a question about kernel copy userspace data

[linwenyuan]

hello ,
i am programming a trial firewall based on netfilter ,which needs the module to access the data of user space ,so i use copy_from_user() but it can't work ,the code(simple test code) is like this:
-------user space program-----
#include <stdlib.h>
#include <stdio.h>

int main(int argc ,char *argv[]) {
char para[100]="insmod test.ko ";
int c=24;
unsigned int point=&c;
printf("%u\n",point);
char b[11]="0000000000"; /*convert the unsigned to chars */
int j=10;
int siz=0;
while(point>0) {
b[--j]=point%10+48;
point=point/10;
siz++;
}
char ips[30]="point=";
strcat(ips,&b[10-siz]);
strcat(para,ips);
puts(para);
system("make");
system(para);
}
-------kernel module-----------
#include <linux/module.h>
#include <linux/netfilter_ipv4.h>
#include <linux/tcp.h>
#include <linux/moduleparam.h>

MODULE_LICENSE("Dual BSD/GPL");

static unsigned int *point;
module_param(point, uint , S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);
MODULE_PARM_DESC(point, "the pointer of user space");

static unsigned int
linuxmag_hook(unsigned int hook, struct sk_buff **pskb,
const struct net_device *indev, const
struct net_device *outdev, int
(*okfn)(struct sk_buff *))
{
printk("world\n");
printk(" %u\n",point);
int *b;
b=kmalloc(sizeof(int),GFP_KERNEL);
copy_from_user(b,(int *)point,sizeof(int));
printk("user space's value=%u\n",*b);
}

static struct nf_hook_ops linuxmag_ops
= { .hook = linuxmag_hook,
.owner = THIS_MODULE,
.pf = PF_INET,
.hooknum = NF_IP_LOCAL_OUT,
.priority = NF_IP_PRI_FILTER-1
};

static int __init init(void)
{
return nf_register_hook(&linuxmag_ops);
}

static void __exit fini(void)
{
nf_unregister_hook(&linuxmag_ops);
}
module_init(init);
module_exit(fini);

i check the /var/log/messages.
the output: the user space's value is 0
not the right value 24 which i defined in main().
this question puzzled me for a few days,and i am a fresh man about linux ,especially the kernel.is there anyone can help me?
any answer would be appreciated very much .

[dilbert]

A few threads below yours is this one:

http://www.linuxforums.org/forum/lin...ss-kernel.html

In there, I inserted an example that is not complete as I simply deleted much of company specific code.

But If you make first an if-statement like in this example, then you can also print out with printk the return value from copy_from_user(). There is presumably something more than only EFAULT.

If this doesn't help, look into your kernel source for the search string "copy_from_user". There are many examples. Surely, difficult to sort out what is useful for you as many examples won't, but that's the best way I think.

I only remember that the structures you are using in this function, must be of a special type. "caddr_xxx" or so.

I think there were also manpages, not of copy_from_user() function name, but maybe you'll find something.

I'm sorry, but out of my head I cannot say more.

One last hint:
When I tried out this ioctl feature, I disabled the device, rmmod'ed the module and loaded it again with a short script. So, I compiled the driver, launched the script, and tested it, over and over without restarting the box.

[linwenyuan]

ok ,thank you very much ,i will try under your guide .but as you say,it is difficult to sort out useful info from google ,i tried that ,but a few of them also are unsuccessful example....
anyhow,thanks

[dilbert]

Yes, in this case, reading the source code in the driver section of your Linux distribution can be helpful.

I thought this is the suggested way to learn Linux.

But if I remember right, the most important thing is the definition of that structure. The special definition is important to share kernel and user space.