From 660438d0f123d87c04dea1eef26a4167123e3390 Mon Sep 17 00:00:00 2001
From: Arturs Artamonovs <dos21h@gmail.com>
Date: Tue, 12 Dec 2023 22:22:28 +0000
Subject: Added notes how to update kernel for raspi4. Usermode helper ready

---
 md/notes/kernel/usermode_helper.md | 164 +++++++++++++++++++++++++++++++++++++
 1 file changed, 164 insertions(+)

(limited to 'md/notes/kernel')

diff --git a/md/notes/kernel/usermode_helper.md b/md/notes/kernel/usermode_helper.md
index dd8155a..5ac7532 100644
--- a/md/notes/kernel/usermode_helper.md
+++ b/md/notes/kernel/usermode_helper.md
@@ -3,6 +3,170 @@ keywords:kernel,linux,threads
 
 # User mode helper
 
+## Adding usermode helper
+
+Here is diff of added new usermode helper
+
+```diff
+diff --git a/kernel/sysctl.c b/kernel/sysctl.c
+index c6d9dec11..5d0b5b0ea 100644
+--- a/kernel/sysctl.c
++++ b/kernel/sysctl.c
+@@ -82,6 +82,8 @@
+ #include <linux/rtmutex.h>
+ #endif
+ 
++#include <linux/hacks.h>
++
+ /* shared constants to be used in various sysctls */
+ const int sysctl_vals[] = { 0, 1, 2, 3, 4, 100, 200, 1000, 3000, INT_MAX, 65535, -1 };
+ EXPORT_SYMBOL(sysctl_vals);
+@@ -2091,6 +2093,13 @@ static struct ctl_table kern_table[] = {
+                .extra2         = SYSCTL_INT_MAX,
+        },
+ #endif
++       {
++               .procname       = "secret_parth",
++                .data           = &sysctl_secret_path,
++                .maxlen         = sizeof(sysctl_secret_path),
++                .mode           = 0644,
++                .proc_handler   = proc_dostring,
++               },
+        { }
+ };
+ ```
+
+create file __hacks.h__ under the __include/linux/hacks.h__
+
+```c
+static char sysctl_secret_path[256] = "/usr/bin/utility";
+```
+
+Add to kernel and rebuild then run system with new kernel.
+After reboot there should be file at __/proc/sys/kernel__
+```sh
+/proc/sys/kernel/sysctl_secret_path
+```
+
+Now we can set values with echo and when kernel module executes the path will be executed
+
+```sh
+echo "/usr/bin/utility" > /proc/sys/kernel/sysctl_secret_path
+```
+
+Lets write utility that dumps time to file at /tmp. 
+
+Dump time utility
+```c
+#include <stdio.h>
+#include <time.h>
+#include <string.h>
+
+int main() {
+	FILE *f=NULL;
+	time_t t;
+	struct tm *tm;
+	char str[128];
+
+	f = fopen("/tmp/date","w+");
+	if (f == NULL) {
+		printf("cannot create file\n");
+	}
+	printf("Start utility\n");
+	printf("Date today\n");
+
+	time(&t);
+	tm = localtime(&t);
+	strftime(str, sizeof(str), "%A %c\n", tm);
+	printf("Date %s\n", str);
+
+	printf("Dump to file /tmp/date\n");
+	
+	fwrite(str, strlen(str), 1, f);
+
+	
+
+	printf("Stop utility\n");
+
+	fclose(f);
+
+	return 0;
+}
+```
+
+Compile and copy under the /usr/bin directory
+
+Call usermod helper from kernel module
+
+## Kernel module starting point
+
+__usermod_helper.c__
+```c
+//http://www.tldp.org/LDP/lkmpg/2.4/html/c147.htm
+#include <linux/module.h>  /* Needed by all modules */
+#include <linux/kernel.h>
+
+int usermod_helper_init( void )
+{
+	printk(KERN_DEBUG "Hello World!\n");
+	return 0;
+}
+
+void usermod_helper_exit( void )
+{
+	printk(KERN_DEBUG "Exit Hello World!\n");
+}
+
+module_init( usermod_helper_init );
+module_exit( usermod_helper_exit );
+
+MODULE_LICENSE("GPL");
+```
+
+
+Call usermode helper from a driver
+
+```c
+...
+#include <linux/hacks.h>
+...
+
+void run_usermode_helper(void) {
+       char *argv[1];
+       char *envp[3];
+       int ret;
+
+       //no args
+       argv[0] = 0;
+
+       //basic env
+       envp[0] = "HOME=/";
+       envp[1] = "PATH=/sbin:/usr/sbin:/usr/bin";
+       envp[2] = 0;
+
+       ret = call_usermodehelper(argv[0], argv, envp, 0);
+}
+
+int usermode_helper_init( void )
+{
+    printk(KERN_DEBUG "Hello usermode!\n");
+
+    run_usermode_helper();
+
+    return 0;
+}
+```
+
+Run 
+```sh
+ sudo insmod usermode_helper.ko
+```
+
+Now there should be file /tmp/date that is created with utility called by driver
 
 ## Links
 
+https://www.raspberrypi.com/documentation/computers/linux_kernel.html#updating-your-kernel  
+
+
+
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