It seems that casting a void* pointer (allocated by kmalloc) to unsigned long long changes it. Printing them with %p and %llx gives different values. Why is it so? Can anyone explain?
Following is a simple repro for that:
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
void* kbuff;
int init_module(void)
{
kbuff = kzalloc(sizeof(char), GFP_KERNEL);
pr_info("%p %llx\n",kbuff, (unsigned long long)kbuff);
return 0;
}
void cleanup_module(void)
{
kfree(kbuff);
}
The dmesg output comes out to be as follows
[67355.673465] 000000003aeb0247 ffff9ef657a58c00
From the documentation of printk() (which pr_info calls):
Pointer Types
Pointers printed without a specifier extension (i.e unadorned %p) are hashed to give a unique identifier without leaking kernel addresses to user space. On 64 bit machines the first 32 bits are zeroed. If you really want the address see %px below.
So, this is a security measure. Use the %px format specifier to print the real address (which should match now)
The documentation is available at www.kernel.org/../printk-formats.html