debugfs.c: extend example with toplevel file, fop and kstrtoull_from_user

kernel_module/debugfs.c

@@ -18,20 +18,49 @@ 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;
 
+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 +69,7 @@ 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;
 }
 

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