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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/
|
