Force some compilers to respect volatile reads
Inspection of the generated assembly showed that before this commit, armcc 5
was optimizing away the successive reads to the volatile local variable that's
used for double-checks. Inspection also reveals that inserting a call to an
external function is enough to prevent it from doing that.
The tested versions of ARM-GCC, Clang and Armcc 6 (aka armclang) all keep the
double read, with our without a call to an external function in the middle.
The inserted function can also be changed to insert a random delay if
desired in the future, as it is appropriately places between the reads.
diff --git a/library/platform_util.c b/library/platform_util.c
index 1a0fefa..97dfe73 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -168,6 +168,15 @@
#endif
}
+/* Some compilers (armcc 5 for example) optimize away successive reads from a
+ * volatile local variable (which we use as a counter-measure to fault
+ * injection attacks), unless there is a call to an external function between
+ * them. This functions doesn't need to do anything, it just needs to be
+ * in another compilation unit. So here's a function that does nothing. */
+void mbedtls_platform_enforce_volatile_reads( void )
+{
+}
+
#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
#include <time.h>
#if !defined(_WIN32) && (defined(unix) || \