[PATCH net v5 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
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Rongzheng Wang <testwrzwrz@gmail.com>
---
 net/xdp/xsk.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 33475b180ea6..27ec1d931fe8 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 +1022,15 @@ 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 (unlikely(pool->tx_sw_csum &&
+		     skb->ip_summed == CHECKSUM_PARTIAL))
+		return skb_checksum_help(skb);
+
+	return 0;
+}
+
 static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 					      struct xdp_desc *desc)
 {
@@ -1277,6 +1278,12 @@ static int __xsk_generic_xmit(struct sock *sk)
 			continue;
 		}
 
+		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);
 		if  (err == NETDEV_TX_BUSY) {
 			/* Tell user-space to retry the send */
-- 
2.39.5 (Apple Git-154)