[libFuzzer]Add a test for defeating a hash sum.

[krasin]

Summary

Add a test for a data followed by 4-byte hash value.
I use a slightly modified Jenkins hash function,
as described in https://en.wikipedia.org/wiki/Jenkins_hash_function

The modification is to ensure that hash(zeros) != 0.

[kcc]

LGTM

[krasin]

Reenabling the now passing test.

[kcc]

LGTM++

lib/Fuzzer/test/CMakeLists.txt

set(Tests
   CxxTokensTest
   FourIndependentBranchesTest
   FullCoverageSetTest
   InfiniteTest
   MemcmpTest
   NullDerefTest
   SimpleCmpTest
   SimpleDictionaryTest
+  SimpleHashTest
   SimpleTest
   StrcmpTest
   StrncmpTest
   SwitchTest
   TimeoutTest
   )
 
 set(CustomMainTests

lib/Fuzzer/test/SimpleHashTest.cpp

+// This test computes a checksum of the data (all but the last 4 bytes),
+// and then compares the last 4 bytes with the computed value.
+// A fuzzer with cmp traces is expected to defeat this check.
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <cstdio>
+
+// A modified jenkins_one_at_a_time_hash initialized by non-zero,
+// so that simple_hash(0) != 0. See also
+// https://en.wikipedia.org/wiki/Jenkins_hash_function
+static uint32_t simple_hash(const uint8_t *Data, size_t Size) {
+  uint32_t Hash = 0x12039854;
+  for (uint32_t i = 0; i < Size; i++) {
+    Hash += Data[i];
+    Hash += (Hash << 10);
+    Hash ^= (Hash >> 6);
+  }
+  Hash += (Hash << 3);
+  Hash ^= (Hash >> 11);
+  Hash += (Hash << 15);
+  return Hash;
+}
+
+extern "C" void LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  if (Size < 14)
+    return;
+
+  uint32_t Hash = simple_hash(&Data[0], Size - 4);
+  uint32_t Want = reinterpret_cast<const uint32_t *>(&Data[Size - 4])[0];
+  if (Hash != Want)
+    return;
+  fprintf(stderr, "BINGO; simple_hash defeated: %x == %x\n", (unsigned int)Hash,
+          (unsigned int)Want);
+  exit(1);
+}

lib/Fuzzer/test/fuzzer.test

 RUN: not LLVMFuzzer-StrcmpTest -use_traces=1 -seed=1 -runs=100000   2>&1 | FileCheck %s
 RUN:     LLVMFuzzer-StrcmpTest               -seed=1 -runs=1000000  2>&1 | FileCheck %s --check-prefix=Done1000000
 
 RUN: not LLVMFuzzer-SwitchTest -use_traces=1 -seed=1 -runs=1000000  2>&1 | FileCheck %s
 RUN:     LLVMFuzzer-SwitchTest               -seed=1 -runs=1000000  2>&1 | FileCheck %s --check-prefix=Done1000000
 
 RUN: not LLVMFuzzer-SimpleDictionaryTest -dict=%S/dict1.txt -seed=1 -runs=1000000  2>&1 | FileCheck %s
 RUN:     LLVMFuzzer-SimpleDictionaryTest                    -seed=1 -runs=1000000  2>&1 | FileCheck %s --check-prefix=Done1000000
+
+RUN: not LLVMFuzzer-SimpleHashTest -use_traces=1 -seed=1 -runs=100000  2>&1 | FileCheck %s
+RUN:     LLVMFuzzer-SimpleHashTest               -seed=1 -runs=1000000 2>&1 | FileCheck %s --check-prefix=Done1000000