RE: NULL Pointer Deference: NFS & Telnet

Eric Dumazet
2010-05-26T05:29:01Z

Le mardi 25 mai 2010 à 21:02 -0500, Arce, Abraham a écrit :
> Thanks David,
>
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index f8abf68..eb81f76 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -334,7 +334,7 @@ static void skb_release_data(struct sk_buff *skb)
> > > if (!skb->cloned ||
> > > !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
> > > &skb_shinfo(skb)->dataref)) {
> > > - 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?
>
> Best Regards
> Abraham
> --

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.

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