Merge branch 'master' of github.com:cirosantilli/linux-kernel-module-cheat

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

@@ -9,7 +9,7 @@
 .. link:params.c[]
 .. link:vermagic.c[]
 .. link:vermagic_fail.c[]
-.. link:module_version.c[]
+.. link:module_info.c[]
 . Pseudo filesystems
 .. link:debugfs.c[]
 .. link:procfs.c[]
@@ -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

@@ -18,20 +18,51 @@ Only the more basic fops can be implemented in debugfs, e.g. mmap is never calle
 #include <linux/module.h>
 #include <uapi/linux/stat.h> /* S_IRUSR */
 
-static struct dentry *dir;
+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;
+	unsigned long long res;
+	/* https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel */
+	ret = kstrtoull_from_user(buf, len, 10, &res);
+	if (ret) {
+		/* Negative error code. */
+		return ret;
+	} else {
+		value = res;
+		*off= len;
+		return len;
+	}
+}
+
+static const struct file_operations fops = {
+	.owner = THIS_MODULE,
+	.write = write,
+};
+
 static int myinit(void)
 {
 	struct dentry *file;
 	dir = debugfs_create_dir("lkmc_debugfs", 0);
 	if (!dir) {
-		printk(KERN_ALERT "debugfs_create_dir failed");
+		pr_alert("debugfs_create_dir failed");
 		return -1;
 	}
-	file = debugfs_create_u32("myfile", S_IRUSR, dir, &value);
+	file = debugfs_create_u32("myfile", S_IRUSR | S_IWUSR, dir, &value);
 	if (!file) {
-		printk(KERN_ALERT "debugfs_create_u32 failed");
+		pr_alert("debugfs_create_u32 failed");
+		return -1;
+	}
+
+	/* Created on the toplevel of the debugfs mount,
+	 * and with explicit fops instead of a fixed integer value. */
+	toplevel_file = debugfs_create_file(
+		"lkmc_debugfs_file", S_IWUSR, NULL, NULL, &fops);
+	if (!toplevel_file) {
 		return -1;
 	}
 	return 0;
@@ -40,6 +71,7 @@ static int myinit(void)
 static void myexit(void)
 {
 	debugfs_remove_recursive(dir);
+	debugfs_remove(toplevel_file);
 }
 
 module_init(myinit)

kernel_module/hello.c

@@ -14,6 +14,9 @@ Hello world module.
 static int myinit(void)
 {
 	pr_info("hello init\n");
+	/* 0 for success, any negative value means failure,
+	 * E* consts if you want to specify failure cause.
+	 * https://www.linux.com/learn/kernel-newbie-corner-loadable-kernel-modules-coming-and-going */
 	return 0;
 }
 

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");

kernel_module/module_info.c

@@ -0,0 +1,32 @@
+/*
+- https://stackoverflow.com/questions/4839024/how-to-find-the-version-of-a-compiled-kernel-module/42556565#42556565
+- https://stackoverflow.com/questions/19467150/significance-of-this-module-in-linux-driver/49812248#49812248
+
+Usage:
+
+    insmod /module_info.ko
+	# dmesg => name    = module_info
+	# dmesg => version = 1.0
+    cat /sys/module/module_info/version
+    # => module_info
+    modinfo /module_info.ko | grep -E '^version:'
+    # => version:        1.0
+*/
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int myinit(void)
+{
+	/* Set by default based on the module file name. */
+	pr_info("name    = %s\n", THIS_MODULE->name);
+	pr_info("version = %s\n", THIS_MODULE->version);
+	return 0;
+}
+
+static void myexit(void) {}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_VERSION("1.0");
+MODULE_LICENSE("GPL");

kernel_module/module_version.c

@@ -1,28 +0,0 @@
-/*
-https://stackoverflow.com/questions/4839024/how-to-find-the-version-of-a-compiled-kernel-module/42556565#42556565
-
-    insmod /module_version.ko
-    cat /sys/modules/module_version/version
-    # => 1.0
-    cat /sys/module/module_version/srcversion
-    # => AB0F06618BC3A36B687CDC5
-    modinfo /module_version.ko | grep -E '^(src|)version'
-    # => version:        1.0
-    # => srcversion:     AB0F06618BC3A36B687CDC5
-*/
-
-#include <linux/module.h>
-#include <linux/kernel.h>
-
-static int myinit(void)
-{
-	pr_info(__FILE__ "\n");
-	return 0;
-}
-
-static void myexit(void) {}
-
-module_init(myinit)
-module_exit(myexit)
-MODULE_VERSION("1.0");
-MODULE_LICENSE("GPL");

rootfs_overlay/debugfs.sh

@@ -1,12 +1,22 @@
 #!/bin/sh
 set -ex
-mkdir -p /debugfs
+d=/debugfs
+mkdir -p "$d"
 # We also added a fstab entry that mounts this under /sys/kernel/debug autmoatically.
 # That is the most common place to mount it.
 # The /sys/kernel/debug directory gets created automatically when debugfs is
 # compiled into the kernel, but it does not get mounted automatically.
 mount -t debugfs none /debugfs
 insmod /debugfs.ko
-cd /debugfs/lkmc_debugfs
+cd "${d}/lkmc_debugfs"
+
 cat myfile
 # => 42
+
+echo 13 > myfile
+cat myfile
+# => 13
+
+echo 666 > "${d}/lkmc_debugfs_file"
+cat myfile
+# => 666

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