Make it easier to define MBEDTLS_PARAM_FAILED as assert

Introduce a new configuration option MBEDTLS_CHECK_PARAMS_ASSERT,
which is disabled by default. When this option is enabled,
MBEDTLS_PARAM_FAILED defaults to assert rather than to a call to
mbedtls_param_failed, and <assert.h> is included.

This fixes Mbed-TLS#2671 (no easy way to make MBEDTLS_PARAM_FAILED assert)
without breaking backward compatibility. With this change,
`config.pl full` runs tests with MBEDTLS_PARAM_FAILED set to assert,
so the tests will fail if a validation check fails, and programs don't
need to provide their own definition of mbedtls_param_failed().

ChangeLog

@@ -57,6 +57,8 @@ Changes
    * Change wording in the `mbedtls_ssl_conf_max_frag_len()`'s documentation to
      improve clarity. Fixes #2258.
    * Replace multiple uses of MD2 by SHA-256 in X.509 test suite. Fixes #821.
+   * Make it easier to define MBEDTLS_PARAM_FAILED as assert (which config.h
+     suggests). #2671
 
 = mbed TLS 2.16.1 branch released 2019-03-19
 

include/mbedtls/config.h

@@ -276,28 +276,52 @@
  * For example, when a function accepts as input a pointer to a buffer that may
  * contain untrusted data, and its documentation mentions that this pointer
  * must not be NULL:
- * - the pointer is checked to be non-NULL only if this option is enabled
- * - the content of the buffer is always validated
+ * - The pointer is checked to be non-NULL only if this option is enabled.
+ * - The content of the buffer is always validated.
  *
  * When this flag is defined, if a library function receives a parameter that
- * is invalid, it will:
- * - invoke the macro MBEDTLS_PARAM_FAILED() which by default expands to a
- *   call to the function mbedtls_param_failed()
- * - immediately return (with a specific error code unless the function
- *   returns void and can't communicate an error).
- *
- * When defining this flag, you also need to:
- * - either provide a definition of the function mbedtls_param_failed() in
- *   your application (see platform_util.h for its prototype) as the library
- *   calls that function, but does not provide a default definition for it,
- * - or provide a different definition of the macro MBEDTLS_PARAM_FAILED()
- *   below if the above mechanism is not flexible enough to suit your needs.
- *   See the documentation of this macro later in this file.
+ * is invalid:
+ * 1. The function will invoke the macro MBEDTLS_PARAM_FAILED().
+ * 2. If MBEDTLS_PARAM_FAILED() did not terminate the program, the function
+ *   will immediately return. If the function returns an Mbed TLS error code,
+ *   the error code in this case is MBEDTLS_ERR_xxx_BAD_INPUT_DATA.
+ *
+ * When defining this flag, you also need to arrange a definition for
+ * MBEDTLS_PARAM_FAILED(). You can do this by any of the following methods:
+ * - By default, the library defines MBEDTLS_PARAM_FAILED() to call a
+ *   function mbedtls_param_failed(), but the library does not define this
+ *   function. If you do not make any other arrangements, you must provide
+ *   the function mbedtls_param_failed() in your application.
+ *   See `platform_util.h` for its prototype.
+ * - If you enable the macro #MBEDTLS_CHECK_PARAMS_ASSERT, then the
+ *   library defines #MBEDTLS_PARAM_FAILED(\c cond) to be `assert(cond)`.
+ *   You can still supply an alternative definition of
+ *   MBEDTLS_PARAM_FAILED(), which may call `assert`.
+ * - If you define a macro MBEDTLS_PARAM_FAILED() before including `config.h`
+ *   or you uncomment the definition of MBEDTLS_PARAM_FAILED() in `config.h`,
+ *   the library will call the macro that you defined and will not supply
+ *   its own version. Note that if MBEDTLS_PARAM_FAILED() calls `assert`,
+ *   you need to enable #MBEDTLS_CHECK_PARAMS_ASSERT so that library source
+ *   files include `<assert.h>`.
  *
  * Uncomment to enable validation of application-controlled parameters.
  */
 //#define MBEDTLS_CHECK_PARAMS
 
