blobmsg_json: add new functions blobmsg_format_json_value*

The current blobmsg_format_json* functions will return invalid JSON when
the "list" argument is given as false (blobmsg_format_element() will
output the name of the blob_attr as if the value is printed as part of a
JSON object).

To avoid breaking software relying on this behaviour, introduce new
functions which will never print the blob_attr name and thus always
produce valid JSON.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cosmetic style fix]

blobmsg_json.c

@@ -214,7 +214,7 @@ static void blobmsg_format_string(struct strbuf *s, const char *str)
 
 static void blobmsg_format_json_list(struct strbuf *s, struct blob_attr *attr, int len, bool array);
 
-static void blobmsg_format_element(struct strbuf *s, struct blob_attr *attr, bool array, bool head)
+static void blobmsg_format_element(struct strbuf *s, struct blob_attr *attr, bool without_name, bool head)
 {
 	const char *data_str;
 	char buf[32];
@@ -224,7 +224,7 @@ static void blobmsg_format_element(struct strbuf *s, struct blob_attr *attr, boo
 	if (!blobmsg_check_attr(attr, false))
 		return;
 
-	if (!array && blobmsg_name(attr)[0]) {
+	if (!without_name && blobmsg_name(attr)[0]) {
 		blobmsg_format_string(s, blobmsg_name(attr));
 		blobmsg_puts(s, ": ", s->indent ? 2 : 1);
 	}
@@ -293,27 +293,31 @@ static void blobmsg_format_json_list(struct strbuf *s, struct blob_attr *attr, i
 	blobmsg_puts(s, (array ? "]" : "}"), 1);
 }
 
+static void setup_strbuf(struct strbuf *s, struct blob_attr *attr, blobmsg_json_format_t cb, void *priv, int indent)
+{
+	s->len = blob_len(attr);
+	s->buf = malloc(s->len);
+	s->pos = 0;
+	s->custom_format = cb;
+	s->priv = priv;
+	s->indent = false;
+
+	if (indent >= 0) {
+		s->indent = true;
+		s->indent_level = indent;
+	}
+}
+
 char *blobmsg_format_json_with_cb(struct blob_attr *attr, bool list, blobmsg_json_format_t cb, void *priv, int indent)
 {
 	struct strbuf s;
 	bool array;
 	char *ret;
 
-	s.len = blob_len(attr);
-	s.pos = 0;
-	s.custom_format = cb;
-	s.priv = priv;
-	s.indent = false;
-
-	s.buf = malloc(s.len);
+	setup_strbuf(&s, attr, cb, priv, indent);
 	if (!s.buf)
 		return NULL;
 
-	if (indent >= 0) {
-		s.indent = true;
-		s.indent_level = indent;
-	}
-
 	array = blob_is_extended(attr) &&
 		blobmsg_type(attr) == BLOBMSG_TYPE_ARRAY;
 
@@ -337,3 +341,30 @@ char *blobmsg_format_json_with_cb(struct blob_attr *attr, bool list, blobmsg_jso
 
 	return ret;
 }
+
+char *blobmsg_format_json_value_with_cb(struct blob_attr *attr, blobmsg_json_format_t cb, void *priv, int indent)
+{
+	struct strbuf s;
+	char *ret;
+
+	setup_strbuf(&s, attr, cb, priv, indent);
+	if (!s.buf)
+		return NULL;
+
+	blobmsg_format_element(&s, attr, true, false);
+
+	if (!s.len) {
+		free(s.buf);
+		return NULL;
+	}
+
+	ret = realloc(s.buf, s.pos + 1);
+	if (!ret) {
+		free(s.buf);
+		return NULL;
+	}
+
+	ret[s.pos] = 0;
+
+	return ret;
+}

blobmsg_json.h

@@ -42,4 +42,18 @@ static inline char *blobmsg_format_json_indent(struct blob_attr *attr, bool list
 	return blobmsg_format_json_with_cb(attr, list, NULL, NULL, indent);
 }
 
+char *blobmsg_format_json_value_with_cb(struct blob_attr *attr,
+					blobmsg_json_format_t cb, void *priv,
+					int indent);
+
+static inline char *blobmsg_format_json_value(struct blob_attr *attr)
+{
+	return blobmsg_format_json_value_with_cb(attr, NULL, NULL, -1);
+}
+
+static inline char *blobmsg_format_json_value_indent(struct blob_attr *attr, int indent)
+{
+	return blobmsg_format_json_value_with_cb(attr, NULL, NULL, indent);
+}
+
 #endif