[PATCH net v2 1/2] xsk: defer software checksum until the skb is complete

[Rongzheng Wang <testwrzwrz@gmail.com>]

When XDP_UMEM_TX_SW_CSUM is used with multi-buffer TX, xsk_skb_metadata()
calls skb_checksum_help() while only the first descriptor has been added to
the skb. The checksum therefore covers just the first fragment, and
skb_checksum_help() sets ip_summed to CHECKSUM_NONE, so the remaining
fragments are never included in the checksum.

Move the software checksum calculation to the point where the last
descriptor has been added to the skb.

On a checksum failure the packet is now dropped instead of being returned
to the application for a retry. This path is not reachable with the
existing metadata validation, but the behavior change is intentional.

Fixes: 11614723af26 ("xsk: Add option to calculate TX checksum in SW")
Cc: stable@vger.kernel.org
Signed-off-by: Rongzheng Wang <testwrzwrz@gmail.com>
---
 net/xdp/xsk.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 33475b180ea6..65be2a368325 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -1011,14 +1011,6 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
 		skb->csum_start = hr + csum_start;
 		skb->csum_offset = csum_offset;
 		skb->ip_summed = CHECKSUM_PARTIAL;
-
-		if (unlikely(pool->tx_sw_csum)) {
-			int err;
-
-			err = skb_checksum_help(skb);
-			if (err)
-				return err;
-		}
 	}
 
 	if (flags & XDP_TXMD_FLAGS_LAUNCH_TIME)
@@ -1030,6 +1022,14 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
 	return 0;
 }
 
+static int xsk_skb_sw_csum(struct sk_buff *skb, struct xsk_buff_pool *pool)
+{
+	if (!pool->tx_sw_csum || skb->ip_summed != CHECKSUM_PARTIAL)
+		return 0;
+
+	return skb_checksum_help(skb);
+}
+
 static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 					      struct xdp_desc *desc)
 {
@@ -1275,6 +1275,12 @@ static int __xsk_generic_xmit(struct sock *sk)
 		if (xp_mb_desc(&desc)) {
 			xs->skb = skb;
 			continue;
+		} else {
+			err = xsk_skb_sw_csum(skb, xs->pool);
+			if (unlikely(err)) {
+				xsk_drop_skb(skb);
+				goto out;
+			}
 		}
 
 		err = __dev_direct_xmit(skb, xs->queue_id);
-- 
2.39.5 (Apple Git-154)