ioctl example complete!

kernel_module/ioctl.c

@@ -1,13 +1,12 @@
 /*
-Give integer and a pointer to the kernel, and get one positive integer out.
+Input: an integer (with some annoying restrictions) and a pointer
 
-Feels like an archaic API... so many weird restrictions for something that could be so simple!
+Output:
 
-TODO: must the ioctl numbers be globally unique? How to ensure that?
+- positive integer return value, which for sanity should only be used with negative values for success
+- the input pointer data may be overwritten to contain output
 
-Ioctl is super picky about the ioctl numbers, it is very annoying: https://stackoverflow.com/questions/10071296/ioctl-is-not-called-if-cmd-2
-
-See how do_vfs_ioctl highjacks several values. This "forces" use to use the _IOx macros...
+Feels like an archaic API... so many weird restrictions and types for something that could be so simple!
 
 Documentation/ioctl/ioctl-number.txt has some info:
 
@@ -17,18 +16,54 @@ Documentation/ioctl/ioctl-number.txt has some info:
     _IOWR  an ioctl with both write and read parameters.
 */
 
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/printk.h> /* printk */
 
+#include "ioctl.h"
+
 MODULE_LICENSE("GPL");
 
 static struct dentry *dir;
 
-static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long argp)
 {
-	pr_info("cmd = %u\n", cmd);
-	return cmd + 1;
+	void __user *arg_user;
+	union {
+		int i;
+		lkmc_ioctl_struct s;
+	} arg_kernel;
+
+	arg_user = (void __user *)argp;
+	pr_info("cmd = %x\n", cmd);
+	switch (cmd) {
+		case LKMC_IOCTL_INC:
+			if (copy_from_user(&arg_kernel.i, arg_user, sizeof(arg_kernel.i))) {
+				return -EFAULT;
+			}
+			pr_info("0 arg = %d\n", arg_kernel.i);
+			arg_kernel.i += 1;
+			if (copy_to_user(arg_user, &arg_kernel.i, sizeof(arg_kernel.i))) {
+				return -EFAULT;
+			}
+		break;
+		case LKMC_IOCTL_INC_DEC:
+			if (copy_from_user(&arg_kernel.s, arg_user, sizeof(arg_kernel.s))) {
+				return -EFAULT;
+			}
+			pr_info("1 arg = %d %d\n", arg_kernel.s.i, arg_kernel.s.j);
+			arg_kernel.s.i += 1;
+			arg_kernel.s.j -= 1;
+			if (copy_to_user(arg_user, &arg_kernel.s, sizeof(arg_kernel.s))) {
+				return -EFAULT;
+			}
+		break;
+		default:
+			return -EINVAL;
+		break;
+	}
+	return 0;
 }
 
 static const struct file_operations fops = {

kernel_module/ioctl.h

@@ -3,6 +3,30 @@
 
 #include <linux/ioctl.h>
 
-#define IOCTL0 _IOW(123, 0, char *)
+/* Structs are the way to pass multiple arguments. */
+typedef struct {
+	int i;
+	int j;
+} lkmc_ioctl_struct;
+
+/* Some random number I can't understand how to choose. */
+#define LKMC_IOCTL_MAGIC 0x33
+
+/*
+ * I think those number does not *need* to be unique across, that is just to help debugging:
+ * https://stackoverflow.com/questions/22496123/what-is-the-meaning-of-this-macro-iormy-macig-0-int
+ *
+ * However, the ioctl syscall highjacks several low values at do_vfs_ioctl, e.g.
+ * This "forces" use to use the _IOx macros...
+ * https://stackoverflow.com/questions/10071296/ioctl-is-not-called-if-cmd-2
+ *
+ * Some of those magic low values are used for fnctl, which can also be used on regular files:
+ * e.g. FIOCLEX for close-on-exec:
+ * https://stackoverflow.com/questions/6125068/what-does-the-fd-cloexec-fcntl-flag-do
+ *
+ * TODO are the W or R of _IOx and type functional, or only to help with uniqueness?
+ * */
+#define LKMC_IOCTL_INC     _IOWR(LKMC_IOCTL_MAGIC, 0, int)
+#define LKMC_IOCTL_INC_DEC _IOWR(LKMC_IOCTL_MAGIC, 1, lkmc_ioctl_struct)
 
 #endif

kernel_module/user/ioctl.c

@@ -12,7 +12,8 @@
 
 int main(int argc, char **argv)
 {
-	int fd, ret;
+	int fd, arg_int, ret;
+	lkmc_ioctl_struct arg_struct;
 
 	if (argc < 4) {
 		puts("Usage: ./prog <ioctl-file> <cmd> <arg>");
@@ -23,13 +24,32 @@ int main(int argc, char **argv)
 		perror("open");
 		return EXIT_FAILURE;
 	}
-	ret = ioctl(fd, IOCTL0, argv[3]);
-	if (ret == -1) {
-		perror("ioctl");
-		return EXIT_FAILURE;
+	/* 0 */
+	{
+		arg_int = 1;
+		ret = ioctl(fd, LKMC_IOCTL_INC, &arg_int);
+		if (ret == -1) {
+			perror("ioctl");
+			return EXIT_FAILURE;
+		}
+		printf("arg = %d\n", arg_int);
+		printf("ret = %d\n", ret);
+		printf("errno = %d\n", errno);
+	}
+	puts("");
+	/* 1 */
+	{
+		arg_struct.i = 1;
+		arg_struct.j = 1;
+		ret = ioctl(fd, LKMC_IOCTL_INC_DEC, &arg_struct);
+		if (ret == -1) {
+			perror("ioctl");
+			return EXIT_FAILURE;
+		}
+		printf("arg = %d %d\n", arg_struct.i, arg_struct.j);
+		printf("ret = %d\n", ret);
+		printf("errno = %d\n", errno);
 	}
-	printf("ret = %d\n", ret);
-	printf("errno = %d\n", errno);
 	close(fd);
 	return EXIT_SUCCESS;
 }