mixer: Fix default_unit switching with mixers that have no devices

Apparently it's possible for a mixer to have no devices:

$ mixer -f /dev/mixer2
pcm2:mixer: <USB audio> at ? kld snd_uaudio (rec)
$

If this is the default sound device, an attempt to change the default
unit using mixer -d fails with a segfault because mod_dunit is called
with a NULL device pointer, which is dereferenced to get the parent
mixer.

ctl_dunit seems to be a dummy, i.e., we don't actually need it and can
simply pass the mixer to mod_dunit() directly.  This patch removes that
structure and associated indirection to fix the crash.

Reviewed by:	christos, hselasky
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D38060

1 files changed, 5 insertions, 14 deletions

diff --git a/usr.sbin/mixer/mixer.c b/usr.sbin/mixer/mixer.c
index e216efe3313c..6095f7550ea8 100644
--- a/usr.sbin/mixer/mixer.c
+++ b/usr.sbin/mixer/mixer.c
@@ -42,8 +42,8 @@ static void printall(struct mixer *, int);
 static void printminfo(struct mixer *, int);
 static void printdev(struct mixer *, int);
 static void printrecsrc(struct mixer *, int); /* XXX: change name */
+static int set_dunit(struct mixer *, int);
 /* Control handlers */
-static int mod_dunit(struct mix_dev *, void *);
 static int mod_volume(struct mix_dev *, void *);
 static int mod_mute(struct mix_dev *, void *);
 static int mod_recsrc(struct mix_dev *, void *);
@@ -51,14 +51,6 @@ static int print_volume(struct mix_dev *, void *);
 static int print_mute(struct mix_dev *, void *);
 static int print_recsrc(struct mix_dev *, void *);
 
-static const mix_ctl_t ctl_dunit = {
-	.parent_dev	= NULL,
-	.id		= -1,
-	.name		= "default_unit",
-	.mod		= mod_dunit,
-	.print		= NULL
-};
-
 int
 main(int argc, char *argv[])
 {
@@ -125,7 +117,7 @@ main(int argc, char *argv[])
 
 	initctls(m);
 
-	if (dflag && ctl_dunit.mod(m->dev, &dunit) < 0)
+	if (dflag && set_dunit(m, dunit) < 0)
 		goto parse;
 	if (sflag) {
 		printrecsrc(m, oflag);
@@ -316,20 +308,19 @@ printrecsrc(struct mixer *m, int oflag)
 }
 
 static int
-mod_dunit(struct mix_dev *d, void *p)
+set_dunit(struct mixer *m, int dunit)
 {
-	int dunit = *((int *)p);
 	int n;
 
 	if ((n = mixer_get_dunit()) < 0) {
 		warn("cannot get default unit");
 		return (-1);
 	}
-	if (mixer_set_dunit(d->parent_mixer, dunit) < 0) {
+	if (mixer_set_dunit(m, dunit) < 0) {
 		warn("cannot set default unit to: %d", dunit);
 		return (-1);
 	}
-	printf("%s: %d -> %d\n", ctl_dunit.name, n, dunit);
+	printf("default_unit: %d -> %d\n", n, dunit);
 
 	return (0);
 }