Re: GFP_ATOMIC vs GFP_KERNEL in netfilter module

[Pablo Neira <pablo@eurodev.net>]

You are in interrupt context, so GFP_KERNEL is illegal since it can sleep.

--
Pablo

Roberto Jung Drebes wrote:

> Hello there,
>
> I am testing a simple netfilter module which delays every 5th IP 
> packet received. I am having problems allocating the timer structure. 
> Here is the netfilter hook I register:
>
> static unsigned int
> comfirm_rx_hook(unsigned int hook,
>                 struct sk_buff **pskb,
>                 const struct net_device *in,
>                 const struct net_device *out,
>                 int (*okfn) (struct sk_buff *))
> {
>     static int counter;
>     struct timer_list *tl;
>     struct cf_delay_parms *dp;
>
>     if ((*pskb)->nfmark != CF_MAGIC) {
>         counter++;
>
>         if (!(counter % 5)) {
>             dp = kmalloc(sizeof(struct cf_delay_parms), GFP_KERNEL);
>             tl = kmalloc(sizeof(struct timer_list), GFP_KERNEL);
>             dp->timer = tl;
>             (*pskb)->nfmark = CF_MAGIC;
>             dp->skb = *pskb;
>             tl->data = (unsigned long) dp;
>             tl->function = (void *) cf_delay_rx;
>             tl->expires = jiffies + 100;
>             init_timer(tl);
>             add_timer(tl);
>             printk("packet delayed\n");
>             return NF_STOLEN;
>         } else
>             return NF_ACCEPT;
>     } else
>         return NF_ACCEPT;
> }
>
> void cf_delay_rx(struct cf_delay_parms *parms)
> {
>     ip_rcv(parms->skb, parms->skb->dev, NULL);
>     kfree(parms->timer);
>     kfree(parms);
> }
>
> If I use GFP_KERNEL with the kmallocs, I get errors like
>
> Debug: sleeping function called from invalid context at mm/slab.c:2000
> in_atomic():1[expected: 0], irqs_disabled():0
>  [<0211b765>] __might_sleep+0x82/0x8c
>  [<02146dbc>] kmem_cache_alloc+0x1d/0x4c
>  [<2204b047>] comfirm_rx_hook+0x47/0xd8 [mymodule]
>  [<022a68eb>] nf_iterate+0x40/0x89
>  [<022b5d63>] ip_rcv_finish+0x0/0x1d9
>  [<022a6bac>] nf_hook_slow+0x47/0xb1
>  [<022b5d63>] ip_rcv_finish+0x0/0x1d9
>  [<022b5be7>] ip_rcv+0x36d/0x3a1
>  [<022b5d63>] ip_rcv_finish+0x0/0x1d9
>  [<0229eef3>] netif_receive_skb+0x1b0/0x1dd
>  [<0229ef8c>] process_backlog+0x6c/0xd9
>  [<0229f056>] net_rx_action+0x5d/0xcd
>  [<02122fa1>] __do_softirq+0x35/0x73
>  [<02108a7b>] do_softirq+0x46/0x4d
>  =======================
>  [<02107e67>] do_IRQ+0x2f7/0x303
>  [<021fc4c1>] acpi_processor_idle+0xd3/0x1c5
>  [<0210408c>] cpu_idle+0x1f/0x34
>  [<0239b6ae>] start_kernel+0x221/0x224
>
> If I use GFP_ATOMIC, I don't get the error, but I think timers are not 
> being called after the delay. I have a similar code for transmition, 
> which works OK with GFP_KERNEL (delays messages) but with GFP_ATOMIC 
> it does also not delay.
>
> I test delay with ping, and I am running kernel 2.6.8-1.521 from 
> Fedora Core 2.
>
> What am I doing wrong?
>
> TIA,
>
> ps: Please CC me any reply from the linux-kernel mailing list.
>