Linux kernel: why does this call to kstrtol crash?

Question

I am learning kernel programming and have a simple call to kstrtol I am using to convert a string to a number. However, everytime I compile this module and use insmod to place it in the kernel, I get "BUG: unable to handle kernel paging request at f862b026" and then a register and stack dump.

I'm following the definition from here: https://www.kernel.org/doc/htmldocs/kernel-api/API-kstrtol.html . It seems like a really simple call. What am I doing wrong here?

#include <linux/kernel.h>

static int __init convert(void)
{
    long myLong;
    char *myNumber = "342";
    myNumber[2] = '\0'; //Overwriting the '2', just so I know for sure I have a terminating '\0'

    if (kstrtol(myNumber, 10, &myLong) == 0)
    {
        printk("We have a number!\n");
    }
return 0;
}

static void __exit convert_exit(void)
{
    printk("Module unloaded\n");
}

module_init(convert);
module_exit(convert_exit); 

Answer 1

You cannot modify string literals. Copy it into an array firstly.

edit: use this instead

char mystr[] = "abdc";

edit2: the underlying reason for this is, that a char pointer to a string literal points to a data segment, usually readonly. If you alter this memory you might get a crash. When you create an array of chars instead, the string literal gets copied into the array on the stack, where you safely can modify it.