bpftool: Avoid leaking the JSON writer prepared for program metadata

Bpftool creates a new JSON object for writing program metadata in plain
text mode, regardless of metadata being present or not. Then this writer
is freed if any metadata has been found and printed, but it leaks
otherwise. We cannot destroy the object unconditionally, because the
destructor prints an undesirable line break. Instead, make sure the
writer is created only after we have found program metadata to print.

Found with valgrind.

Fixes: aff52e6 ("bpftool: Support dumping metadata")
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211022094743.11052-1-quentin@isovalent.com

tools/bpf/bpftool/prog.c

@@ -307,18 +307,12 @@ static void show_prog_metadata(int fd, __u32 num_maps)
 		if (printed_header)
 			jsonw_end_object(json_wtr);
 	} else {
-		json_writer_t *btf_wtr = jsonw_new(stdout);
+		json_writer_t *btf_wtr;
 		struct btf_dumper d = {
 			.btf = btf,
-			.jw = btf_wtr,
 			.is_plain_text = true,
 		};
 
-		if (!btf_wtr) {
-			p_err("jsonw alloc failed");
-			goto out_free;
-		}
-
 		for (i = 0; i < vlen; i++, vsi++) {
 			t_var = btf__type_by_id(btf, vsi->type);
 			name = btf__name_by_offset(btf, t_var->name_off);
@@ -328,6 +322,14 @@ static void show_prog_metadata(int fd, __u32 num_maps)
 
 			if (!printed_header) {
 				printf("\tmetadata:");
+
+				btf_wtr = jsonw_new(stdout);
+				if (!btf_wtr) {
+					p_err("jsonw alloc failed");
+					goto out_free;
+				}
+				d.jw = btf_wtr,
+
 				printed_header = true;
 			}