New configuration option MBEDTLS_CHECK_RETURN_WARNING

MBEDTLS_CHECK_RETURN_TYPICAL defaults off, but is enabled if
MBEDTLS_CHECK_RETURN_WARNING is enabled at compile time.
(MBEDTLS_CHECK_RETURN_CRITICAL is always enabled.)

The default is off so that a plausible program that builds with one version
of Mbed TLS in the default configuration will still build under the next
version.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/mbedtls_config.h

@@ -520,6 +520,29 @@
  */
 //#define MBEDTLS_CAMELLIA_SMALL_MEMORY
 
+/**
+ * \def MBEDTLS_CHECK_RETURN_WARNING
+ *
+ * If this macro is defined, emit a compile-time warning if application code
+ * calls a function without checking its return value, but the return value
+ * should generally be checked in portable applications.
+ *
+ * This is only supported on platforms where #MBEDTLS_CHECK_RETURN is
+ * implemented. Otherwise this option has no effect.
+ *
+ * Uncomment to get warnings on using fallible functions without checking
+ * their return value.
+ *
+ * \note  This feature is a work in progress.
+ *        Warnings will be added to more functions in the future.
+ *
+ * \note  A few functions are considered critical, and ignoring the return
+ *        value of these functions will trigger a warning even if this
+ *        macro is not defined. To completely disable return value check
+ *        warnings, define #MBEDTLS_CHECK_RETURN with an empty expansion.
+ */
+//#define MBEDTLS_CHECK_RETURN_WARNING
+
 /**
  * \def MBEDTLS_CIPHER_MODE_CBC
  *