netdev - [PATCH net-next 1/2] net: fix call location in kfree_skb_list

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Date:   Thu, 05 Jan 2023 16:42:42 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        pabeni@...hat.com
Subject: [PATCH net-next 1/2] net: fix call location in kfree_skb_list_reason

The SKB drop reason uses __builtin_return_address(0) to give the call
"location" to trace_kfree_skb() tracepoint skb:kfree_skb.

To keep this stable for compilers kfree_skb_reason() is annotated with
__fix_address (noinline __noclone) as fixed in commit c205cc7534a9
("net: skb: prevent the split of kfree_skb_reason() by gcc").

The function kfree_skb_list_reason() invoke kfree_skb_reason(), which
cause the __builtin_return_address(0) "location" to report the
unexpected address of kfree_skb_list_reason.

Example output from 'perf script':
 kpktgend_0  1337 [000]    81.002597: skb:kfree_skb: skbaddr=0xffff888144824700 protocol=2048 location=kfree_skb_list_reason+0x1e reason: QDISC_DROP

Patch creates an __always_inline __kfree_skb_reason() helper call that
is called from both kfree_skb_list() and kfree_skb_list_reason().
Suggestions for solutions that shares code better are welcome.

As preparation for next patch move __kfree_skb() invocation out of
this helper function.

Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---
 net/core/skbuff.c |   34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4a0eb5593275..007a5fbe284b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -932,6 +932,21 @@ void __kfree_skb(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(__kfree_skb);
 
+static __always_inline
+bool __kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
+{
+	if (unlikely(!skb_unref(skb)))
+		return false;
+
+	DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
+
+	if (reason == SKB_CONSUMED)
+		trace_consume_skb(skb);
+	else
+		trace_kfree_skb(skb, __builtin_return_address(0), reason);
+	return true;
+}
+
 /**
  *	kfree_skb_reason - free an sk_buff with special reason
  *	@skb: buffer to free
@@ -944,26 +959,19 @@ EXPORT_SYMBOL(__kfree_skb);
 void __fix_address
 kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
 {
-	if (unlikely(!skb_unref(skb)))
-		return;
-
-	DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
-
-	if (reason == SKB_CONSUMED)
-		trace_consume_skb(skb);
-	else
-		trace_kfree_skb(skb, __builtin_return_address(0), reason);
-	__kfree_skb(skb);
+	if (__kfree_skb_reason(skb, reason))
+		__kfree_skb(skb);
 }
 EXPORT_SYMBOL(kfree_skb_reason);
 
-void kfree_skb_list_reason(struct sk_buff *segs,
-			   enum skb_drop_reason reason)
+void __fix_address
+kfree_skb_list_reason(struct sk_buff *segs, enum skb_drop_reason reason)
 {
 	while (segs) {
 		struct sk_buff *next = segs->next;
 
-		kfree_skb_reason(segs, reason);
+		if (__kfree_skb_reason(segs, reason))
+			__kfree_skb(segs);
 		segs = next;
 	}
 }