Merge remote-tracking branch 'public/pr/1927' into development-restricted

include/mbedtls/config.h

@@ -137,12 +137,21 @@
 /**
  * \def MBEDTLS_HAVE_TIME_DATE
  *
- * System has time.h and time(), gmtime() and the clock is correct.
+ * System has time.h, time(), and an implementation for
+ * mbedtls_platform_gmtime_r() (see below).
  * The time needs to be correct (not necesarily very accurate, but at least
  * the date should be correct). This is used to verify the validity period of
  * X.509 certificates.
  *
  * Comment if your system does not have a correct clock.
+ *
+ * \note mbedtls_platform_gmtime_r() is an abstraction in platform_util.h that
+ * behaves similarly to the gmtime_r() function from the C standard. Refer to
+ * the documentation for mbedtls_platform_gmtime_r() for more information.
+ *
+ * \note It is possible to configure an implementation for
+ * mbedtls_platform_gmtime_r() at compile-time by using the macro
+ * MBEDTLS_PLATFORM_GMTIME_R_ALT.
  */
 #define MBEDTLS_HAVE_TIME_DATE
 
@@ -3100,6 +3109,25 @@
  */
 //#define MBEDTLS_PLATFORM_ZEROIZE_ALT
 
+/**
+ * Uncomment the macro to let Mbed TLS use your alternate implementation of
+ * mbedtls_platform_gmtime_r(). This replaces the default implementation in
+ * platform_util.c.
+ *
+ * gmtime() is not a thread-safe function as defined in the C standard. The
+ * library will try to use safer implementations of this function, such as
+ * gmtime_r() when available. However, if Mbed TLS cannot identify the target
+ * system, the implementation of mbedtls_platform_gmtime_r() will default to
+ * using the standard gmtime(). In this case, calls from the library to
+ * gmtime() will be guarded by the global mutex mbedtls_threading_gmtime_mutex
+ * if MBEDTLS_THREADING_C is enabled. We recommend that calls from outside the
+ * library are also guarded with this mutex to avoid race conditions. However,
+ * if the macro MBEDTLS_PLATFORM_GMTIME_R_ALT is defined, Mbed TLS will
+ * unconditionally use the implementation for mbedtls_platform_gmtime_r()
+ * supplied at compile time.
+ */
+//#define MBEDTLS_PLATFORM_GMTIME_R_ALT
+
 /* \} name SECTION: Customisation configuration options */
 
 /* Target and application specific configurations */