[PATCH 2/3] tcp, udp, conf: Don't silently ignore listens on unsupported IP versions

[David Gibson <david@gibson.dropbear.id.au>]

Currently, it's possible to explicitly ask for forwarding from an IPv4
address, while disabling IPv4:
    $ pasta -t 192.0.2.1/12345 -6
or vice versa:
    $ pasta -t 2001:db8::1/12345 -4

Currently, the impossible to implement forwarding option will be silently
ignored.  That's potentially confusing since in a complex setup, it might
not be obvious why the requested forward isn't taking effect.

Specifically, it's ignored at a fairly low level: tcp_listen() and
udp_listen() ignore it and return 0.  Those run kind of late to give a
good error message.  Change the low-level functions to return -EACCES
(chosen because that's what the kernel will return if you request IPv6
when it's disabled by sysctl).  Most callers of {tcp,udp}_listen() ignore
the return code, so this is a no-op for them.  In the remaining caller,
conf_ports_range_except() check for the case explicitly, and provide a
meaningful error message.

Of itself, this bug is insignificant, but this is a roadblock to having
{tcp,udp}_listen() return socket fds, which in turn is a roadblock to my
flexible forwarding work.  So, might as well fix it.

Link: https://bugs.passt.top/show_bug.cgi?id=186

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 conf.c | 10 ++++++++++
 tcp.c  |  6 ++----
 udp.c  |  6 ++----
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/conf.c b/conf.c
index 70ea168c..cc3c20a9 100644
--- a/conf.c
+++ b/conf.c
@@ -162,6 +162,16 @@ static void conf_ports_range_except(const struct ctx *c, char optname,
 		    optname, optarg);
 	}
 
+	if (addr) {
+		if (!c->ifi4 && inany_v4(addr)) {
+			die("IPv4 is disabled, can't use -%c %s",
+			    optname, optarg);
+		} else if (!c->ifi6 && !inany_v4(addr)) {
+			die("IPv6 is disabled, can't use -%c %s",
+			    optname, optarg);
+		}
+	}
+
 	for (i = first; i <= last; i++) {
 		if (bitmap_isset(exclude, i))
 			continue;
diff --git a/tcp.c b/tcp.c
index e7fa85f3..67007c05 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2700,16 +2700,14 @@ int tcp_listen(const struct ctx *c, uint8_t pif,
 			/* Restrict to v6 only */
 			addr = &inany_any6;
 		else if (inany_v4(addr))
-			/* Nothing to do */
-			return 0;
+			return -EACCES;
 	}
 	if (!c->ifi6) {
 		if (!addr)
 			/* Restrict to v4 only */
 			addr = &inany_any4;
 		else if (!inany_v4(addr))
-			/* Nothing to do */
-			return 0;
+			return -EACCES;
 	}
 
 	if (pif == PIF_HOST) {
diff --git a/udp.c b/udp.c
index eda55c39..8cfa1e1f 100644
--- a/udp.c
+++ b/udp.c
@@ -1162,16 +1162,14 @@ int udp_listen(const struct ctx *c, uint8_t pif,
 			/* Restrict to v6 only */
 			addr = &inany_any6;
 		else if (inany_v4(addr))
-			/* Nothing to do */
-			return 0;
+			return -EACCES;
 	}
 	if (!c->ifi6) {
 		if (!addr)
 			/* Restrict to v4 only */
 			addr = &inany_any4;
 		else if (!inany_v4(addr))
-			/* Nothing to do */
-			return 0;
+			return -EACCES;
 	}
 
 	s = pif_sock_l4(c, EPOLL_TYPE_UDP_LISTEN, pif,
-- 
2.52.0