title:Create sysfs entry
keywords:kernel,linux,threads

# Create sysfs entry

## Starting point

__Makefile__
```Makefile
obj-m += sysfs_entry.o

KDIR ?= /lib/modules/$(shell uname -r)/build

all:
	make -C $(KDIR) M=$(PWD) modules

clean:
	make -C $(KDIR) M=$(PWD) clean
```

__sysfs_entry.c__
```c
#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>

int sysfs_entry_init( void )
{
	printk(KERN_DEBUG "Hello Sysfs!\n");
	return 0;
}

void sysfs_entry_exit( void )
{
	printk(KERN_DEBUG "Exit Sysfs!\n");
}

module_init( sysfs_entry_init );
module_exit( sysfs_entry_exit );

MODULE_LICENSE("GPL");
```

## Creating entries

### Kobject structure

```c
struct kobject {
	const char		*name;
	struct list_head	entry;
	struct kobject		*parent;
	struct kset		*kset;
	const struct kobj_type	*ktype;
	struct kernfs_node	*sd; /* sysfs directory entry */
	struct kref		kref;
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
	struct delayed_work	release;
#endif
	unsigned int state_initialized:1;
	unsigned int state_in_sysfs:1;
	unsigned int state_add_uevent_sent:1;
	unsigned int state_remove_uevent_sent:1;
	unsigned int uevent_suppress:1;
};
```

### Add entry to /sys/kernel

__sysfs_entry.c__
```c
struct kobject *sysfs_kobj;

int sysfs_entry_init( void )
{
	...
	
	sysfs_kobj = kobject_create_and_add("secret", kernel_kobj);

	...
	return 0;
}

void sysfs_entry_exit( void )
{
	...

	kobject_put(sysfs_kobj);
}

```

When loading module with 
```sh
sudo insmod sysfs_entry.ko
```

There should be new directory under /sys/kernel called secret.

### Add file under the directory

__sysfs_entry.c__
```c

...
volatile int secret_value;

struct kobject *sysfs_kobj;
struct kobj_attribute secret_attr = __ATTR(secret_value, 0660, NULL, NULL);
...

int sysfs_entry_init( void )
{
	...
	
	if(sysfs_create_file(sysfs_kobj, &secret_attr.attr)) {
		pr_err("Cant create secret file");
	}


	...
	return 0;
}

void sysfs_entry_exit( void )
{
	...
	sysfs_remove_file(sysfs_kobj, &secret_attr.attr);
	...
	
}
```

Now we are able to find secret_value file and attempt to read values

```sh
sudo cat /sys/kernel/secret/secret_value 
cat: /sys/kernel/secret/secret_value: Input/output error
```

### Read write value from the syfs entry

__sysfs_entry.c__
```c
...
static ssize_t secret_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) {
	printk(KERN_DEBUG "Show value %d\n", secret_value);
	return sprintf(buf, "%d", secret_value);
}

static ssize_t secret_store(struct kobject *kobj, struct kobj_attribute *attr,const char *buf, size_t count) {
	printk(KERN_DEBUG "Set value %s %ldd\n", buf, count);
	sscanf(buf, "%d", &secret_value);
	return count;
}

struct kobj_attribute secret_attr = __ATTR(secret_value, 0660, secret_show, secret_store);
...

int sysfs_entry_init( void )
{
	...
	
	...
	return 0;
}

void sysfs_entry_exit( void )
{
	...
	sysfs_remove_file(sysfs_kobj, &secret_attr.attr);
	kobject_put(sysfs_kobj);
	...
}
```

Now we are able to set and read values from sys file


```sh
sudo cat /sys/kernel/secret/secret_value
```
or
```sh
echo 1 | sudo tee -a /sys/kernel/secret/secret_value
```

## List of functions used

| Function name | Note |
|---|---|
| kobject_create_and_add |  |
| sysfs_create_file |  |
| sysfs_remove_file |  |
| kobject_put |  |
|  |  |



## Final result

__sysfs_entry.c__
```c
#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>

volatile int secret_value;

struct kobject *sysfs_kobj;

static ssize_t secret_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) {
	printk(KERN_DEBUG "Show value %d\n", secret_value);
	return sprintf(buf, "%d", secret_value);
}


static ssize_t secret_store(struct kobject *kobj, struct kobj_attribute *attr,const char *buf, size_t count) {
	printk(KERN_DEBUG "Set value %s %ldd\n", buf, count);
	sscanf(buf, "%d", &secret_value);
	return count;
}

struct kobj_attribute secret_attr = __ATTR(secret_value, 0660, secret_show, secret_store);


int sysfs_entry_init( void )
{
	printk(KERN_DEBUG "Hello Syscfs!\n");

	sysfs_kobj = kobject_create_and_add("secret", kernel_kobj);

	if(sysfs_create_file(sysfs_kobj, &secret_attr.attr)) {
		pr_err("Cant create secret file");
	}

	return 0;
}

void sysfs_entry_exit( void )
{
	printk(KERN_DEBUG "Exit Sysfs!\n");

	sysfs_remove_file(sysfs_kobj, &secret_attr.attr);
	kobject_put(sysfs_kobj);
}

module_init( sysfs_entry_init );
module_exit( sysfs_entry_exit );

MODULE_LICENSE("GPL");

```

## Links

https://man7.org/linux/man-pages/man5/sysfs.5.html  
https://embetronicx.com/tutorials/linux/device-drivers/sysfs-in-linux-kernel/  
https://elixir.bootlin.com/linux/v6.1.64/source/include/linux/sysfs.h  
https://docs.kernel.org/filesystems/sysfs.html  
https://elixir.bootlin.com/linux/v6.1.64/source/include/linux/kobject.h#L64  
https://pradheepshrinivasan.github.io/2015/07/02/Creating-an-simple-sysfs/