kstrto: move doc to README

Author: cirosantilli

README.adoc

@@ -4076,6 +4076,30 @@ The module also shows which handlers are registered for each IRQ, as we have obs
 
 When in text mode, we can also observe interrupt line 4 with handler `ttyS0` increase continuously as IO goes through the UART.
 
+=== Linux kernel utility functions
+
+==== kstrto
+
+Convert a string to an integer:
+
+....
+/kstrto.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/kstrto.c[]
+* link:rootfs_overlay/kstrto.sh[]
+
+Bibliography: https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel/49811658#49811658
+
 === Linux kernel tracing
 
 Good overviews:

kernel_module/README.adoc

@@ -12,8 +12,6 @@ Our kernel modules!
 . Misc
 .. link:virt_to_phys.c[]
 .. link:netlink.c[]
-. Utilities
-.. link:kstrto.c[]
 . Hardening
 .. link:strlen_overflow.c[]
 . Tracing

kernel_module/kstrto.c

@@ -1,36 +1,48 @@
-/*
-https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel/49811658#49811658
-
-Usage:
-
-	/kstrto.sh
-*/
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#kstrto */
 
 #include <linux/debugfs.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
-#include <uapi/linux/stat.h> /* S_IRUSR */
+#include <linux/seq_file.h>
+#include <linux/uaccess.h> /* copy_from_user, copy_to_user */
+#include <uapi/linux/stat.h> /* S_IWUSR */
 
 static struct dentry *toplevel_file;
+static char read_buf[1024];
+
+static int show(struct seq_file *m, void *v)
+{
+	seq_printf(m, read_buf);
+	return 0;
+}
+
+static int open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show, NULL);
+}
 
 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) {
+	ssize_t ret;
+	int kstrto_return;
+	unsigned long long kstrto_result;
+	kstrto_return = kstrtoull_from_user(buf, len, 10, &kstrto_result);
+	if (kstrto_return) {
 		/* Negative error code. */
-		pr_info("ko = %d\n", ret);
-		return ret;
+		ret = kstrto_return;
 	} else {
-		pr_info("ok = %llu\n", res);
-		*off= len;
-		return len;
+		ret = len;
 	}
+	snprintf(read_buf, sizeof(read_buf), "%llu", kstrto_result + 1);
+	return ret;
 }
 
 static const struct file_operations fops = {
+	.llseek = seq_lseek,
+	.open = open,
 	.owner = THIS_MODULE,
+	.read = seq_read,
+	.release = single_release,
 	.write = write,
 };
 

rootfs_overlay/kstrto.sh

@@ -1,7 +1,8 @@
 #!/bin/sh
+set -e
+f=/sys/kernel/debug/lkmc_kstrto
 insmod /kstrto.ko
-cd /sys/kernel/debug
-echo 1234 > lkmc_kstrto
-# 1234
-echo foobar > lkmc_kstrto
-# -22
+printf 123 > "$f"
+[ "$(cat "$f")" = 124 ]
+echo foobar > "$f" && exit 1
+rmmod kstrto

rootfs_overlay/test_all.sh

@@ -8,6 +8,7 @@ for test in \
   /fops.sh \
   /init_module.sh \
   /ioctl.sh \
+  /kstrto.sh \
   /mmap.sh \
   /params.sh \
   /procfs.sh \