ct: adds chained memcmp test

test/ct.cpp

@@ -83,3 +83,30 @@ TEST(ConstantTime, CtCheck_memcmp) {
     ct_purify(&ret, 1);
     ASSERT_EQ(ret,1);
 }
+
+TEST(ConstantTime, CtCheck_memcmp_chained) {
+    unsigned char a[16], b[16], c[16], d[16];
+    memset(a, 42, sizeof(a));
+    memset(b, 42, sizeof(b));
+    memset(d, 42, sizeof(b));
+    memset(c, 41, sizeof(c));
+    uint8_t ret;
+
+    ct_poison(a, 16);
+
+    ct_expect_umr();
+    // obviously must generate UMR if if first check fails
+    // and second is not done
+    ret = (ct_memcmp(a,c,16)==0) && (ct_memcmp(a,b,16)==0);
+    ct_require_umr();
+    ct_purify(&ret, 1);
+    ASSERT_EQ(ret,0);
+
+    ct_expect_umr();
+    // it's still UMR even if both checks are OK
+    ret = (ct_memcmp(a,d,16)==0) && (ct_memcmp(a,b,16)==0);
+    ct_require_umr();
+
+    ct_purify(&ret, 1);
+    ASSERT_EQ(ret,1);
+}