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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
|
