From: David Gwynne <david@gwynne.id.au>
Subject: let bpf pick the first bpf_if, not the bpf_if with the lowest dlt
To: tech@openbsd.org
Date: Wed, 20 Nov 2024 10:18:35 +1000

this lets bpf_setif pick the first found dlt in the bpf_if list when
attaching a bpf descriptor to an interface. back in bpf.c r1.52
claudio changed bpf_setif so it would prefer lower numbered DLTs
when attaching a bpf descriptor to an interface. before that it would
pick the last bpf_if that was added because of how the bpf_if list was
ordered.

claudio made the change so things would show the "most common" DLT
by default, and the example given was an atw(4) wifi interface.
wifi interfaces provide at least two and often more DLTs,

wifi interfaces attach a DLT_EN10MB interface first, which is what
people expect when they tcpdump on an interface, so this doesn't break
the change in 1.52.

however, i want to attach multiple DLTs to some other interfaces, but
control which one is the default regardless of the number used for the
DLT identifier. letting bpf pick the first attached dlt on an interface
is the simplest fix for this, and doesn't seem to break existing use
cases.

ok?

Index: bpf.c
===================================================================
RCS file: /cvs/src/sys/net/bpf.c,v
diff -u -p -r1.227 bpf.c
--- bpf.c	19 Nov 2024 23:26:35 -0000	1.227
+++ bpf.c	19 Nov 2024 23:29:03 -0000
@@ -1180,22 +1180,19 @@ bpf_setf(struct bpf_d *d, struct bpf_pro
 int
 bpf_setif(struct bpf_d *d, struct ifreq *ifr)
 {
-	struct bpf_if *bp, *candidate = NULL;
+	struct bpf_if *bp;
 	int error = 0;
 
 	/*
 	 * Look through attached interfaces for the named one.
 	 */
 	TAILQ_FOREACH(bp, &bpf_iflist, bif_next) {
-		if (strcmp(bp->bif_name, ifr->ifr_name) != 0)
-			continue;
-
-		if (candidate == NULL || candidate->bif_dlt > bp->bif_dlt)
-			candidate = bp;
+		if (strcmp(bp->bif_name, ifr->ifr_name) == 0)
+			break;
 	}
 
 	/* Not found. */
-	if (candidate == NULL)
+	if (bp == NULL)
 		return (ENXIO);
 
 	/*
@@ -1208,12 +1205,12 @@ bpf_setif(struct bpf_d *d, struct ifreq 
 		if ((error = bpf_allocbufs(d)))
 			goto out;
 	}
-	if (candidate != d->bd_bif) {
+	if (bp != d->bd_bif) {
 		/*
 		 * Detach if attached to something else.
 		 */
 		bpf_detachd(d);
-		bpf_attachd(d, candidate);
+		bpf_attachd(d, bp);
 	}
 	bpf_resetd(d);
 out: