host/start-vm: tap_open: return name of tap

This is important when using %d to dynamically choose a tap name in
the kernel.  It's not needed for the start-vm program, but supporting
dynamic names makes writing tests easier.

3 files changed, 21 insertions, 9 deletions

diff --git a/host/start-vm/net-util.c b/host/start-vm/net-util.c
index cc6b950..f4c454f 100644
--- a/host/start-vm/net-util.c
+++ b/host/start-vm/net-util.c
@@ -1,10 +1,11 @@
 // SPDX-License-Identifier: EUPL-1.2
 // SPDX-FileCopyrightText: 2022 Alyssa Ross <hi@alyssa.is>
 
+#include "net-util.h"
+
 #include <err.h>
 #include <errno.h>
 #include <fcntl.h>
-#include <net/if.h>
 #include <string.h>
 #include <unistd.h>
 
@@ -126,7 +127,7 @@ int bridge_delete(const char *name)
 	return r;
 }
 
-int tap_open(const char *name, int flags)
+int tap_open(char name[static IFNAMSIZ], int flags)
 {
 	struct ifreq ifr;
 	int fd;
@@ -141,9 +142,11 @@ int tap_open(const char *name, int flags)
 
 	if ((fd = open("/dev/net/tun", O_RDWR)) == -1)
 		return -1;
-	if (!ioctl(fd, TUNSETIFF, &ifr))
-		return fd;
+	if (ioctl(fd, TUNSETIFF, &ifr) == -1) {
+		close(fd);
+		return -1;
+	}
 
-	close(fd);
-	return -1;
+	strncpy(name, ifr.ifr_name, IFNAMSIZ);
+	return fd;
 }
diff --git a/host/start-vm/net-util.h b/host/start-vm/net-util.h
index 85d7833..e1899eb 100644
--- a/host/start-vm/net-util.h
+++ b/host/start-vm/net-util.h
@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: EUPL-1.2
 // SPDX-FileCopyrightText: 2022 Alyssa Ross <hi@alyssa.is>
 
+#include <net/if.h>
+
 int if_up(const char *name);
 int if_rename(const char *name, const char *newname);
 int if_down(const char *name);
@@ -10,4 +12,4 @@ int bridge_add_if(const char *brname, const char *ifname);
 int bridge_remove_if(const char *brname, const char *ifname);
 int bridge_delete(const char *name);
 
-int tap_open(const char *name, int flags);
+int tap_open(char name[static IFNAMSIZ], int flags);
diff --git a/host/start-vm/tests/tap_open.c b/host/start-vm/tests/tap_open.c
index ba9b9ec..19bc145 100644
--- a/host/start-vm/tests/tap_open.c
+++ b/host/start-vm/tests/tap_open.c
@@ -6,10 +6,16 @@
 #include <assert.h>
 #include <errno.h>
 #include <sched.h>
+#include <string.h>
+
+#include <sys/ioctl.h>
+
+#include <linux/if_tun.h>
 
 int main(void)
 {
-	char name[] = "tap%d";
+	char name[IFNAMSIZ] = "tap%d";
+	struct ifreq ifr;
 	int fd;
 
 	unshare(CLONE_NEWUSER|CLONE_NEWNET);
@@ -17,5 +23,6 @@ int main(void)
 	fd = tap_open(name, 0);
 	if (fd == -1 && errno == EPERM)
 		return 77;
-	assert(fd != -1);
+	assert(!ioctl(fd, TUNGETIFF, &ifr));
+	assert(!strcmp(name, ifr.ifr_name));
 }