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 11:57:26 -0800

On 06 Jan 11:54, Saeed Mahameed wrote:
>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
>

I see now, it's because of the next patch, you will use a different
deallocator in kfree_skb_list_reason.

Reviewed-by: Saeed Mahameed <[email-masked]>