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/tinycrypt/ecc_dsa.c b/tinycrypt/ecc_dsa.c
index 687ea98..6c171c3 100644
--- a/tinycrypt/ecc_dsa.c
+++ b/tinycrypt/ecc_dsa.c
@@ -67,6 +67,7 @@
#if defined(MBEDTLS_USE_TINYCRYPT)
#include <tinycrypt/ecc.h>
#include <tinycrypt/ecc_dsa.h>
+#include "mbedtls/platform_util.h"
#if default_RNG_defined
static uECC_RNG_Function g_rng_function = &default_CSPRNG;
@@ -304,6 +305,7 @@
/* Accept only if v == r. */
diff = uECC_vli_equal(rx, r);
if (diff == 0) {
+ mbedtls_platform_enforce_volatile_reads();
if (diff == 0) {
return UECC_SUCCESS;
}