RE: NULL Pointer Deference: NFS & Telnet

[Eric Dumazet <eric.dumazet@gmail.com>]

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.