[PATCH v4] conf: Support CIDR notation for -a/--address option

[Jon Maloy <jmaloy@redhat.com>]

Extend the -a/--address option to accept addresses in CIDR notation
(e.g., 192.168.1.1/24 or 2001:db8::1/64) as an alternative to using
separate -a and -n options.

We add a new conf_addr_prefix_len() helper function that:
- Parses address strings with optional /prefix_len suffix
- Validates prefix length based on address family (0-32 for IPv4,
  0-128 for IPv6), including handling of IPv4-to-IPv6 mapping case.
- Returns address family via union inany_addr output parameter

For IPv4, the prefix length is stored in ip4.prefix_len when provided.
Mixing -n and CIDR notation results in an error to catch likely user
mistakes.

Also fix a bug in conf_ip4_prefix() that was incorrectly using the
global 'optarg' instead of its 'arg' parameter.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

---
v3: Fixes after feedback from Laurent, David and Stefano
    Notably, updated man page for the -a option

v4: Fixes based on feedback from David G:
  - Handling prefix length adjustment when IPv4-to-IPv6 mapping
  - Removed redundant !IN6_IS_ADDR_V4MAPPED(&addr.a6) test
  - Simplified tests of acceptable address types
  - Merged documentation and code commits
  - Some documentation text clarifications
---
 conf.c  | 91 +++++++++++++++++++++++++++++++++++++++++++++++----------
 inany.c | 29 ++++++++++++++++++
 inany.h |  1 +
 passt.1 | 17 +++++++----
 4 files changed, 118 insertions(+), 20 deletions(-)

diff --git a/conf.c b/conf.c
index 2942c8c..b36abf3 100644
--- a/conf.c
+++ b/conf.c
@@ -682,7 +682,7 @@ static int conf_ip4_prefix(const char *arg)
 			return -1;
 	} else {
 		errno = 0;
-		len = strtoul(optarg, NULL, 0);
+		len = strtoul(arg, NULL, 0);
 		if (len > 32 || errno)
 			return -1;
 	}
@@ -690,6 +690,50 @@ static int conf_ip4_prefix(const char *arg)
 	return len;
 }
 
+/**
+ * conf_addr_prefix_len() - Parse address with optional prefix length
+ * @arg:	Address string, optionally with /prefix_len suffix (modified)
+ * @addr:	Output for parsed address
+ * @prefix_len: Output for prefix length (0 if not specified)
+ *
+ * Return: AF_INET for IPv4, AF_INET6 for IPv6, -1 on error
+ */
+static int conf_addr_prefix_len(char *arg, union inany_addr *addr,
+				int *prefix_len)
+{
+	char *slash;
+
+	*prefix_len = 0;
+
+	/* Check for /prefix_len suffix */
+	slash = strchr(arg, '/');
+	if (slash) {
+		unsigned long len;
+		char *end;
+
+		*slash = '\0';
+		errno = 0;
+		len = strtoul(slash + 1, &end, 10);
+		if (errno || *end)
+			return -1;
+
+		*prefix_len = len;
+	}
+
+	if (!inany_prefix_pton(arg, addr, prefix_len))
+		return -1;
+
+	if (inany_v4(addr)) {
+		if (*prefix_len > 32)
+			return -1;
+		return AF_INET;
+	}
+
+	if (*prefix_len > 128)
+		return -1;
+	return AF_INET6;
+}
+
 /**
  * conf_ip4() - Verify or detect IPv4 support, get relevant addresses
  * @ifi:	Host interface to attempt (0 to determine one)
@@ -896,7 +940,7 @@ static void usage(const char *name, FILE *f, int status)
 		"    a zero value disables assignment\n"
 		"    default: 65520: maximum 802.3 MTU minus 802.3 header\n"
 		"                    length, rounded to 32 bits (IPv4 words)\n"
-		"  -a, --address ADDR	Assign IPv4 or IPv6 address ADDR\n"
+		"  -a, --address ADDR	Assign IPv4 or IPv6 address ADDR[/PREFIXLEN]\n"
 		"    can be specified zero to two times (for IPv4 and IPv6)\n"
 		"    default: use addresses from interface with default route\n"
 		"  -n, --netmask MASK	Assign IPv4 MASK, dot-decimal or bits\n"
@@ -1499,6 +1543,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt";
 	char userns[PATH_MAX] = { 0 }, netns[PATH_MAX] = { 0 };
 	bool copy_addrs_opt = false, copy_routes_opt = false;
+	bool prefix_from_cidr = false, prefix_from_opt = false;
 	enum fwd_ports_mode fwd_default = FWD_NONE;
 	bool v4_only = false, v6_only = false;
 	unsigned dns4_idx = 0, dns6_idx = 0;
@@ -1808,23 +1853,36 @@ void conf(struct ctx *c, int argc, char **argv)
 			c->mtu = mtu;
 			break;
 		}
-		case 'a':
-			if (inet_pton(AF_INET6, optarg, &c->ip6.addr)	&&
-			    !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr)	&&
-			    !IN6_IS_ADDR_LOOPBACK(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_V4MAPPED(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_V4COMPAT(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_MULTICAST(&c->ip6.addr)) {
+		case 'a': {
+			union inany_addr addr;
+			const struct in_addr *a4;
+			int prefix_len = 0;
+			int af;
+
+			af = conf_addr_prefix_len(optarg, &addr, &prefix_len);
+
+			if (inany_is_unspecified(&addr) ||
+			    inany_is_multicast(&addr) ||
+			    inany_is_loopback(&addr) ||
+			    IN6_IS_ADDR_V4COMPAT(&addr.a6))
+				die("Invalid address: %s", optarg);
+
+			if (af == AF_INET6) {
+				c->ip6.addr = addr.a6;
 				if (c->mode == MODE_PASTA)
 					c->ip6.no_copy_addrs = true;
 				break;
 			}
 
