BUG: kernel NULL pointer dereference when using linux linked list

Question

I was trying to write the assignment in Operating System Concept 9th, chapter 2, which is writing a simple linux module. I write the code following the example in the book, but got a null pointer dereference error while deleting items in the linked list. Here is my code.

  1 #include <linux/module.h>
  2 #include <linux/kernel.h>
  3 #include <linux/list.h>
  4 #include <linux/slab.h>
  5 /* This function is called when the module is loaded. */
  6 
  7 struct birthday{
  8     int day;
  9     int month;
 10     int year;
 11     struct list_head list;
 12 };
 13 
 14 struct list_head birthday_list;
 15 
 16 int simple_init(void)
 17 {
 18        printk(KERN_INFO "Loading Module\n");
 19        LIST_HEAD(birthday_list);
 20        struct birthday *person;
 21        person = kmalloc(sizeof(*person), GFP_KERNEL);
 22        person->day=24;
 23        person->month=1;
 24        person->year=2000;
 25        INIT_LIST_HEAD(&person->list);
 26        list_add_tail(&person->list, &birthday_list); 
 27        struct birthday *ptr;
 28        list_for_each_entry(ptr, &birthday_list, list){
 29            printk(KERN_INFO "Birthday: %d/%d/%d\n", ptr->year, ptr->month, ptr->day);
 30        }
 31        return 0;
 32 }
 33 
 34 /* This function is called when the module is removed. */
 35 void simple_exit(void) {
 36         printk(KERN_INFO "Removing Module\n");
 37         struct birthday *ptr, *next;
 38 
 39         list_for_each_entry_safe(ptr, next, &birthday_list, list){
 40             printk(KERN_INFO "Birthday: %d/%d/%d\n", ptr->year, ptr->month, ptr->day);
 41             list_del(&(ptr->list));
 42             kfree(ptr);
 43         }
 44 
 45         printk(KERN_INFO "Removing Successfully\n");
 46 }
 47 module_init( simple_init );
 48 module_exit( simple_exit );
 49 
 50 MODULE_LICENSE("GPL");
 51 MODULE_DESCRIPTION("Simple Module");
 52 MODULE_AUTHOR("SGG");```

Ian Abbott · Accepted Answer · 2021-03-31 17:07:29Z

2

Initialize global variable `birthday_list`

The global variable birthday_list has implicit initialization {NULL, NULL} which is not a valid empty list. That is the cause of the null pointer dereference in simple_exit(). A valid empty list has the head node's next and prev members pointing to the head node itself.

You can define and initialize the global birthday_list variable using the LIST_HEAD macro:

LIST_HEAD(birthday_list);

However, it is preferable to declare it static:

static LIST_HEAD(birthday_list);

Remove `LIST_HEAD(birthday_list);` from `simple_init()`

In simple_init(), LIST_HEAD(birthday_list); is creating a local variable birthday_list, but you should be using the global variable of the same name. So just remove LIST_HEAD(birthday_list); from simple_init().

answered Mar 31, 2021 at 17:07

Ian Abbott

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BUG: kernel NULL pointer dereference when using linux linked list

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Initialize global variable `birthday_list`

Remove `LIST_HEAD(birthday_list);` from `simple_init()`

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1 Answer 1

Initialize global variable birthday_list

Remove LIST_HEAD(birthday_list); from simple_init()

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Initialize global variable `birthday_list`

Remove `LIST_HEAD(birthday_list);` from `simple_init()`