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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
|
