[RFC PATCH 2/6] telemetry: fix escaping of invalid json characters

[Bruce Richardson]

For string values returned from telemetry, escape any values that cannot
normally appear in a json string. According to the json spec[1], the
characters than need to be handled are control chars (char value < 0x20)
and '"' and '\' characters.

To handle this, we replace the snprintf call with a separate string
copying and encapsulation routine which checks each character as it
copies it to the final array.

[1] https://www.rfc-editor.org/rfc/rfc8259.txt

Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
---
 lib/telemetry/telemetry_json.h | 48 +++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/lib/telemetry/telemetry_json.h b/lib/telemetry/telemetry_json.h
index db70690274..13df5d07e3 100644
--- a/lib/telemetry/telemetry_json.h
+++ b/lib/telemetry/telemetry_json.h
@@ -44,6 +44,52 @@ __json_snprintf(char *buf, const int len, const char *format, ...)
 	return 0; /* nothing written or modified */
 }
 
+static const char control_chars[0x20] = {
+		['\n'] = 'n',
+		['\r'] = 'r',
+		['\t'] = 't',
+};
+
+/**
+ * @internal
+ * Does the same as __json_snprintf(buf, len, "\"%s\"", str)
+ * except that it does proper escaping as necessary.
+ * Drops any invalid characters we don't support
+ */
+static inline int
+__json_format_str(char *buf, const int len, const char *str)
+{
+	char tmp[len];
+	int tmpidx = 0;
+
+	tmp[tmpidx++] = '"';
+	while (*str != '\0') {
+		if (*str < (int)RTE_DIM(control_chars)) {
+			int idx = *str;  /* compilers don't like char type as index */
+			if (control_chars[idx] != 0) {
+				tmp[tmpidx++] = '\\';
+				tmp[tmpidx++] = control_chars[idx];
+			}
+		} else if (*str == '"' || *str == '\\') {
+			tmp[tmpidx++] = '\\';
+			tmp[tmpidx++] = *str;
+		} else
+			tmp[tmpidx++] = *str;
+		/* we always need space for closing quote and null character.
+		 * Ensuring at least two free characters also means we can always take an
+		 * escaped character like "\n" without overflowing
+		 */
+		if (tmpidx > len - 2)
+			return 0;
+		str++;
+	}
+	tmp[tmpidx++] = '"';
+	tmp[tmpidx] = '\0';
+
+	strcpy(buf, tmp);
+	return tmpidx;
+}
+
 /* Copies an empty array into the provided buffer. */
 static inline int
 rte_tel_json_empty_array(char *buf, const int len, const int used)
@@ -62,7 +108,7 @@ rte_tel_json_empty_obj(char *buf, const int len, const int used)
 static inline int
 rte_tel_json_str(char *buf, const int len, const int used, const char *str)
 {
-	return used + __json_snprintf(buf + used, len - used, "\"%s\"", str);
+	return used + __json_format_str(buf + used, len - used, str);
 }
 
 /* Appends a string into the JSON array in the provided buffer. */
-- 
2.34.1