tools: bpftool: fix crash on bad parameters with JSON

If bad or unrecognised parameters are specified after JSON output is
requested, `usage()` will try to output null JSON object before the
writer is created.

To prevent this, create the writer as soon as the `--json` option is
parsed.

Fixes: 004b45c0e51a ("tools: bpftool: provide JSON output for all possible commands")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
index d6e4762170a4464d029415edc189030f55841844..14ad54a1c404a8d7145b1646476d78d4a0c91f72 100644 (file)
--- a/tools/bpf/bpftool/main.c
+++ b/tools/bpf/bpftool/main.c
@@ -291,7 +291,15 @@ int main(int argc, char **argv)
                        pretty_output = true;
                        /* fall through */
                case 'j':
-                       json_output = true;
+                       if (!json_output) {
+                               json_wtr = jsonw_new(stdout);
+                               if (!json_wtr) {
+                                       p_err("failed to create JSON writer");
+                                       return -1;
+                               }
+                               json_output = true;
+                       }
+                       jsonw_pretty(json_wtr, pretty_output);
                        break;
                case 'f':
                        show_pinned = true;
@@ -306,15 +314,6 @@ int main(int argc, char **argv)
        if (argc < 0)
                usage();
 
-       if (json_output) {
-               json_wtr = jsonw_new(stdout);
-               if (!json_wtr) {
-                       p_err("failed to create JSON writer");
-                       return -1;
-               }
-               jsonw_pretty(json_wtr, pretty_output);
-       }
-
        bfd_init();
 
        ret = cmd_select(cmds, argc, argv, do_help);