kstrto

README.adoc

@@ -1239,6 +1239,8 @@ although `-E` is smarter:
 
 so you should almost always use it, unless you are really counting each cycle ;-)
 
+This method prevents the BusyBox' init from launching a shell, so you cannot interact with the system afterwards. If you also want that, use: <<init-busybox>>.
+
 If the script is large, you can add it to a gitignored file and pass that to `-E` as in:
 
 ....
@@ -1286,6 +1288,7 @@ But this fails when we are `init` itself!
 
 but why not just use your super simple and effective `/poweroff.out` and be done with it?
 
+[[init-busybox]]
 === Run command at the end of BusyBox init
 
 If you rely on something that BusyBox' init set up for you like networking, you could do:

kernel_module/README.adoc

@@ -36,6 +36,8 @@
 .. link:character_device.c[]
 .. link:character_device_create.c[]
 .. link:virt_to_phys.c[]
+. Utilities
+.. kstrto
 . Hardware device drivers
 .. link:pci_min.c[]
 .. link:pci.c[]

kernel_module/debugfs.c

@@ -21,6 +21,8 @@ Only the more basic fops can be implemented in debugfs, e.g. mmap is never calle
 static struct dentry *dir, *toplevel_file;
 static u32 value = 42;
 
+/* This basically re-implents the write operation of debugfs_create_u32,
+ * it is just an excuse to illustrate a fop. */
 static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
 {
 	int ret;

kernel_module/kstrto.c

@@ -0,0 +1,53 @@
+/*
+https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel/49811658#49811658
+
+Usage:
+
+	/kstrto.sh
+*/
+
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <uapi/linux/stat.h> /* S_IRUSR */
+
+static struct dentry *toplevel_file;
+
+static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
+{
+	int ret;
+	unsigned long long res;
+	ret = kstrtoull_from_user(buf, len, 10, &res);
+	if (ret) {
+		/* Negative error code. */
+		pr_info("ko = %d\n", ret);
+		return ret;
+	} else {
+		pr_info("ok  = %llu\n", res);
+		*off= len;
+		return len;
+	}
+}
+
+static const struct file_operations fops = {
+	.owner = THIS_MODULE,
+	.write = write,
+};
+
+static int myinit(void)
+{
+	toplevel_file = debugfs_create_file("lkmc_kstrto", S_IWUSR, NULL, NULL, &fops);
+	if (!toplevel_file) {
+		return -1;
+	}
+	return 0;
+}
+
+static void myexit(void)
+{
+	debugfs_remove(toplevel_file);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");

rootfs_overlay/kstrto.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+insmod /kstrto.ko
+cd /sys/kernel/debug
+echo 1234 > lkmc_kstrto
+# 1234
+echo foobar > lkmc_kstrto
+# -22