From 91ecd00cc83cf701fe7f73b1aa244c27174e8386 Mon Sep 17 00:00:00 2001
From: Kacper <kacper@mail.openlinux.dev>
Date: Tue, 16 Dec 2025 00:36:30 +0100
Subject: Add init prog, chroot syscall and fix menuconfig build

---
 boot/Kbuild      |   1 +
 boot/init/Kbuild |   3 ++
 boot/init/init   | Bin 0 -> 145432 bytes
 boot/init/init.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 130 insertions(+)
 create mode 100644 boot/Kbuild
 create mode 100644 boot/init/Kbuild
 create mode 100755 boot/init/init
 create mode 100644 boot/init/init.c

(limited to 'boot')

diff --git a/boot/Kbuild b/boot/Kbuild
new file mode 100644
index 00000000..01daa04b
--- /dev/null
+++ b/boot/Kbuild
@@ -0,0 +1 @@
+obj-y += init/
diff --git a/boot/init/Kbuild b/boot/init/Kbuild
new file mode 100644
index 00000000..461a0101
--- /dev/null
+++ b/boot/init/Kbuild
@@ -0,0 +1,3 @@
+bin-y := init
+obj-y := init.o
+libs-y := $(srctree)/lib/libc/libc.a
diff --git a/boot/init/init b/boot/init/init
new file mode 100755
index 00000000..87e091d3
Binary files /dev/null and b/boot/init/init differ
diff --git a/boot/init/init.c b/boot/init/init.c
new file mode 100644
index 00000000..4e643a34
--- /dev/null
+++ b/boot/init/init.c
@@ -0,0 +1,126 @@
+#include <dirent.h>
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/fs.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+struct rootfs {
+	char *dev;
+	int flags;
+};
+
+int get_rootfs(struct rootfs *root)
+{
+	int fd;
+	ssize_t len;
+	char buf[1024];
+
+	root->dev = NULL;
+	root->flags = 0;
+
+	fd = open("/proc/cmdline", O_RDONLY);
+	if (fd < 0) {
+		return -1;
+	}
+
+	len = read(fd, buf, sizeof(buf) - 1);
+	if (len < 0) {
+		close(fd);
+		return -1;
+	}
+	buf[len] = '\0';
+	close(fd);
+
+	// Read cmdline by words
+	// not using strtok
+	char *p = buf;
+	while (*p) {
+		while (*p == ' ')
+			p++;
+
+		if (*p == '\0')
+			break;
+
+		char *start = p;
+		while (*p && *p != ' ')
+			p++;
+		size_t word_len = p - start;
+
+		if (strncmp(start, "root=", 5) == 0) {
+			size_t dev_len = word_len - 5;
+			root->dev = strndup(start + 5, dev_len);
+		} else if (strncmp(start, "ro", 2) == 0) {
+			root->flags |= MS_RDONLY;
+		} else if (strncmp(start, "rw", 2) == 0) {
+			root->flags &= ~MS_RDONLY;
+		}
+	}
+
+	return 0;
+}
+
+int main(void)
+{
+	struct rootfs root;
+	char *const argv[] = { "/bin/init", NULL };
+
+	if (mkdir("/dev", 0755) < 0 && errno != EEXIST) {
+		perror("mkdir /dev");
+		return -1;
+	}
+
+	if (mount("devtmpfs", "/dev", "devtmpfs", 0, NULL) < 0) {
+		perror("mount devtmpfs");
+		return -1;
+	}
+
+	if (mkdir("/proc", 0755) < 0 && errno != EEXIST) {
+		perror("mkdir /proc");
+		return -1;
+	}
+
+	if (mount("proc", "/proc", "proc", 0, NULL) < 0) {
+		perror("mount proc");
+		return -1;
+	}
+
+	/* Get root filesystem info from kernel cmdline */
+	if (get_rootfs(&root) < 0) {
+		err(1, "init: get rootfs info");
+	}
+
+	if (mount(root.dev, "/root", "ext4", root.flags, NULL) < 0)
+		err(1, "init: mount rootfs %s", root.dev);
+
+	if (mkdir("/root/dev", 0755) < 0)
+		err(1, "init: mkdir /root/dev");
+
+	if (mount("/dev", "/root/dev", NULL, MS_BIND | MS_REC, NULL) < 0)
+		err(1, "init: mount --bind /dev");
+
+	if (mkdir("/root/proc", 0755) < 0)
+		err(1, "init: chdir /root");
+
+	if (mount("/proc", "/root/proc", NULL, MS_BIND | MS_REC, NULL) < 0)
+		err(1, "init: mount --bind /proc");
+
+	if (mkdir("/root/sys", 0755) < 0)
+		err(1, "init: chdir /root");
+
+	if (mount("sysfs", "/root/sys", "sysfs", 0, NULL) < 0)
+		err(1, "init: mount --bind /sys");
+
+	if (chroot("/root") < 0)
+		err(1, "init: chroot /root");
+
+	if (chdir("/") < 0)
+		err(1, "init: chdir /");
+
+	execve("/bin/init", argv, NULL);
+	err(1, "init: execve /bin/init");
+}
-- 
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