From: Yan Zhai <yan@xxxxxxxxxxxxxx>
Date: Fri, 31 Jan 2025 12:32:57 -0800
> Hello,
>
> We encountered a panic when tracing kfree_skb with RAW_TP. The problematic
> argument was introduced in commit ba8de796baf4 ("net: introduce
> sk_skb_reason_drop function"). It turns out that the verifier still accepted
> the program despite it didn't test sk == NULL. And this caused kernel panic. I
> attached a small reproducer and panic trace at the end. It's stably
> reproducible when packets are dropped without a receiver (e.g. run iperf2 UDP
> test toward localhost), in both 6.12.11 release and a recent bpf-next master
> snapshot (I was using commit c03320a6768c).
>
> As a contrast, for another tracepoint like tcp_send_reset, if sk is not checked
> before dereferencing, the verifier will complain and reject the program as
> expected. So this feels like some annotation is missing? Appreciate if someone
> could help me figure out.
Maybe __nullable is missing given we annotated skb for tcp_send_reset ?
https://lore.kernel.org/netdev/20240911033719.91468-4-lulie@xxxxxxxxxxxxxxxxx/
completely untested:
---8<---
diff --git a/include/trace/events/skb.h b/include/trace/events/skb.h
index b877133cd93a..34accc5929d6 100644
--- a/include/trace/events/skb.h
+++ b/include/trace/events/skb.h
@@ -24,14 +24,14 @@ DEFINE_DROP_REASON(FN, FN)
TRACE_EVENT(kfree_skb,
TP_PROTO(struct sk_buff *skb, void *location,
- enum skb_drop_reason reason, struct sock *rx_sk),
+ enum skb_drop_reason reason, struct sock *rx_sk__nullable),
- TP_ARGS(skb, location, reason, rx_sk),
+ TP_ARGS(skb, location, reason, rx_sk__nullable),
TP_STRUCT__entry(
__field(void *, skbaddr)
__field(void *, location)
- __field(void *, rx_sk)
+ __field(void *, rx_sk__nullable)
__field(unsigned short, protocol)
__field(enum skb_drop_reason, reason)
),
@@ -39,7 +39,7 @@ TRACE_EVENT(kfree_skb,
TP_fast_assign(
__entry->skbaddr = skb;
__entry->location = location;
- __entry->rx_sk = rx_sk;
+ __entry->rx_sk = rx_sk__nullable;
__entry->protocol = ntohs(skb->protocol);
__entry->reason = reason;
),
---8<---
