[PATCH v2] tcp: fix tp->undo_retrans accounting in tcp_sacktag_one()

Commit a71d77e6be1e ("tcp: fix segment accounting when DSACK range covers
multiple segments") fix some DSACK accounting for multiple segments.
In tcp_sacktag_one(), we should also use the actual DSACK rang(pcount)
for tp->undo_retrans accounting.

Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")

Before that commit, the assumption underlying the tp->undo_retrans-- 
seems correct, AFAICT.

Signed-off-by: zhenggy <redacted>
---
 net/ipv4/tcp_input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3f7bd7a..141e85e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -1346,7 +1346,7 @@ static u8 tcp_sacktag_one(struct sock *sk,
 	if (dup_sack && (sacked & TCPCB_RETRANS)) {
 		if (tp->undo_marker && tp->undo_retrans > 0 &&
 		    after(end_seq, tp->undo_marker))
-			tp->undo_retrans--;
+			tp->undo_retrans = max_t(int, 0, tp->undo_retrans - pcount);
 		if ((sacked & TCPCB_SACKED_ACKED) &&
 		    before(start_seq, state->reord))
 				state->reord = start_seq;

-- 
1.8.3.1

On Thu, Sep 9, 2021 at 10:40 PM zhenggy [off-list ref] wrote:

Commit a71d77e6be1e ("tcp: fix segment accounting when DSACK range covers
multiple segments") fix some DSACK accounting for multiple segments.
In tcp_sacktag_one(), we should also use the actual DSACK rang(pcount)

nit: typo; I'd suggest: "range (pcount)"

for tp->undo_retrans accounting.

Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")

Before that commit, the assumption underlying the tp->undo_retrans--
seems correct, AFAICT.

Signed-off-by: zhenggy <redacted>

Please take a look at the standard formatting in the "git log"
history. The Fixes: tag should be in the footers section, typically
the first footer, preceding fields like Reported-by: and Cc: and
Signed-off-by:.

Please consider something like:

----
Since 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")
this logic can no longer assume that a retransmitted packet is a single segment.

Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")
Signed-off-by: zhenggy <redacted>
---

thanks,
neal