Linux Kernel Module : Problem with kernel_write function - c

Question

I have a problem using kernel_write function while I am developing LKM for Linux 4.14.73 version.
This is the part of my module where I am facing the issue :
void change_led_state(char *led_path, int led_value)
{
printk("we are in change_led_state\n");
size_t len = sizeof(led_value);
char lpath[64];
ssize_t rk=0;
strncpy(lpath, led_path, sizeof(lpath) - 1);
lpath[sizeof(lpath) - 1] = '\0';
f_led = filp_open(lpath, O_WRONLY , 0);
if (f_led == NULL) {
printk("Unable to access led\n");
return;
}
rk = kernel_write(f_led, led_value, len, &f_led->f_pos);
printk("rk = %d\n", rk);
filp_close(f_led, NULL);
}
rk is usually getting a negative value. I think it must be positive. What's wrong with the function please ? How can I correct it ?
For more informations, I have the same code but in user-space program, and it's working fine, I just want to replace it to kernel space. This is the user space program :
void change_led_state(char *led_path, int led_value)
{
char lpath[64];
FILE *led_fd;
strncpy(lpath, led_path, sizeof(lpath) - 1);
lpath[sizeof(lpath) - 1] = '\0';
led_fd = fopen(lpath, "w");
if (led_fd == NULL) {
fprintf(stderr, "simplekey: unable to access led\n");
return;
}
fprintf(led_fd, "%d\n", led_value);
fclose(led_fd);
}
void reset_leds(void)
{
change_led_state(LED_PATH "/" green "/brightness", 0);
}
void eval_keycode(int code)
{
static int green_state = 0;
switch (code) {
case 260:
printf("BTN pressed\n");
// figure out red state
green_state = green_state ? 0 : 1;
change_led_state(LED_PATH "/" green "/brightness", green_state);
break;
default :
printf("Wrong Button was pressed\n");
break;
}
}
In the main fuction I just call eval_keycode function and it's working. What mistakes I have made ?

Answer 1

In your call:
rk = kernel_write(f_led, led_value, len, &f_led->f_pos);
The second parameter is supposed to be a char *, but you are passing an int.
In addition, when writing to the "/sys/class/leds/[device]/brightness" file, the code that handles this expects a pointer to a string of decimal digits, terminated by some non-digit character (such as a newline) or a null terminator.
You need to "print" your led_value to a char array as a decimal number and write the array contents to the file:
char led_valstr[20]; /* should be large enough */
len = scnprintf(led_valstr, sizeof(led_valstr), "%d\n", led_value);
rk = kernel_write(f_led, led_valstr, len, &f_led->f_pos);
(EDIT: corrected call to scnprintf.)