title:User mode helper
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