netdev - [PATCH net v2] udp: Compute L4 checksum as usual when not segmenting the skb

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Message-Id: <20241011-uso-swcsum-fixup-v2-1-6e1ddc199af9@cloudflare.com>
Date: Fri, 11 Oct 2024 14:17:30 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, kernel-team@...udflare.com, 
 Ivan Babrou <ivan@...udflare.com>, Jakub Sitnicki <jakub@...udflare.com>, 
 stable@...r.kernel.org
Subject: [PATCH net v2] udp: Compute L4 checksum as usual when not
 segmenting the skb

If:

  1) the user requested USO, but
  2) there is not enough payload for GSO to kick in, and
  3) the egress device doesn't offer checksum offload, then

we want to compute the L4 checksum in software early on.

In the case when we are not taking the GSO path, but it has been requested,
the software checksum fallback in skb_segment doesn't get a chance to
compute the full checksum, if the egress device can't do it. As a result we
end up sending UDP datagrams with only a partial checksum filled in, which
the peer will discard.

Fixes: 10154dbded6d ("udp: Allow GSO transmit from devices with no checksum offload")
Reported-by: Ivan Babrou <ivan@...udflare.com>
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
Acked-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: stable@...r.kernel.org
---
Changes in v2:
- Fix typo in patch description
- Link to v1: https://lore.kernel.org/r/20241010-uso-swcsum-fixup-v1-1-a63fbd0a414c@cloudflare.com
---
 net/ipv4/udp.c | 4 +++-
 net/ipv6/udp.c | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 8accbf4cb295..2849b273b131 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -951,8 +951,10 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
 			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
 			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
 								 cork->gso_size);
+
+			/* Don't checksum the payload, skb will get segmented */
+			goto csum_partial;
 		}
-		goto csum_partial;
 	}

 	if (is_udplite)  				 /*     UDP-Lite      */
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 52dfbb2ff1a8..0cef8ae5d1ea 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1266,8 +1266,10 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
 			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
 			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
 								 cork->gso_size);
+
+			/* Don't checksum the payload, skb will get segmented */
+			goto csum_partial;
 		}
-		goto csum_partial;
 	}

 	if (is_udplite)