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

# Create procfs entry

Lets extend hello world driver. 

## Starting point

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

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

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

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

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

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

void procfs_entry_exit( void )
{
	printk(KERN_DEBUG "Exit Procfs!\n");
}

module_init( procfs_entry_init );
module_exit( procfs_entry_exit );

MODULE_LICENSE("GPL");
```

## Adding procfs directory

Create proc directory with

```c
dir = proc_mkdir(DIR_NAME, PARENT_DIR);
```

under the directory create file that will contain data

```c
proc_create("file", MODE, PARENT_DIR, PROC_FOPS)
```

Lets create /proc/secret/file. And define basic read/write operations. That at this 
stage will do nothing.

```c
#include <linux/proc_fs.h>

static struct proc_dir_entry *procdir, *procfile;

static int secret_open(struct inode *inode, struct file *file) {
        return 0;
}

static int secret_release(struct inode *inode, struct file *file) {
        return 0;
}

static ssize_t secret_read(struct file *file, char __user *buffer, size_t length, loff_t *offset) {
        return 0;
}

static ssize_t secret_write(struct file *file, const char *buffer, size_t length, loff_t *offset) {
        return 0;
}

static struct proc_ops procfs_ops = {
        .proc_open = secret_open,
        .proc_read = secret_read,
        .proc_write = secret_write,
        .proc_release = secret_release
};


int procfs_entry_init( void ) {
...
procdir = proc_mkdir("secret", NULL);
if (procdir == NULL) {
	pr_err("Cant create secret directory\n");
	return -1;
}

procfile = proc_create("file", 0644, procdir, &procfs_ops);
if (procfile == NULL) {
	pr_err("Cant create file under the secret\n");
	return -1;
}
...
}

void procfs_entry_exit( void )
{
	...
	//remove entries after unloading module
	proc_remove(procdir);
	proc_remove(procfile);
	...
}
```

Now module can be run and there is one file under the /proc/secret

Next step add data to read from file entry/

Update the file operation functions, so its possible to read some data from /proc/secret/file

```c
char file_data[256] = "This is data\n";
static int read_once=1;


static int secret_open(struct inode *inode, struct file *file) {
        return 0;
}

static int secret_release(struct inode *inode, struct file *file) {
        return 0;
}

static ssize_t secret_read(struct file *file, char __user *buffer, size_t length, loff_t *offset) {
       int len=0;
       if (read_once) {
               read_once=0;
       } else {
               read_once=1;
               return 0;
       }
       len = strlen(file_data);
       if (copy_to_user(buffer, &file_data, len)) {
               pr_err("secreate_read: Error\n");
       }
       return len;
}

static ssize_t secret_write(struct file *file, const char *buffer, size_t length, loff_t *offset) {
        return 0;
}
```

Now some data is exposed if module is loaded and run command

```sh
cat /proc/secret/file
```

output will be

```sh
This is data
```



## List of procfs functions used

| Function name | Note |
|---|---|
| proc_mkdir | Create procfs directory |
| proc_create | Create procfs file |
| proc_remove | Remove procfs entry |

## Final result


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

static struct proc_dir_entry *procdir, *procfile;
char file_data[256] = "This is data\n";
static int read_once=1;

static int secret_open(struct inode *inode, struct file *file) {
	return 0;
}

static int secret_release(struct inode *inode, struct file *file) {
	return 0;
}

static ssize_t secret_read(struct file *file, char __user *buffer, size_t length, loff_t *offset) {
        int len=0;
	if (read_once) {
		read_once=0;
	} else {
		read_once=1;
		return 0;
	}
	len = strlen(file_data);
	if (copy_to_user(buffer, &file_data, len)) {
		pr_err("secreate_read: Error\n");
	}
	return len;
}

static ssize_t secret_write(struct file *file, const char *buffer, size_t length, loff_t *offset) {
	return 0;
}

static struct proc_ops procfs_ops = {
        .proc_open = secret_open,
        .proc_read = secret_read,
        .proc_write = secret_write,
        .proc_release = secret_release
};


int procfs_entry_init( void )
{
	printk(KERN_DEBUG "Hello Procfs!\n");
	procdir = proc_mkdir("secret", NULL);
	if (procdir == NULL) {
		pr_err("Cant create secret directory\n");
		return -1;
	}

	procfile = proc_create("file", 0644, procdir, &procfs_ops);
	if (procfile == NULL) {
		pr_err("Cant create file under the secret\n");
		return -1;
	}

	return 0;
}

void procfs_entry_exit( void )
{
	printk(KERN_DEBUG "Exit Procfs!\n");
	proc_remove(procdir);
	proc_remove(procfile);
}

module_init( procfs_entry_init );
module_exit( procfs_entry_exit );

MODULE_LICENSE("GPL")
```

## Links

https://elixir.bootlin.com/linux/v6.1.21/source/include/linux/proc_fs.h