Subject: Re: [PATCH net-next 1/2] net: fix call location in kfree_skb_list_reason

From: Saeed Mahameed <[email-masked]>
Date: Fri, 6 Jan 2023 11:54:00 -0800

On 05 Jan 16:42, Jesper Dangaard Brouer wrote:
>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 <[email-masked]>
>---
> 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;

why not just call __kfree_skb(skb); here instead of the boolean return ? 
if it because __kfree_skb() makes a call to
skb_release_all()->..->kfree_skb_list_reason()
then it's already too deep and the return address in that case isn't
predictable, so you're not avoiding any breakage by keeping
direct calls to __kfree_skb() from kfree_skb_reason and+kfree_skb_list_reason

>+}
>+
> /**
>  *	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;
> 	}
> }
>
>