git: c8b3b605ae85 - main - tcp: fix checksum calculation bug

The branch main has been updated by tuexen:

URL: https://cgit.FreeBSD.org/src/commit/?id=c8b3b605ae854ead6c8804e0400d80cb8fa73fdf

commit c8b3b605ae854ead6c8804e0400d80cb8fa73fdf
Author:     Timo Völker <timo.voelker@fh-muenster.de>
AuthorDate: 2025-12-19 16:50:20 +0000
Commit:     Michael Tuexen <tuexen@FreeBSD.org>
CommitDate: 2025-12-19 16:50:20 +0000

    tcp: fix checksum calculation bug
    
    The new function in_delayed_cksum_o() was introduced to compute
    the checksum in the case the mbuf chain does not start with the
    IP header. The offset of the IP header is specified by the
    parameter iph_offset.
    If iph_offset was positive, the function computed an incorrect
    checksum.
    
    Reviewed by:            sobomax, tuexen
    Fixes:                  5feb38e37847 ("netinet: provide "at offset" variant of the in_delayed_cksum() API")
    MFC after:              1 week
    Differential Revision:  https://reviews.freebsd.org/D54269
---
 sys/netinet/ip_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
index a6b29c6a7612..f1aa3c498ef7 100644
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1066,8 +1066,8 @@ in_delayed_cksum_o(struct mbuf *m, uint16_t iph_offset)
 		if (csum == 0)
 			csum = 0xffff;
 	} else {
-		cklen = ntohs(ip->ip_len);
-		csum = in_cksum_skip(m, cklen, offset);
+		cklen = ntohs(ip->ip_len) - (ip->ip_hl << 2);
+		csum = in_cksum_skip(m, cklen + offset, offset);
 	}
 	offset += m->m_pkthdr.csum_data;	/* checksum offset */