[PATCH net-next v8 2/9] net: Enable BIG TCP with partial GSO

[Alice Mikityanska <alice.kernel@fastmail.im>]

From: Alice Mikityanska <alice@isovalent.com>

skb_segment is called for partial GSO, when netif_needs_gso returns true
in validate_xmit_skb. Partial GSO is needed, for example, when
segmentation of tunneled traffic is offloaded to a NIC that only
supports inner checksum offload.

Currently, skb_segment clamps the segment length to 65534 bytes, because
gso_size == 65535 is a special value GSO_BY_FRAGS, and we don't want
to accidentally assign mss = 65535, as it would fall into the
GSO_BY_FRAGS check further in the function.

This implementation, however, artificially blocks len > 65534, which is
possible since the introduction of BIG TCP. To allow bigger lengths and
avoid resegmentation of BIG TCP packets, store the gso_by_frags flag in
the beginning and don't use a special value of mss for this purpose
after mss was modified.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
 net/core/skbuff.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 18dabb4e9cfa..6ae4c2b205f2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4775,6 +4775,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 	struct sk_buff *tail = NULL;
 	struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list;
 	unsigned int mss = skb_shinfo(head_skb)->gso_size;
+	bool gso_by_frags = mss == GSO_BY_FRAGS;
 	unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
 	unsigned int offset = doffset;
 	unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
@@ -4790,7 +4791,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 	int nfrags, pos;
 
 	if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) &&
-	    mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
+	    !gso_by_frags && mss != skb_headlen(head_skb)) {
 		struct sk_buff *check_skb;
 
 		for (check_skb = list_skb; check_skb; check_skb = check_skb->next) {
@@ -4818,7 +4819,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 	sg = !!(features & NETIF_F_SG);
 	csum = !!can_checksum_protocol(features, proto);
 
-	if (sg && csum && (mss != GSO_BY_FRAGS))  {
+	if (sg && csum && !gso_by_frags)  {
 		if (!(features & NETIF_F_GSO_PARTIAL)) {
 			struct sk_buff *iter;
 			unsigned int frag_len;
@@ -4852,9 +4853,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		/* GSO partial only requires that we trim off any excess that
 		 * doesn't fit into an MSS sized block, so take care of that
 		 * now.
-		 * Cap len to not accidentally hit GSO_BY_FRAGS.
 		 */
-		partial_segs = min(len, GSO_BY_FRAGS - 1) / mss;
+		partial_segs = len / mss;
 		if (partial_segs > 1)
 			mss *= partial_segs;
 		else
@@ -4878,7 +4878,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		int hsize;
 		int size;
 
-		if (unlikely(mss == GSO_BY_FRAGS)) {
+		if (unlikely(gso_by_frags)) {
 			len = list_skb->len;
 		} else {
 			len = head_skb->len - offset;
-- 
2.54.0