git: 26370413d43b - stable/12 - tcp: fix computation of offset

The branch stable/12 has been updated by rscheff:

URL: https://cgit.FreeBSD.org/src/commit/?id=26370413d43bfd65500270ff331ae6bdf0f54133

commit 26370413d43bfd65500270ff331ae6bdf0f54133
Author:     Michael Tuexen <tuexen@FreeBSD.org>
AuthorDate: 2022-09-19 10:42:43 +0000
Commit:     Richard Scheffenegger <rscheff@FreeBSD.org>
CommitDate: 2022-09-25 08:41:54 +0000

    tcp: fix computation of offset
    
    Only update the offset if actually retransmitting from the
    scoreboard. If not done correctly, this may result in
    trying to (re)-transmit data not being being in the socket
    buffe and therefore resulting in a panic.
    
    PR:                     264257
    PR:                     263445
    PR:                     260393
    Reviewed by:            rscheff@
    MFC after:              3 days
    Sponsored by:           Netflix, Inc.
    Differential Revision:  https://reviews.freebsd.org/D36626
    
    (cherry picked from commit 6d9e911fbadf3b409802a211c1dae9b47cb5a2b8)
---
 sys/netinet/tcp_output.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index b486f6250508..e8d585a15d1c 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -317,10 +317,10 @@ again:
 			len = ((int32_t)ulmin(cwin,
 			    SEQ_SUB(p->end, p->rxmit)));
 		}
-		off = SEQ_SUB(p->rxmit, tp->snd_una);
-		KASSERT(off >= 0,("%s: sack block to the left of una : %d",
-		    __func__, off));
 		if (len > 0) {
+			off = SEQ_SUB(p->rxmit, tp->snd_una);
+			KASSERT(off >= 0,("%s: sack block to the left of una : %d",
+			    __func__, off));
 			sack_rxmit = 1;
 			sendalot = 1;
 			TCPSTAT_INC(tcps_sack_rexmits);