json: Work around segfault when encountering xt stmt

When trying to convert an xt stmt into JSON, print() callback was
called. Though the code in src/xt.c does not respect output_fp,
therefore buffer wasn't filled as expected making libjansson to puke:

| # nft -j list ruleset
| warning: stmt ops xt have no json callback
| nft: json.c:169: stmt_print_json: Assertion `__out' failed.
| Aborted (core dumped)

Avoid this by detecting xt stmt ops and returning a stub.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/json.c b/src/json.c
index 431d38af..f08a3b64 100644
--- a/src/json.c
+++ b/src/json.c
@@ -166,6 +166,12 @@ static json_t *stmt_print_json(const struct stmt *stmt, struct output_ctx *octx)
 	char buf[1024];
 	FILE *fp;
 
+	/* XXX: Can't be supported at this point:
+	 * xt_stmt_xlate() ignores output_fp.
+	 */
+	if (stmt->ops->type == STMT_XT)
+		return json_pack("{s:n}", "xt");
+
 	if (stmt->ops->json)
 		return stmt->ops->json(stmt, octx);