create a /proc file

Makefile

ifneq ($(KERNELRELEASE),)
   obj-m := proc.o
else

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

PWD := $(shell pwd)
KBUILD_CFLAGS += $(call cc-option,-Wno-error,)

default:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif
clean:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) clean

proc.c

#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>

static int hello_proc_show(struct seq_file *m, void *v) {
  const int val = 13;
  //seq_printf(m, "Hello proc!\n");
  seq_printf(m, "%d!\n", val);
  return 0;
}

static int hello_proc_open(struct inode *inode, struct  file *file) {
  return single_open(file, hello_proc_show, NULL);
}

static const struct proc_ops hello_proc_fops = {
  .proc_open = hello_proc_open,
  .proc_read = seq_read,
  .proc_lseek = seq_lseek,
  .proc_release = single_release,
};

static int __init hello_proc_init(void) {
  proc_create("hello_proc", 0, NULL, &hello_proc_fops);
  return 0;
}

static void __exit hello_proc_exit(void) {
  remove_proc_entry("hello_proc", NULL);
}

MODULE_LICENSE("GPL");
module_init(hello_proc_init);
module_exit(hello_proc_exit);

for recent kernels >5.10