linux kernel module assignment makes integer from pointer without a cast - c

Question

this is simple sys_call_table hooking code
#include <asm/unistd.h>
#include <linux/autoconf.h>
#include <linux/in.h>
#include <linux/init_task.h>
#include <linux/ip.h>
#include <linux/kernel.h>
#include <linux/kmod.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/skbuff.h>
#include <linux/stddef.h>
#include <linux/string.h>
#include <linux/syscalls.h>
#include <linux/tcp.h>
#include <linux/types.h>
#include <linux/unistd.h>
#include <linux/version.h>
#include <linux/workqueue.h>
ssize_t *sys_call_table = (ssize_t *)0xc0026e04;
asmlinkage ssize_t (*orig_open)(const char *pathname, int flags);
asmlinkage ssize_t hacked_open(const char *pathname, int flags)
{
printk(KERN_INFO "SYS_OPEN called : %s\n", pathname);
return orig_open(pathname, flags);
}
int init_module(void)
{
orig_open = sys_call_table[__NR_open]; /* line 33 */
sys_call_table[__NR_open] = hacked_open; /* line 34 */
return 0;
}
void cleanup_module(void)
{
sys_call_table[__NR_open] = orig_open; /* line 40 */
}
MODULE_LICENSE("GPL");
i got an warning like below
this code works fine but i want to delete warnings. how can i do?
/home/tester/tools/lkm/a.c: In function 'init_module':
/home/tester/tools/lkm/a.c:33: warning: assignment makes integer from pointer without a cast
/home/tester/tools/lkm/a.c:34: warning: assignment makes integer from pointer without a cast
/home/tester/tools/lkm/a.c: In function 'cleanup_module':
/home/tester/tools/lkm/a.c:40: warning: assignment makes integer from pointer without a cast

Answer 1

If you want to silence your compiler, you have to add typecasts (even if it is often a bad idea, this is how your compiler turns it).
ssize_t *sys_call_table = (ssize_t *)0xc0026e04;
typedef ssize_t (*ftype)(const char *, int);
ftype orig_open;
ssize_t hacked_open(const char *pathname, int flags)
{
printf("SYS_OPEN called : %s\n", pathname);
return orig_open(pathname, flags);
}
int init_module(void)
{
orig_open = (ftype)sys_call_table[__NR_open];
sys_call_table[__NR_open] = (ssize_t)hacked_open;
return 0;
}

Answer 2

When you look at line 33 you will see the problem:
orig_open = sys_call_table[__NR_open];
You have defined sys_call_table to be a pointer to integer. That's the reason for the warning.
Same problem with the other lines. If you define sys_call_table properly, the warnings will go away.
You should at least define it as an array of pointers or pointer to pointers, because if ssize_t is only 32 bit on 64 bit system, you might truncate the 64 bit addresses to a 32 bit integer.