ip_gre: remove CRC flag from dev features in gre_gso_segment

This patch is to let it always do CRC checksum in sctp_gso_segment()
by removing CRC flag from the dev features in gre_gso_segment() for
SCTP over GRE, just as it does in Commit 527beb8 ("udp: support
sctp over udp in skb_udp_tunnel_segment") for SCTP over UDP.

It could set csum/csum_start in GSO CB properly in sctp_gso_segment()
after that commit, so it would do checksum with gso_make_checksum()
in gre_gso_segment(), and Commit 622e32b ("net: gre: recompute
gre csum for sctp over gre tunnels") can be reverted now.

Note that when need_csum is false, we can still leave CRC checksum
of SCTP to HW by not clearing this CRC flag if it's supported, as
Jakub and Alex noticed.

v1->v2:
  - improve the changelog.
  - fix "rev xmas tree" in varibles declaration.
v2->v3:
  - remove CRC flag from dev features only when need_csum is true.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/00439f24d5f69e2c6fa2beadc681d056c15c258f.1610772251.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/gre_offload.c

@@ -15,10 +15,10 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)
 {
 	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
-	bool need_csum, need_recompute_csum, gso_partial;
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	u16 mac_offset = skb->mac_header;
 	__be16 protocol = skb->protocol;
+	bool need_csum, gso_partial;
 	u16 mac_len = skb->mac_len;
 	int gre_offset, outer_hlen;
 
@@ -41,10 +41,11 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 	skb->protocol = skb->inner_protocol;
 
 	need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_GRE_CSUM);
-	need_recompute_csum = skb->csum_not_inet;
 	skb->encap_hdr_csum = need_csum;
 
 	features &= skb->dev->hw_enc_features;
+	if (need_csum)
+		features &= ~NETIF_F_SCTP_CRC;
 
 	/* segment inner packet. */
 	segs = skb_mac_gso_segment(skb, features);
@@ -99,15 +100,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 		}
 
 		*(pcsum + 1) = 0;
-		if (need_recompute_csum && !skb_is_gso(skb)) {
-			__wsum csum;
-
-			csum = skb_checksum(skb, gre_offset,
-					    skb->len - gre_offset, 0);
-			*pcsum = csum_fold(csum);
-		} else {
-			*pcsum = gso_make_checksum(skb, 0);
-		}
+		*pcsum = gso_make_checksum(skb, 0);
 	} while ((skb = skb->next));
 out:
 	return segs;