sp example: Switch to self-made memcmp

Author: real-or-random

examples/examples_util.h

@@ -106,3 +106,21 @@ static void secure_erase(void *ptr, size_t len) {
     volatile_memset(ptr, 0, len);
 #endif
 }
+
+/** Semantics like memcmp. Variable-time.
+ *
+ * We use this to avoid possible compiler bugs with memcmp, e.g.
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189
+ */
+static /*SECP256K1_INLINE*/ int secp256k1_memcmp_var(const void *s1, const void *s2, size_t n) {
+    const unsigned char *p1 = s1, *p2 = s2;
+    size_t i;
+
+    for (i = 0; i < n; i++) {
+        int diff = p1[i] - p2[i];
+        if (diff != 0) {
+            return diff;
+        }
+    }
+    return 0;
+}

examples/silentpayments.c

@@ -101,7 +101,7 @@ const unsigned char* label_lookup(
     const struct labels_cache* cache = (const struct labels_cache*)cache_ptr;
     size_t i;
     for (i = 0; i < cache->entries_used; i++) {
-        if (memcmp(cache->entries[i].label, label33, 33) == 0) {
+        if (secp256k1_memcmp_var(cache->entries[i].label, label33, 33) == 0) {
             return cache->entries[i].label_tweak;
         }
     }