Re: mac80211 truesize bugs

[David Miller <davem@davemloft.net>]

From: Johannes Berg <johannes@sipsolutions.net>
Date: Sun, 04 May 2008 01:24:48 +0200

> 
> > > @@ -699,6 +699,12 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> > >  	if (skb_shared(skb))
> > >  		BUG();
> > >  
> > > +	if (unlikely((nhead || ntail) && skb->sk)) {
> > > +		printk(KERN_ERR "SKB BUG: Illegal pskb expand (%d:%d) "
> > > +		       "with socket attached\n",
> > > +		       nhead, ntail);
> > > +	}
> > > +
> > >  	size = SKB_DATA_ALIGN(size);
> > 
> > Ok I think I'm starting to understand this a little better. However,
> > shouldn't this function update skb->truesize so if the skb is later
> > attached to a different socket again it has the right size?
> 
> Judging from some of the callers, the caller should. Why?!

Relax :-)

We certainly could check that there is no socket attached here,
and make the truesize adjustment right at this spot.

It just never happened before in practice in a way that matters.

That's why we have the truesize assertion, to discover situations
like this and thus be able to fix it.