Document ssl_write_real() behaviour in detail
@@ -7231,8 +7231,16 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
}
/*
- * Send application data to be encrypted by the SSL layer,
- * taking care of max fragment length and buffer size
+ * Send application data to be encrypted by the SSL layer, taking care of max
+ * fragment length and buffer size.
+ *
+ * According to RFC 5246 Section 6.2.1:
+ *
+ * Zero-length fragments of Application data MAY be sent as they are
+ * potentially useful as a traffic analysis countermeasure.
+ *
+ * Therefore, it is possible that the input message length is 0 and the
+ * corresponding return code is 0 on success.
*/
static int ssl_write_real( mbedtls_ssl_context *ssl,
const unsigned char *buf, size_t len )
@@ -7260,6 +7268,12 @@ static int ssl_write_real( mbedtls_ssl_context *ssl,
if( ssl->out_left != 0 )
{
+ /*
+ * The user has previously tried to send the data and
+ * MBEDTLS_ERR_SSL_WANT_WRITE or the message was only partially
+ * written. In this case, we expect the high-level write function
+ * (e.g. mbedtls_ssl_write()) to be called with the same parameters
+ */
if( ( ret = mbedtls_ssl_flush_output( ssl ) ) != 0 )
{
MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_flush_output", ret );
@@ -7268,6 +7282,11 @@ static int ssl_write_real( mbedtls_ssl_context *ssl,
}
else
{
+ /*
+ * The user is trying to send a message the first time, so we need to
+ * copy the data into the internal buffers and setup the data structure
+ * to keep track of partial writes
+ */
ssl->out_msglen = len;
ssl->out_msgtype = MBEDTLS_SSL_MSG_APPLICATION_DATA;
memcpy( ssl->out_msg, buf, len );
