (pid == 0)
-        {
-            execl("/bin/sh", "sh", nullptr);
-            perror("exec");
-            _exit(127);
-        }
+  pid_t pid = fork();
 
-        if (pid > 0)
-        {
-            waitpid(pid, nullptr, 0);
-        }
-    }
+  if (pid == 0) {
+    execl("/bin/sh", "sh", nullptr);
+    perror("exec");
+    _exit(127);
+  }
+
+  if (pid > 0) {
+    waitpid(pid, nullptr, 0);
+  }
+
+  sync();
+  reboot(RB_POWER_OFF);
 }