[PATCH net-next v3 6/6] tcp: fastopen: extend retransmit-queue trimming to handle linear sk_buff

[Jeremy Harris <jgh@exim.org>]

A corner-case for the 3rd-ack after a data-on-synack is for
some but not all of the data to be acked. Support this by
adding to the retransmit-queue trim routine to handle a
linear sk_buff.

Signed-off-by: Jeremy Harris <jgh@exim.org>
---
 net/ipv4/tcp_output.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c50553c1c795..bff5934ff04b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1708,8 +1708,22 @@ static int __pskb_trim_head(struct sk_buff *skb, int len)
 	struct skb_shared_info *shinfo;
 	int i, k, eat;
 
-	DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
-	eat = len;
+	eat = skb_headlen(skb);
+	if (unlikely(eat)) {
+		if (len < eat)
+			eat = len;
+		skb->head += eat;
+		skb->len -= eat;
+		if (skb->data_len)
+			skb->data_len -= eat;
+
+		eat = len - eat;
+		if (eat == 0)
+			return len;
+	} else {
+		eat = len;
+	}
+
 	k = 0;
 	shinfo = skb_shinfo(skb);
 	for (i = 0; i < shinfo->nr_frags; i++) {
-- 
2.49.0