Fix libFuzzer array alignment with empty modules

compiler-rt/lib/fuzzer/FuzzerTracePC.cpp

@@ -69,6 +69,9 @@ void TracePC::HandleInline8bitCountersInit(uint8_t *Start, uint8_t *Stop) {
 }
 
 void TracePC::HandlePCsInit(const uintptr_t *Start, const uintptr_t *Stop) {
+  if (Start == Stop) {
+    return;
+  }
   const PCTableEntry *B = reinterpret_cast<const PCTableEntry *>(Start);
   const PCTableEntry *E = reinterpret_cast<const PCTableEntry *>(Stop);
   if (NumPCTables && ModulePCTable[NumPCTables - 1].Start == B) return;

compiler-rt/test/fuzzer/SimulateEmptyModuleTest.cpp

@@ -0,0 +1,60 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Like SimpleTest, but simulates an "empty" module (i.e. one without any functions to instrument).
+// This reproduces a previous bug (when libFuzzer is compiled with assertions enabled).
+
+#include <assert.h>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <iostream>
+#include <ostream>
+
+extern "C" {
+void __sanitizer_cov_8bit_counters_init(uint8_t *Start, uint8_t *Stop);
+void __sanitizer_cov_pcs_init(const uintptr_t *pcs_beg,
+                              const uintptr_t *pcs_end);
+}
+
+void dummy_func() {}
+
+uint8_t empty_8bit_counters[0];
+uintptr_t empty_pcs[0];
+
+uint8_t fake_8bit_counters[1] = {0};
+uintptr_t fake_pcs[2] = {reinterpret_cast<uintptr_t>(&dummy_func),
+                         reinterpret_cast<uintptr_t>(&dummy_func)};
+
+// Register two modules at program launch (same time they'd normally be registered).
+// Triggering the bug requires loading an empty module, then a non-empty module after it.
+bool dummy = []() {
+  // First, simulate loading an empty module.
+  __sanitizer_cov_8bit_counters_init(empty_8bit_counters, empty_8bit_counters);
+  __sanitizer_cov_pcs_init(empty_pcs, empty_pcs);
+
+  // Next, simulate loading a non-empty module.
+  __sanitizer_cov_8bit_counters_init(fake_8bit_counters,
+                                     fake_8bit_counters + 1);
+  __sanitizer_cov_pcs_init(fake_pcs, fake_pcs + 2);
+
+  return true;
+}();
+
+static volatile int Sink;
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  assert(Data);
+  if (Size > 0 && Data[0] == 'H') {
+    Sink = 1;
+    if (Size > 1 && Data[1] == 'i') {
+      Sink = 2;
+      if (Size > 2 && Data[2] == '!') {
+        std::cout << "BINGO; Found the target, exiting\n" << std::flush;
+        exit(0);
+      }
+    }
+  }
+  return 0;
+}

compiler-rt/test/fuzzer/empty-module.test

@@ -0,0 +1,7 @@
+CHECK: BINGO
+RUN: %cpp_compiler %S/SimulateEmptyModuleTest.cpp -o %t-SimulateEmptyModuleTest
+
+RUN: not %run %t-SimulateEmptyModuleTest         2>&1 | FileCheck %s
+
+# only_ascii mode. Will perform some minimal self-validation.
+RUN: not %run %t-SimulateEmptyModuleTest -only_ascii=1 2>&1