json: Fix osf ttl support

Having to use numerical values for ttl property in JSON is not
practical as these values are arbitrary and meaningful only in
netfilter. Instead align JSON output/input with standard API, accepting
names for TTL matching strategy.

Also add missing documentation in libnftables-json man page and fix JSON
equivalent in tests/py.

Fixes: 03eafe098d5ee ("osf: add ttl option support")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

4 files changed, 83 insertions, 8 deletions

diff --git a/doc/libnftables-json.adoc b/doc/libnftables-json.adoc
index 98303b35..ea5fbe81 100644
--- a/doc/libnftables-json.adoc
+++ b/doc/libnftables-json.adoc
@@ -1288,3 +1288,27 @@ ____
 ____
 
 Construct a reference to packet's socket.
+
+=== OSF
+[verse]
+____
+*{ "osf": {
+	"key":* 'OSF_KEY'*,
+	"ttl":* 'OSF_TTL'
+*}}*
+
+'OSF_KEY' := *"name"*
+'OSF_TTL' := *"loose"* | *"skip"*
+____
+
+Perform OS fingerprinting. This expression is typically used in LHS of a *match*
+statement.
+
+*key*::
+	What part of the fingerprint info to match against. At this point, only
+	the OS name is supported.
+*ttl*::
+	Define how packet's TTL value is to be matched. This property is
+	optional. If omitted, TTL value has to match exactly. A value of *loose*
+	accepts TTL values less than the fingerprint one. A value of *skip*
+	omits TTL value comparison entirely.
diff --git a/src/json.c b/src/json.c
index cea9f19c..d21536af 100644
--- a/src/json.c
+++ b/src/json.c
@@ -857,7 +857,18 @@ json_t *socket_expr_json(const struct expr *expr, struct output_ctx *octx)
 
 json_t *osf_expr_json(const struct expr *expr, struct output_ctx *octx)
 {
-	return json_pack("{s:{s:i, s:s}}", "osf", "ttl", expr->osf.ttl, "key", "name");
+	json_t *root = json_pack("{s:s}", "key", "name");
+
+	switch (expr->osf.ttl) {
+	case 1:
+		json_object_set_new(root, "ttl", json_string("loose"));
+		break;
+	case 2:
+		json_object_set_new(root, "ttl", json_string("skip"));
+		break;
+	}
+
+	return json_pack("{s:o}", "osf", root);
 }
 
 json_t *xfrm_expr_json(const struct expr *expr, struct output_ctx *octx)
diff --git a/src/parser_json.c b/src/parser_json.c
index fc0dc9a9..46a02fe1 100644
--- a/src/parser_json.c
+++ b/src/parser_json.c
@@ -375,14 +375,25 @@ static struct expr *json_parse_meta_expr(struct json_ctx *ctx,
 static struct expr *json_parse_osf_expr(struct json_ctx *ctx,
 					const char *type, json_t *root)
 {
-	const char *key;
-	uint8_t ttl;
+	const char *key, *ttl;
+	uint8_t ttlval = 0;
 
-	if (json_unpack_err(ctx, root, "{s:i, s:s}", "ttl", ttl,"key", &key))
+	if (json_unpack_err(ctx, root, "{s:s}", "key", &key))
 		return NULL;
 
+	if (!json_unpack(root, "{s:s}", "ttl", &ttl)) {
+		if (!strcmp(ttl, "loose")) {
+			ttlval = 1;
+		} else if (!strcmp(ttl, "skip")) {
+			ttlval = 2;
+		} else {
+			json_error(ctx, "Invalid osf ttl option '%s'.", ttl);
+			return NULL;
+		}
+	}
+
 	if (!strcmp(key, "name"))
-		return osf_expr_alloc(int_loc, ttl);
+		return osf_expr_alloc(int_loc, ttlval);
 
 	json_error(ctx, "Invalid osf key value.");
 	return NULL;
diff --git a/tests/py/inet/osf.t.json b/tests/py/inet/osf.t.json
index 45335cab..452f3023 100644
--- a/tests/py/inet/osf.t.json
+++ b/tests/py/inet/osf.t.json
@@ -4,7 +4,6 @@
         "match": {
             "left": {
                 "osf": {
-                    "ttl": 0,
                     "key": "name"
                 }
             },
@@ -14,13 +13,44 @@
     }
 ]
 
+# osf ttl loose name "Linux"
+[
+    {
+        "match": {
+            "left": {
+                "osf": {
+                    "key": "name",
+                    "ttl": "loose"
+                }
+            },
+            "op": "==",
+            "right": "Linux"
+        }
+    }
+]
+
+# osf ttl skip name "Linux"
+[
+    {
+        "match": {
+            "left": {
+                "osf": {
+                    "key": "name",
+                    "ttl": "skip"
+                }
+            },
+            "op": "==",
+            "right": "Linux"
+        }
+    }
+]
+
 # osf name { "Windows", "MacOs" }
 [
     {
         "match": {
             "left": {
                 "osf": {
-                    "ttl": 0,
                     "key": "name"
                 }
             },
@@ -60,7 +90,6 @@
                     },
                     "key": {
                         "osf": {
-                            "ttl": 0,
                             "key": "name"
                         }
                     }