RE: NULL Pointer Deference: NFS & Telnet

Arce, Abraham
2010-05-26T20:19:07Z

Thanks Eric, David,

[..]

> > > > - if (skb_shinfo(skb)->nr_frags) {
> > > > + if (skb_shinfo(skb)->nr_frags && skb_has_frags(skb)) {
> > > > int i;
> > > > for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
> > > > put_page(skb_shinfo(skb)->frags[i].page);
> > >
> > > skb_shinfo(skb)->nr_frags counts the number of entries contained
> > > in the skb_shinfo(skb)->frags[] array.
> > >
> > > This has nothing to do with the frag list pointer,
> > > skb_shinfo(skb)->frag_list, which is what skb_has_frags()
> > > tests.
> > >
> > > You've got some kind of memory corruption going on and it
> > > appears to have nothing to do with the code paths you're
> > > playing with here.
> >
> > Do you have any recommendation on debugging technique/tool for this memory
> corruption issue?

[..]

> It seems quite strange. You have a skb->nr_frags > 0 value, but a
> frags[i].page = 0 value
>
> You might add following function :
>
> shinfo_check(struct sk_buff *skb)
> {
> struct skb_shared_info *shinfo = skb_shinfo(skb);
> int i;
>
> WARN_ON(shinfo->nr_frags >= MAX_SKB_FRAGS);
> for (i = 0; i < shinfo->nr_frags; i++)
> WARN_ON(!shinfo->frags[i].page);
> }
>
> And call it from various points, to check who corrupts your skb.

By increasing the allocation length of our rx skbuff the corruption issue is fixed... I have increased it by 2... Were we writing outside our boundaries of skb data?

Please let me know about this approach...

diff --git a/drivers/net/ks8851.c b/drivers/net/ks8851.c
index b4fb07a..6da81e1 100644
--- a/drivers/net/ks8851.c
+++ b/drivers/net/ks8851.c
@@ -504,7 +504,7 @@ static void ks8851_rx_pkts(struct ks8851_net *ks)
ks->rc_rxqcr | RXQCR_SDA | RXQCR_ADRFE);

if (rxlen > 0) {
- skb = netdev_alloc_skb(ks->netdev, rxlen + 2 + 8);
+ skb = netdev_alloc_skb(ks->netdev, rxlen + 4 + 8);
if (!skb) {

Best Regards
Abraham