git: 1019354b5416 - main - carp: deal with negative net.inet.carp.demotion

The branch main has been updated by kp:

URL: https://cgit.FreeBSD.org/src/commit/?id=1019354b54161c140d10a8203ff1e849c09c8010

commit 1019354b54161c140d10a8203ff1e849c09c8010
Author:     Marius Halden <marius.halden@modirum.com>
AuthorDate: 2021-10-31 20:18:42 +0000
Commit:     Kristof Provost <kp@FreeBSD.org>
CommitDate: 2021-11-01 16:08:23 +0000

    carp: deal with negative net.inet.carp.demotion
    
    Given nodes 1 and 2, where node 1 has an advskew of 0 and node 2 has an
    advskew of 100, making them master and backup respectively.
    
    If net.inet.carp.demotion is set to a negative value on node 1, node 2
    might become master while node 1 still retains it master status. Wether
    or not node 2 becomes master seems to depend on the nodes advskew and
    what the demotion sysctl was set to on node 1.
    
    The reason for node 2 becoming master seems to be that the calculated
    advskew taking demotion into account is truncated to a single unsigned
    byte when copied into the carp header for sending, and node 1 stays
    master since it takes uses the whole non-truncated calculated advskew
    when deciding wether to stay master.
    
    PR:             259528
    Reviewed by:    donner, glebius
    MFC after:      3 weeks
    Sponsored by:   Modirum MDPay
    Differential Revision:  https://reviews.freebsd.org/D32759
---
 sys/netinet/ip_carp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/sys/netinet/ip_carp.c b/sys/netinet/ip_carp.c
index 9a551a70c3ad..7554becb974e 100644
--- a/sys/netinet/ip_carp.c
+++ b/sys/netinet/ip_carp.c
@@ -304,7 +304,9 @@ SYSCTL_VNET_PCPUSTAT(_net_inet_carp, OID_AUTO, stats, struct carpstats,
 
 #define	DEMOTE_ADVSKEW(sc)					\
     (((sc)->sc_advskew + V_carp_demotion > CARP_MAXSKEW) ?	\
-    CARP_MAXSKEW : ((sc)->sc_advskew + V_carp_demotion))
+    CARP_MAXSKEW :						\
+        (((sc)->sc_advskew + V_carp_demotion < 0) ?		\
+        0 : ((sc)->sc_advskew + V_carp_demotion)))
 
 static void	carp_input_c(struct mbuf *, struct carp_header *, sa_family_t);
 static struct carp_softc