Example 2 - avoid optimizing out stack variable clearing by using the 'volatile' keyword

isle2983 · isle2983 · commit cdd46814c4ab · 2017-01-29T14:47:15.000-07:00
diff --git a/src/group_impl.h b/src/group_impl.h
@@ -214,8 +214,13 @@ static void secp256k1_ge_clear(secp256k1_ge *r) {
     secp256k1_fe_clear(&r->y);
 }
 
-static void __attribute__((optimize("O0"))) secp256k1_secure_clear(void *s, size_t n) {
-    (void) memset(s, 0, n);
+typedef void *(*memset_t)(void *, int, size_t);
+
+static volatile memset_t secure_memset = memset;
+
+static void secp256k1_secure_clear(void *s, size_t n) {
+    (void) secure_memset(s, 0, n);
+
 }
 
 static void secp256k1_int_clear(int *r) {

Original file line number	Diff line number	Diff line change
`@@ -214,8 +214,13 @@ static void secp256k1_ge_clear(secp256k1_ge *r) {`
`214`	`214`	`secp256k1_fe_clear(&r->y);`
`215`	`215`	`}`
`216`	`216`
`217`		`-static void __attribute__((optimize("O0"))) secp256k1_secure_clear(void *s, size_t n) {`
`218`		`- (void) memset(s, 0, n);`
	`217`	`+typedef void (memset_t)(void *, int, size_t);`
	`218`	`+`
	`219`	`+static volatile memset_t secure_memset = memset;`
	`220`	`+`
	`221`	`+static void secp256k1_secure_clear(void *s, size_t n) {`
	`222`	`+ (void) secure_memset(s, 0, n);`
	`223`	`+`
`219`	`224`	`}`
`220`	`225`
`221`	`226`	`static void secp256k1_int_clear(int *r) {`