+/**
+ * \def MBEDTLS_CHECK_PARAMS_ASSERT
+ *
+ * Allow MBEDTLS_PARAM_FAILED() to call `assert`, and make it default to
+ * `assert`. This macro is only used if #MBEDTLS_CHECK_PARAMS is defined.
+ *
+ * If this macro is not defined, then MBEDTLS_PARAM_FAILED() defaults to
+ * calling a function mbedtls_param_failed(). See the documentation of
+ * #MBEDTLS_CHECK_PARAMS for details.
+ *
+ * Uncomment to allow MBEDTLS_PARAM_FAILED() to call `assert`.
+ */
+//#define MBEDTLS_CHECK_PARAMS_ASSERT
+
 /* \} name SECTION: System support */
 
 /**
@@ -3060,13 +3084,16 @@
 
 /**
  * \brief       This macro is invoked by the library when an invalid parameter
- *              is detected that is only checked with MBEDTLS_CHECK_PARAMS
+ *              is detected that is only checked with #MBEDTLS_CHECK_PARAMS
  *              (see the documentation of that option for context).
  *
- *              When you leave this undefined here, a default definition is
- *              provided that invokes the function mbedtls_param_failed(),
- *              which is declared in platform_util.h for the benefit of the
- *              library, but that you need to define in your application.
+ *              When you leave this undefined here, the library provides
+ *              a default definition. If the macro #MBEDTLS_CHECK_PARAMS_ASSERT
+ *              is defined, the default definition is `assert(cond)`,
+ *              otherwise the default definition calls a function
+ *              mbedtls_param_failed(). This function is declared in
+ *              `platform_util.h` for the benefit of the library, but
+ *              you need to define in your application.
  *
  *              When you define this here, this replaces the default
  *              definition in platform_util.h (which no longer declares the
@@ -3075,6 +3102,9 @@
  *              particular, that all the necessary declarations are visible
  *              from within the library - you can ensure that by providing
  *              them in this file next to the macro definition).
+ *              If you define this macro to call `assert`, also define
+ *              #MBEDTLS_CHECK_PARAMS_ASSERT so that library source files
+ *              include `<assert.h>`.
  *
  *              Note that you may define this macro to expand to nothing, in
  *              which case you don't have to worry about declarations or

include/mbedtls/platform_util.h

@@ -43,13 +43,24 @@ extern "C" {
 
 #if defined(MBEDTLS_CHECK_PARAMS)
 
+#if defined(MBEDTLS_CHECK_PARAMS_ASSERT)
+/* Allow the user to define MBEDTLS_PARAM_FAILED to something like assert
+ * (which is what our config.h suggests). */
+#include <assert.h>
+#endif /* MBEDTLS_CHECK_PARAMS_ASSERT */
+
 #if defined(MBEDTLS_PARAM_FAILED)
 /** An alternative definition of MBEDTLS_PARAM_FAILED has been set in config.h.
  *
  * This flag can be used to check whether it is safe to assume that
  * MBEDTLS_PARAM_FAILED() will expand to a call to mbedtls_param_failed().
  */
 #define MBEDTLS_PARAM_FAILED_ALT
+
+#elif defined(MBEDTLS_CHECK_PARAMS_ASSERT)
+#define MBEDTLS_PARAM_FAILED( cond ) assert( cond )
+#define MBEDTLS_PARAM_FAILED_ALT
+
 #else /* MBEDTLS_PARAM_FAILED */
 #define MBEDTLS_PARAM_FAILED( cond ) \
     mbedtls_param_failed( #cond, __FILE__, __LINE__ )

library/version_features.c

@@ -87,6 +87,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_CHECK_PARAMS)
     "MBEDTLS_CHECK_PARAMS",
 #endif /* MBEDTLS_CHECK_PARAMS */
+#if defined(MBEDTLS_CHECK_PARAMS_ASSERT)
+    "MBEDTLS_CHECK_PARAMS_ASSERT",
+#endif /* MBEDTLS_CHECK_PARAMS_ASSERT */
 #if defined(MBEDTLS_TIMING_ALT)
     "MBEDTLS_TIMING_ALT",
 #endif /* MBEDTLS_TIMING_ALT */

programs/ssl/query_config.c

@@ -274,6 +274,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_CHECK_PARAMS */
 
+#if defined(MBEDTLS_CHECK_PARAMS_ASSERT)
+    if( strcmp( "MBEDTLS_CHECK_PARAMS_ASSERT", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_CHECK_PARAMS_ASSERT );
+        return( 0 );
+    }
+#endif /* MBEDTLS_CHECK_PARAMS_ASSERT */
+
 #if defined(MBEDTLS_TIMING_ALT)
     if( strcmp( "MBEDTLS_TIMING_ALT", config ) == 0 )
     {