Creating a simple write only proc entry in kernel - c

Question

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include<linux/sched.h>
#include <asm/uaccess.h>
#include <linux/slab.h>
char *msg;
ssize_t write_proc(struct file *filp,const char *buf,size_t count,loff_t *offp)
{
copy_from_user(msg,buf,count);
printk(KERN_INFO "%s",msg);
return count;
}
struct file_operations proc_fops = {
write: write_proc
};
int proc_init (void) {
proc_create("write",0,NULL,&proc_fops);
return 0;
}
void proc_cleanup(void) {
remove_proc_entry("write",NULL);
}
MODULE_LICENSE("GPL");
module_init(proc_init);
module_exit(proc_cleanup);
When I used the command echo 'hello' > /proc/write Nothing show up on terminal . Can you help me to find mistakes in the code ? The string that I writed on it should have show up on terminal.
Example :
$ echo 'hello' > /proc/write
hello

Answer 1

Here are some simple modifications on your code:
#define MSG_SIZE (512)
static char *msg;
#define ourmin(a,b) (((a)<(b)) ? (a) : (b))
ssize_t write_proc(struct file *filp,const char *buf,size_t count,loff_t *offp)
{
unsigned long actual_len = ourmin(count, MSG_SIZE-1);
memset(msg, 0, MSG_SIZE);
copy_from_user(msg, buf, actual_len);
printk(KERN_DEBUG "Got: %s",msg);
return count;
}
int proc_init (void) {
// Allocate space for msg
if ((msg = kmalloc(MSG_SIZE, GFP_KERNEL)) == NULL)
return -ENOMEM;
// Should check the output of this too
proc_create("write",0,NULL,&proc_fops);
return 0;
}
void proc_cleanup(void) {
remove_proc_entry("write",NULL);
kfree(msg);
}
I could retrieve the output in the kernel log (dmesg for instance).