Re: [PATCH net 2/2] net: Warn when overriding referenced dst entry

[Stanislav Fomichev <stfomichev@gmail.com>]

On 07/22, Jakub Kicinski wrote:
> On Tue, 22 Jul 2025 14:02:56 -0700 Stanislav Fomichev wrote:
> > Add WARN_ON_ONCE with CONFIG_NET_DEBUG to warn on overriding referenced
> > dst entries. This should get a better signal than tracing leaked
> > objects from kmemleak.
> 
> Looks like we're tripping the severe nastiness in icmp.c:
> 
> 		/* Ugh! */
> 		orefdst = skb_in->_skb_refdst; /* save old refdst */
> 		skb_dst_set(skb_in, NULL);
> 		err = ip_route_input(skb_in, fl4_dec.daddr, fl4_dec.saddr,
> 				     dscp, rt2->dst.dev) ? -EINVAL : 0;
> 
> 		dst_release(&rt2->dst);
> 		rt2 = skb_rtable(skb_in);
> 		skb_in->_skb_refdst = orefdst; /* restore old refdst */
> 
> There's more of these around. I think we need a new helper for
> "saving" and "restoring" the refdst? Either way, I reckon the
> patch with the check belongs in net-next.

SG, will run all tests locally and try to weed these out before
reposting for net-next.

> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 5520524c93bf..c89931ac01e5 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -1159,6 +1159,12 @@ static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
> >  	return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
> >  }
> >  
> > +static inline void skb_dst_check(struct sk_buff *skb)
> 
> skb_dst_check_unset() ? Right now we assume the caller will override.
> Or will there be more conditions added?

skb_dst_check_unset sounds good. I was not planning to add more checks.

> > +{
> > +	DEBUG_NET_WARN_ON_ONCE(skb_dst(skb) &&
> > +			       !(skb->_skb_refdst & SKB_DST_NOREF));
> 
> Why not 
> 
> 	DEBUG_NET_WARN_ON_ONCE(skb->_skb_refdst & SKB_DST_PTRMASK);
> 
> ?

That's more precise, agreed!