kernel modules: add a quick scull port from LDD3

Also:

* fix fops.c on both kernels:
  * 5.9: the out of space error code was 1 not 8
  * 6.6: for whatever reason we can't read the user buffer as before on the
         diagnostic print, it leads to segfault and oops
* create memfile.c which is like fops.c but of unlimited size

kernel_modules/memfile.c

@@ -0,0 +1,189 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#fops */
+
+#include <linux/debugfs.h>
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h> /* file_operations */
+#include <linux/kernel.h> /* min */
+#include <linux/module.h>
+#include <linux/printk.h> /* printk */
+#include <linux/string.h> /* strcpy */
+#include <linux/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/rwsem.h>
+#include <uapi/linux/stat.h> /* S_IRUSR */
+
+/* Params */
+static int log = 0;
+module_param(log, int, S_IRUSR | S_IWUSR);
+MODULE_PARM_DESC(log, "enable logging");
+
+/* Dynamic array: https://stackoverflow.com/questions/3536153/c-dynamically-growing-array */
+
+typedef struct {
+	char *buf;
+	size_t used;
+	size_t _size;
+} dyn_arr_t;
+
+int dyn_arr_init(dyn_arr_t *a, size_t size);
+int dyn_arr_init(dyn_arr_t *a, size_t size)
+{
+	a->buf = kvzalloc(size, GFP_KERNEL);
+	if (!a->buf)
+		return -ENOMEM;
+	a->used = 0;
+	a->_size = size;
+	return 0;
+}
+
+/* Reserve the required space for a future data insertion of size len at offset off.
+ * We don't do the actual insertion here as there are multiple possible insertion methods
+ * e.g. copy_from_user or strcpy.
+ */
+int dyn_arr_reserve(dyn_arr_t *a, size_t off, size_t len);
+int dyn_arr_reserve(dyn_arr_t *a, size_t off, size_t len)
+{
+	size_t new_used, new_size;
+	
+	new_used = off + len;
+	if (new_used > a->_size) {
+		new_size = new_used * 2;
+		a->buf = kvrealloc(a->buf, a->_size, new_size, GFP_KERNEL);
+		if (!a->buf)
+			return -ENOMEM;
+		a->_size = new_size;
+	}
+	if (off > a->used)
+		memset(a->buf + a->used, '\0', off - a->used);
+	if (new_used > a->used)
+		a->used = new_used;
+	if (log) pr_info("dyn_arr_reserve _size:=%zu used:=%zu\n", a->_size, a->used);
+	return 0;
+}
+
+void dyn_arr_free(dyn_arr_t *a);
+void dyn_arr_free(dyn_arr_t *a)
+{
+	kvfree(a->buf);
+	a->buf = NULL;
+	a->used = 0;
+	a->_size = 0;
+}
+
+/* Globals. */
+static dyn_arr_t data;
+static struct dentry *debugfs_file;
+struct rw_semaphore rwsem;
+
+static int open(struct inode *inode, struct file *filp)
+{
+	if (log) pr_info("open\n");
+	if ((filp->f_flags & O_TRUNC)) {
+		if (log) pr_info("open O_TRUNC\n");
+		data.used = 0;
+	} else if ((filp->f_flags & O_APPEND)) {
+		if (log) pr_info("open O_APPEND\n");
+		filp->f_pos = data.used;
+	}
+	return 0;
+}
+
+static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
+{
+	ssize_t ret;
+
+	if (log) pr_info("read len=%zu off=%lld\n", len, (long long)*off);
+	down_read(&rwsem);
+	if (data.used <= *off) {
+		ret = 0;
+	} else {
+		ret = min(len, data.used - (size_t)*off);
+		if (copy_to_user(buf, data.buf + *off, ret)) {
+			ret = -EFAULT;
+		} else {
+			*off += ret;
+		}
+	}
+	up_read(&rwsem);
+	if (log) pr_info("read ret:=%lld\n", (long long)ret);
+	return ret;
+}
+
+static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
+{
+	ssize_t ret;
+
+	if (log) pr_info("write len=%zu off=%lld\n", len, (long long)*off);
+	down_write(&rwsem);
+	dyn_arr_reserve(&data, *off, len);
+	if (copy_from_user(data.buf + *off, buf, len)) {
+		ret = -EFAULT;
+	} else {
+		ret = len;
+		*off += ret;
+	}
+	up_write(&rwsem);
+	if (log) pr_info("write ret:=%lld\n", (long long)ret);
+	return ret;
+}
+
+static int release(struct inode *inode, struct file *filp)
+{
+	if (log) pr_info("release\n");
+	return 0;
+}
+
+static loff_t llseek(struct file *filp, loff_t off, int whence)
+{
+	loff_t newpos;
+
+	if (log) pr_info("llseek off=%lld whence=%lld\n", (long long)off, (long long)whence);
+	switch(whence) {
+		case SEEK_SET:
+			newpos = off;
+			break;
+		case SEEK_CUR:
+			newpos = filp->f_pos + off;
+			break;
+		case SEEK_END:
+			newpos = data.used + off;
+			break;
+		default:
+			return -EINVAL;
+	}
+	if (newpos < 0) return -EINVAL;
+	filp->f_pos = newpos;
+	if (log) pr_info("llseek newpos:=%lld\n", (long long)newpos);
+	return newpos;
+}
+
+static const struct file_operations fops = {
+	.owner = THIS_MODULE,
+	.llseek = llseek,
+	.open = open,
+	.read = read,
+	.release = release,
+	.write = write,
+};
+
+static int myinit(void)
+{
+	int ret;
+	
+	ret = dyn_arr_init(&data, 1);
+	if (ret)
+		return ret;
+	init_rwsem(&rwsem);
+	debugfs_file = debugfs_create_file("lkmc_memfile",
+		S_IRUSR | S_IWUSR, NULL, NULL, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	dyn_arr_free(&data);
+	debugfs_remove_recursive(debugfs_file);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");