config: add the ability for plugins to specify that config values should be concealed.

And use it for `exposesecret-passphrase`.  This is probably overly
cautious, but it makes me feel a little better that we won't leak it
to someone with read-only access.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/configvar.h

@@ -62,6 +62,8 @@ struct configvar {
 #define OPT_DYNAMIC (1 << (OPT_USER_START+6))
 /* Keep whitespace at the end of the option argument */
 #define OPT_KEEP_WHITESPACE (1 << (OPT_USER_START+7))
+/* Don't show value in listconfigs */
+#define OPT_CONCEAL (1 << (OPT_USER_START+8))
 
 /* Use this instead of opt_register_*_arg if you want OPT_* from above */
 #define clnopt_witharg(names, type, cb, show, arg, desc)		\

doc/developers-guide/plugin-development/a-day-in-the-life-of-a-plugin.md

@@ -124,10 +124,11 @@ Plugins are free to register any `name` for their `rpcmethod` as long as the nam
 
 There are currently four supported option 'types':
 
-- string: a string
-- bool: a boolean
-- int: parsed as a signed integer (64-bit)
-- flag: no-arg flag option. Presented as `true` if config specifies it.
+- `string`: a string
+- `string-conceal`: a string which will appear as "..." in `listconfigs`.
+- `bool`: a boolean
+- `int`: parsed as a signed integer (64-bit)
+- `flag`: no-arg flag option. Presented as `true` if config specifies it.
 
 In addition, string and int types can specify `"multi": true` to indicate they can be specified multiple times.  These will always be represented in `init` as a (possibly empty) JSON array. "multi" flag types do not make  
 sense.

lightningd/configs.c

@@ -65,6 +65,9 @@ static const char *get_opt_val(const struct opt_table *ot,
 			       char buf[],
 			       const struct configvar *cv)
 {
+	if (ot->type & OPT_CONCEAL)
+		return "...";
+
 	if (ot->show == (void *)opt_show_charp) {
 		/* Don't truncate or quote! */
 		return *(char **)ot->u.carg;

lightningd/options.c

@@ -2095,7 +2095,9 @@ void add_config_deprecated(struct lightningd *ld,
 			if (!opt->show(buf, sizeof(buf) - sizeof("..."), opt->u.carg))
 				buf[0] = '\0';
 
-			if ((opt->type & OPT_SHOWINT)
+			if (opt->type & OPT_CONCEAL) {
+				strcpy(buf, "...");
+			} else if ((opt->type & OPT_SHOWINT)
 				   || (opt->type & OPT_SHOWMSATS)) {
 				if (streq(buf, "")
 				    || strspn(buf, "-0123456789.") != strlen(buf))

lightningd/plugin.c

@@ -1168,7 +1168,10 @@ static const char *plugin_opt_add(struct plugin *plugin, const char *buffer,
 		/* These all take an arg. */
 		char *(*cb_arg)(const char *optarg, void *arg);
 
-		if (json_tok_streq(buffer, typetok, "string")) {
+		if (json_tok_streq(buffer, typetok, "string-conceal")) {
+			optflags |= OPT_CONCEAL;
+			cb_arg = (void *)plugin_opt_string_check;
+		} else if (json_tok_streq(buffer, typetok, "string")) {
 			cb_arg = (void *)plugin_opt_string_check;
 		} else if (json_tok_streq(buffer, typetok, "int")) {
 			cb_arg = (void *)plugin_opt_long_check;

plugins/.gitignore

@@ -18,3 +18,4 @@ cln-renepay
 recover
 cln-askrene
 recklessrpc
+exposesecret

plugins/exposesecret.c

@@ -156,7 +156,7 @@ int main(int argc, char *argv[])
 	plugin_main(argv, init, take(exposesecret),
 		    PLUGIN_RESTARTABLE, true, NULL, commands, ARRAY_SIZE(commands),
 	            NULL, 0, NULL, 0, NULL, 0,
-		    plugin_option("exposesecret-passphrase", "string",
+		    plugin_option("exposesecret-passphrase", "string-conceal",
 				  "Enable exposesecret command to allow HSM Secret backup, with this passphrase",
 				  charp_option, NULL, &exposesecret->exposure_passphrase),
 		    NULL);

tests/test_plugin.py

@@ -4453,6 +4453,10 @@ def test_listchannels_broken_message(node_factory):
 def test_exposesecret(node_factory):
     l1, l2 = node_factory.get_nodes(2, opts=[{'exposesecret-passphrase': "test_exposesecret"}, {}])
 
+    # listconfigs will conceal the value for us, even if we ask directly.
+    l1.rpc.listconfigs()['configs']['exposesecret-passphrase']['value_str'] == '...'
+    l1.rpc.listconfigs('exposesecret-passphrase')['configs']['exposesecret-passphrase']['value_str'] == '...'
+
     # l2 won't expose the secret!
     with pytest.raises(RpcError, match="exposesecrets-passphrase is not set"):
         l2.rpc.exposesecret(passphrase='test_exposesecret')