From 6543fa5a5c47cfbea92586f0994431fc8ba09f6a Mon Sep 17 00:00:00 2001
From: Mitchell Horne <mhorne@FreeBSD.org>
Date: Sat, 14 May 2022 10:25:21 -0300
Subject: [PATCH] dumpon: warn if the configured netdump link is down

Previously we expected the DIOCSKERNELDUMP ioctl to return ENXIO if the
interface was down, but it does not actually do this. Grab the link
status using getifaddrs(3) instead, and downgrade this case from an
error to a warning; the user might bring the link back up at a later
time.

Reviewed by:	cem
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D35196
---
 sbin/dumpon/dumpon.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/sbin/dumpon/dumpon.c b/sbin/dumpon/dumpon.c
index 7d8f81d5eaaf..626350427595 100644
--- a/sbin/dumpon/dumpon.c
+++ b/sbin/dumpon/dumpon.c
@@ -186,6 +186,25 @@ find_gateway(const char *ifname)
 	return (ret);
 }
 
+static void
+check_link_status(const char *ifname)
+{
+	struct ifaddrs *ifap, *ifa;
+
+	if (getifaddrs(&ifap) != 0)
+		err(EX_OSERR, "getifaddrs");
+
+	for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next) {
+		if (strcmp(ifname, ifa->ifa_name) != 0)
+			continue;
+		if ((ifa->ifa_flags & IFF_UP) == 0) {
+			warnx("warning: %s's link is down", ifname);
+		}
+		break;
+	}
+	freeifaddrs(ifap);
+}
+
 static void
 check_size(int fd, const char *fn)
 {
@@ -659,6 +678,9 @@ main(int argc, char *argv[])
 		else
 			error = errno;
 	}
+	/* Emit a warning if the user configured a downed interface. */
+	if (error == 0 && netdump)
+		check_link_status(kdap->kda_iface);
 	explicit_bzero(kdap->kda_encryptedkey, kdap->kda_encryptedkeysize);
 	free(kdap->kda_encryptedkey);
 	explicit_bzero(kdap, sizeof(*kdap));
@@ -669,10 +691,7 @@ main(int argc, char *argv[])
 			 * errors, especially as users don't have any great
 			 * discoverability into which NICs support netdump.
 			 */
-			if (error == ENXIO)
-				errx(EX_OSERR, "Unable to configure netdump "
-				    "because the interface's link is down.");
-			else if (error == ENODEV)
+			if (error == ENODEV)
 				errx(EX_OSERR, "Unable to configure netdump "
 				    "because the interface driver does not yet "
 				    "support netdump.");