-			if (inet_pton(AF_INET, optarg, &c->ip4.addr)	&&
-			    !IN4_IS_ADDR_UNSPECIFIED(&c->ip4.addr)	&&
-			    !IN4_IS_ADDR_BROADCAST(&c->ip4.addr)	&&
-			    !IN4_IS_ADDR_LOOPBACK(&c->ip4.addr)		&&
-			    !IN4_IS_ADDR_MULTICAST(&c->ip4.addr)) {
+			a4 = inany_v4(&addr);
+			if (af == AF_INET && a4) {
+				c->ip4.addr = *a4;
+				if (prefix_len) {
+					if (prefix_from_opt)
+						die("Can't mix CIDR with -n");
+					c->ip4.prefix_len = prefix_len;
+					prefix_from_cidr = true;
+				}
 				if (c->mode == MODE_PASTA)
 					c->ip4.no_copy_addrs = true;
 				break;
@@ -1832,11 +1890,14 @@ void conf(struct ctx *c, int argc, char **argv)
 
 			die("Invalid address: %s", optarg);
 			break;
+		}
 		case 'n':
+			if (prefix_from_cidr)
+				die("Can't use both -n and CIDR prefix length");
 			c->ip4.prefix_len = conf_ip4_prefix(optarg);
 			if (c->ip4.prefix_len < 0)
 				die("Invalid netmask: %s", optarg);
-
+			prefix_from_opt = true;
 			break;
 		case 'M':
 			parse_mac(c->our_tap_mac, optarg);
diff --git a/inany.c b/inany.c
index 7680439..f142a76 100644
--- a/inany.c
+++ b/inany.c
@@ -57,3 +57,32 @@ int inany_pton(const char *src, union inany_addr *dst)
 
 	return 0;
 }
+
+/** inany_prefix_pton - Parse an IPv[46] address with prefix length adjustment
+ * @src:	IPv[46] address string
+ * @dst:	output buffer, filled with parsed address
+ * @prefix_len: pointer to prefix length
+ *
+ * Return: on success, 1, if no parseable address is found, 0
+ */
+int inany_prefix_pton(const char *src, union inany_addr *dst, int *prefix_len)
+{
+	/* First try parsing as plain IPv4 */
+	if (inet_pton(AF_INET, src, &dst->v4mapped.a4)) {
+		memset(&dst->v4mapped.zero, 0, sizeof(dst->v4mapped.zero));
+		memset(&dst->v4mapped.one, 0xff, sizeof(dst->v4mapped.one));
+		return 1;
+	}
+
+	/* Try parsing as IPv6, adjust prefix length if mapped IPv4 address */
+	if (inet_pton(AF_INET6, src, &dst->a6)) {
+		if (inany_v4(dst) && prefix_len && *prefix_len > 0) {
+			if (*prefix_len < 96)
+				return 0;
+			*prefix_len -= 96;
+		}
+		return 1;
+	}
+
+	return 0;
+}
diff --git a/inany.h b/inany.h
index 61b36fb..36865f9 100644
--- a/inany.h
+++ b/inany.h
@@ -295,5 +295,6 @@ static inline void inany_siphash_feed(struct siphash_state *state,
 
 const char *inany_ntop(const union inany_addr *src, char *dst, socklen_t size);
 int inany_pton(const char *src, union inany_addr *dst);
+int inany_prefix_pton(const char *src, union inany_addr *dst, int *prefix_len);
 
 #endif /* INANY_H */
diff --git a/passt.1 b/passt.1
index db0d662..7ca03be 100644
--- a/passt.1
+++ b/passt.1
@@ -156,10 +156,14 @@ By default, the advertised MTU is 65520 bytes, that is, the maximum 802.3 MTU
 minus the length of a 802.3 header, rounded to 32 bits (IPv4 words).
 
 .TP
-.BR \-a ", " \-\-address " " \fIaddr
+.BR \-a ", " \-\-address " " \fIaddr\fR[\fB/\fR\fIprefix_len\fR]
 Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option
 5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3)
 for an IPv6 \fIaddr\fR.
+An optional \fB/\fR\fIprefix_len\fR (0-32 for IPv4, 0-128 for IPv6) can be
+appended in CIDR notation (e.g., 192.168.1.1/24). This is an alternative to
+using the \fB-n\fR, \fB--netmask\fR option. Mixing CIDR notation with
+\fB-n\fR results in an error.
 This option can be specified zero (for defaults) to two times (once for IPv4,
 once for IPv6).
 By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
@@ -172,10 +176,13 @@ is assigned for IPv4, and no additional address will be assigned for IPv6.
 .TP
 .BR \-n ", " \-\-netmask " " \fImask
 Assign IPv4 netmask \fImask\fR, expressed as dot-decimal or number of bits, via
-DHCP (option 1).
-By default, the netmask associated to the host address matching the assigned one
-is used. If there's no matching address on the host, the netmask is determined
-according to the CIDR block of the assigned address (RFC 4632).
+DHCP (option 1). Alternatively, the prefix length can be specified using CIDR
+notation with the \fB-a\fR, \fB--address\fR option (e.g., \fB-a\fR 192.168.1.1/24).
+Mixing \fB-n\fR with CIDR notation results in an error.
+If no address is indicated, the netmask associated with the adopted host address,
+if any, is used. If an address is indicated, but without a prefix length, the
+netmask is determined based on the corresponding network class. In all other
+cases, the netmask is determined by using the indicated prefix length.
 
 .TP
 .BR \-M ", " \-\-mac-addr " " \fIaddr
-- 
2.52.0