util: Make sock_l4() treat empty string ifname like NULL

sock_l4() takes NULL for ifname if you don't want to bind the socket to a
particular interface.  However, for a number of the callers, it's more
natural to use an empty string for that case.  Change sock_l4() to accept
either NULL or an empty string equivalently, and simplify some callers
using that change.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

3 files changed, 7 insertions, 16 deletions

diff --git a/icmp.c b/icmp.c
index d982fda..c82efd0 100644
--- a/icmp.c
+++ b/icmp.c
@@ -187,12 +187,8 @@ int icmp_tap_handler(const struct ctx *c, uint8_t pif, int af,
 		iref.id = id = ntohs(ih->un.echo.id);
 
 		if ((s = icmp_id_map[V4][id].sock) <= 0) {
-			const char *bind_if;
-
-			bind_if = *c->ip4.ifname_out ? c->ip4.ifname_out : NULL;
-
-			s = sock_l4(c, AF_INET, IPPROTO_ICMP,
-				    &c->ip4.addr_out, bind_if, id, iref.u32);
+			s = sock_l4(c, AF_INET, IPPROTO_ICMP, &c->ip4.addr_out,
+				    c->ip4.ifname_out, id, iref.u32);
 			if (s < 0)
 				goto fail_sock;
 			if (s > FD_REF_MAX) {
@@ -237,12 +233,9 @@ int icmp_tap_handler(const struct ctx *c, uint8_t pif, int af,
 
 		iref.id = id = ntohs(ih->icmp6_identifier);
 		if ((s = icmp_id_map[V6][id].sock) <= 0) {
-			const char *bind_if;
-
-			bind_if = *c->ip6.ifname_out ? c->ip6.ifname_out : NULL;
-
 			s = sock_l4(c, AF_INET6, IPPROTO_ICMPV6,
-				    &c->ip6.addr_out, bind_if, id, iref.u32);
+				    &c->ip6.addr_out,
+				    c->ip6.ifname_out, id, iref.u32);
 			if (s < 0)
 				goto fail_sock;
 			if (s > FD_REF_MAX) {
diff --git a/udp.c b/udp.c
index f6e8b37..7057977 100644
--- a/udp.c
+++ b/udp.c
@@ -870,8 +870,7 @@ int udp_tap_handler(struct ctx *c, uint8_t pif,
 			union udp_epoll_ref uref = { .port = src };
 			const char *bind_if = NULL;
 
-			if (!IN6_IS_ADDR_LOOPBACK(&s_in.sin_addr) &&
-			    *c->ip6.ifname_out)
+			if (!IN6_IS_ADDR_LOOPBACK(&s_in.sin_addr))
 				bind_if = c->ip6.ifname_out;
 
 			if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.addr_out) &&
@@ -919,8 +918,7 @@ int udp_tap_handler(struct ctx *c, uint8_t pif,
 			union udp_epoll_ref uref = { .v6 = 1, .port = src };
 			const char *bind_if = NULL;
 
-			if (!IN6_IS_ADDR_LOOPBACK(&s_in6.sin6_addr) &&
-			    *c->ip6.ifname_out)
+			if (!IN6_IS_ADDR_LOOPBACK(&s_in6.sin6_addr))
 				bind_if = c->ip6.ifname_out;
 
 			if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr_out) &&
diff --git a/util.c b/util.c
index 6e90371..d51f50f 100644
--- a/util.c
+++ b/util.c
@@ -187,7 +187,7 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 	if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &y, sizeof(y)))
 		debug("Failed to set SO_REUSEADDR on socket %i", fd);
 
-	if (ifname) {
+	if (ifname && *ifname) {
 		/* Supported since kernel version 5.7, commit c427bfec18f2
 		 * ("net: core: enable SO_BINDTODEVICE for non-root users"). If
 		 * it's unsupported, don't bind the socket at all, because the