Re: Allocating struct sock from kernel module

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Well.....all that u are trying to do is not really allowed, as inside
af_inet.c the variable inetsw_array is defined as static:

/* Upon startup we insert all the elements in inetsw_array[] into
 * the linked list inetsw.
 */
static struct inet_protosw inetsw_array[] =

and so only those functions within af_inet.c can access it, not
anywhere else,  kernel module in particular.

But as a hacker, anything IS POSSIBLE....but u have to be aware of
access contention.   At startup the elements inside inetsw_arrray is
passed to inet_register_protosw() via ptr:

void inet_register_protosw(struct inet_protosw *p)
{
        struct list_head *lh;
        struct inet_protosw *answer;
        int protocol = p->protocol;
        struct list_head *last_perm;

        spin_lock_bh(&inetsw_lock);

And u noticed the spin lock inetsw_lock is used whenever accessing the
"p" inet_protosw ptr is needed.   Another example:

void inet_unregister_protosw(struct inet_protosw *p)
{
        if (INET_PROTOSW_PERMANENT & p->flags) {
                printk(KERN_ERR
                       "Attempt to unregister permanent protocol %d.\n",
                       p->protocol);
        } else {
                spin_lock_bh(&inetsw_lock);
                list_del_rcu(&p->list);
                spin_unlock_bh(&inetsw_lock);

                synchronize_net();
        }

And so u must therefore call spin lock in your code whenever u want to
access the "p" ptr.

How to access the inetsw_array, or spin lock variable in another
function, or in an external kernel module?

BY ADDRESS.

Looking into /proc/kallsyms:

cat /proc/kallsyms |grep inetsw_
c07fb640 d inetsw_array
c0972628 b inetsw_lock

So it is in kernel memory, just set up a ptr to point there, and u are
accessing the variable.   This is generally used to bypass the
"exported symbols" not available problem......: - (.

On Tue, Jun 29, 2010 at 10:37 AM, Jim Sansing <jjsansing@xxxxxxxxxxx> wrote:
> I am trying to send a Raw packet from my kernel module.  I found what I
> hoped was a good example, but it came with the warning that allocating the
> 'struct sock' was the tricky part, which is where I am now.  I want to use
> sk_alloc, which is defined as:
>
>  struct sock *sk_alloc(struct net *net, int family, gfp_t priority, struct
> proto *prot);
>
> I have been looking for how to get to the Raw 'struct proto', but it is
> buried in at least one list, maybe more, and I haven't been able to find how
> to get to it directly.  The only place I have seen it being accessed is in
> af_inet.c, which steps thru the 'struct inet_protosw' queue, checking the
> protocol field (Raw uses the int, IPPROTO_IP).  But the inet_protosw queue
> is not an Exported Symbol.  Is there a way to get the Raw 'struct proto',
> perhaps using the IPPROTO_IP int?
>
> Also, are there any gotchas as far as the 'struct sock' after it has been
> allocated?
>
> Thanks.
>
> Later . . .   Jim
>
>
> --
> To unsubscribe from this list: send an email with
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>
>



-- 
Regards,
Peter Teoh

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