Re: [PATCH net] net: netem: fix error path for corrupted GSO frames

[Jakub Kicinski]

On Thu, 17 Oct 2019 11:10:06 -0700, Cong Wang wrote:
> On Wed, Oct 16, 2019 at 4:22 PM Jakub Kicinski wrote:
> > On Wed, 16 Oct 2019 15:42:28 -0700, Cong Wang wrote:  
> > > > @@ -612,7 +613,7 @@ static int netem_enqueue(struct sk_buff *skb, 
> > > > struct Qdisc *sch,
> > > >                         }
> > > >                         segs = skb2;
> > > >                 }
> > > > -               qdisc_tree_reduce_backlog(sch, -nb, prev_len - len);
> > > > +               qdisc_tree_reduce_backlog(sch, !skb - nb, prev_len - 
> > > > len);  
> > >
> > > Am I the only one has trouble to understand the expression
> > > "!skb - nb"?  
> >
> > The backward logic of qdisc_tree_reduce_backlog() always gives me a
> > pause :S  
> 
> Yeah, reducing with a negative value is actually an add. Feel free
> to add a wrapper for this if you think it is better.

I was avoiding adding the wrapper due to stable, but perhaps it should
be okay.

How does this look?

--->8--------------

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 637548d54b3e..66c2dc6a4742 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -630,6 +630,13 @@ void qdisc_reset(struct Qdisc *qdisc);
 void qdisc_put(struct Qdisc *qdisc);
 void qdisc_put_unlocked(struct Qdisc *qdisc);
 void qdisc_tree_reduce_backlog(struct Qdisc *qdisc, int n, int len);
+
+static inline void
+qdisc_tree_increase_backlog(struct Qdisc *qdisc, int n, int len)
+{
+       qdisc_tree_reduce_backlog(qdisc, -n, -len);
+}
+
 #ifdef CONFIG_NET_SCHED
 int qdisc_offload_dump_helper(struct Qdisc *q, enum tc_setup_type type,
                              void *type_data);
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 0e44039e729c..e22c13b56bfc 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -509,6 +509,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc 
*sch,
                if (skb->ip_summed == CHECKSUM_PARTIAL &&
                    skb_checksum_help(skb)) {
                        qdisc_drop(skb, sch, to_free);
+                       skb = NULL;
                        goto finish_segs;
                }
 
@@ -592,10 +593,13 @@ static int netem_enqueue(struct sk_buff *skb, struct 
Qdisc *sch,
 
 finish_segs:
        if (segs) {
-               unsigned int len, last_len;
+               unsigned int last_len, len = 0;
                int nb = 0;
 
-               len = skb->len;
+               if (skb) {
+                       len += skb->len;
+                       nb++;
+               }
 
                while (segs) {
                        skb2 = segs->next;
@@ -612,7 +616,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc 
*sch,
                        }
                        segs = skb2;
                }
-               qdisc_tree_reduce_backlog(sch, -nb, prev_len - len);
+               /* Parent qdiscs accounted for 1 skb of size @prev_len */
+               qdisc_tree_increase_backlog(sch, nb - 1, len - prev_len);
        }
        return NET_XMIT_SUCCESS;
 }