From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cong Wang Subject: [Patch net-next] net: explain __skb_checksum_complete() with comments Date: Mon, 26 Nov 2018 09:31:26 -0800 Message-ID: <20181126173126.20456-1-xiyou.wangcong@gmail.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Cc: Cong Wang , Herbert Xu To: netdev@vger.kernel.org Return-path: Received: from mail-pf1-f172.google.com ([209.85.210.172]:36079 "EHLO mail-pf1-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726224AbeK0E0Z (ORCPT ); Mon, 26 Nov 2018 23:26:25 -0500 Received: by mail-pf1-f172.google.com with SMTP id b85so6916577pfc.3 for ; Mon, 26 Nov 2018 09:31:38 -0800 (PST) Sender: netdev-owner@vger.kernel.org List-ID: Cc: Herbert Xu Signed-off-by: Cong Wang --- net/core/dev.c | 1 + net/core/skbuff.c | 18 +++++++++++++++++- 2 files changed, 18 insertions(+), 1 deletion(-) diff --git a/net/core/dev.c b/net/core/dev.c index 368dc3b49dc0..ee0a4ac0bbb6 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -5787,6 +5787,7 @@ __sum16 __skb_gro_checksum_complete(struct sk_buff *skb) /* NAPI_GRO_CB(skb)->csum holds pseudo checksum */ sum = csum_fold(csum_add(NAPI_GRO_CB(skb)->csum, wsum)); + /* See comments in __skb_checksum_complete(). */ if (likely(!sum)) { if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) && !skb->csum_complete_sw) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 6f2ea0f0fb75..530097df328f 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -2637,6 +2637,7 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len) __sum16 sum; sum = csum_fold(skb_checksum(skb, 0, len, skb->csum)); + /* See comments in __skb_checksum_complete(). */ if (likely(!sum)) { if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) && !skb->csum_complete_sw) @@ -2648,6 +2649,15 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len) } EXPORT_SYMBOL(__skb_checksum_complete_head); +/* This function assumes skb->csum already holds pseudo header's checksum, + * which has been changed from the hardware checksum, for example, by + * __skb_checksum_validate_complete(). And, the original skb->csum must + * have been validated unsuccessfully for CHECKSUM_COMPLETE case. + * + * It returns non-zero if the recomputed checksum is still invalid, otherwise + * zero. The new checksum is stored back into skb->csum unless the skb is + * shared. + */ __sum16 __skb_checksum_complete(struct sk_buff *skb) { __wsum csum; @@ -2655,8 +2665,14 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb) csum = skb_checksum(skb, 0, skb->len, 0); - /* skb->csum holds pseudo checksum */ sum = csum_fold(csum_add(skb->csum, csum)); + /* This check is inverted, because we already knew the hardware + * checksum is invalid before calling this function. So, if the + * re-computed checksum is valid instead, then we have a mismatch + * between the original skb->csum and skb_checksum(). This means either + * the original hardware checksum is incorrect or we screw up skb->csum + * when moving skb->data around. + */ if (likely(!sum)) { if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) && !skb->csum_complete_sw) -- 2.19.1