Recommit "[libFuzzer] Fix arguments of InsertPartOf/CopyPartOf calls in CrossOver mutator."

The CrossOver mutator is meant to cross over two given buffers (referred to as
the first/second buffer henceforth). Previously InsertPartOf/CopyPartOf calls
used in the CrossOver mutator incorrectly inserted/copied part of the second
buffer into a "scratch buffer" (MutateInPlaceHere of the size
CurrentMaxMutationLen), rather than the first buffer. This is not intended
behavior, because the scratch buffer does not always (i) contain the content of
the first buffer, and (ii) have the same size as the first buffer;
CurrentMaxMutationLen is typically a lot larger than the size of the first
buffer. This patch fixes the issue by using the first buffer instead of the
scratch buffer in InsertPartOf/CopyPartOf calls.

A FuzzBench experiment was run to make sure that this change does not
inadvertently degrade the performance. The performance is largely the same; more
details can be found at:
https://storage.googleapis.com/fuzzer-test-suite-public/fixcrossover-report/index.html

This patch also adds two new tests, namely "cross_over_insert" and
"cross_over_copy", which specifically target InsertPartOf and CopyPartOf,
respectively.

- cross_over_insert.test checks if the fuzzer can use InsertPartOf to trigger
  the crash.

- cross_over_copy.test checks if the fuzzer can use CopyPartOf to trigger the
  crash.

These newly added tests were designed to pass with the current patch, but not
without the it (with 790878f291 these tests do not
pass). To achieve this, -max_len was intentionally given a high value. Without
this patch, InsertPartOf/CopyPartOf will generate larger inputs, possibly with
unpredictable data in it, thereby failing to trigger the crash.

The test pass condition for these new tests is narrowed down by (i) limiting
mutation depth to 1 (i.e., a single CrossOver mutation should be able to trigger
the crash) and (ii) checking whether the mutation sequence of "CrossOver-" leads
to the crash.

Also note that these newly added tests and an existing test (cross_over.test)
all use "-reduce_inputs=0" flags to prevent reducing inputs; it's easier to
force the fuzzer to keep original input string this way than tweaking
cov-instrumented basic blocks in the source code of the fuzzer executable.

Differential Revision: https://reviews.llvm.org/D85554

compiler-rt/lib/fuzzer/FuzzerMutate.cpp

@@ -425,26 +425,26 @@ size_t MutationDispatcher::Mutate_CrossOver(uint8_t *Data, size_t Size,
   if (!CrossOverWith) return 0;
   const Unit &O = *CrossOverWith;
   if (O.empty()) return 0;
-  MutateInPlaceHere.resize(MaxSize);
-  auto &U = MutateInPlaceHere;
   size_t NewSize = 0;
   switch(Rand(3)) {
     case 0:
-      NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
+      MutateInPlaceHere.resize(MaxSize);
+      NewSize = CrossOver(Data, Size, O.data(), O.size(),
+                          MutateInPlaceHere.data(), MaxSize);
+      memcpy(Data, MutateInPlaceHere.data(), NewSize);
       break;
     case 1:
-      NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
+      NewSize = InsertPartOf(O.data(), O.size(), Data, Size, MaxSize);
       if (!NewSize)
-        NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+        NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
       break;
     case 2:
-      NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+      NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
       break;
     default: assert(0);
   }
   assert(NewSize > 0 && "CrossOver returned empty unit");
   assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
-  memcpy(Data, U.data(), NewSize);
   return NewSize;
 }
 

compiler-rt/test/fuzzer/CrossOverTest.cpp

@@ -4,10 +4,11 @@
 
 // Test for a fuzzer. The fuzzer must find the string
 // ABCDEFGHIJ
-// We use it as a test for CrossOver functionality
-// by passing two inputs to it:
-// ABCDE00000
-// ZZZZZFGHIJ
+// We use it as a test for each of CrossOver functionalities
+// by passing the following sets of two inputs to it:
+// {ABCDE00000, ZZZZZFGHIJ}
+// {ABCDEHIJ, ZFG} to specifically test InsertPartOf
+// {ABCDE00HIJ, ZFG} to specifically test CopyPartOf
 //
 #include <assert.h>
 #include <cstddef>
@@ -42,13 +43,11 @@ static const uint32_t ExpectedHash = 0xe1677acb;
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   // fprintf(stderr, "ExpectedHash: %x\n", ExpectedHash);
-  if (Size != 10) return 0;
+  if (Size == 10 && ExpectedHash == simple_hash(Data, Size))
+    *NullPtr = 0;
   if (*Data == 'A')
     Sink++;
   if (*Data == 'Z')
     Sink--;
-  if (ExpectedHash == simple_hash(Data, Size))
-    *NullPtr = 0;
   return 0;
 }
-

compiler-rt/test/fuzzer/cross_over.test

@@ -12,7 +12,7 @@ RUN: echo -n ABCDE00000 > %t-corpus/A
 RUN: echo -n ZZZZZFGHIJ > %t-corpus/B
 
 
-RUN: not %run %t-CrossOverTest -max_len=10 -seed=1 -runs=10000000 %t-corpus
+RUN: not %run %t-CrossOverTest -max_len=10 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus
 
 # Test the same thing but using -seed_inputs instead of passing the corpus dir.
-RUN: not %run %t-CrossOverTest -max_len=10 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B
+RUN: not %run %t-CrossOverTest -max_len=10 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B

compiler-rt/test/fuzzer/cross_over_copy.test

@@ -0,0 +1,20 @@
+# Tests CrossOver CopyPartOf.
+# We want to make sure that the test can find the input
+# ABCDEFGHIJ when given two other inputs in the seed corpus:
+#    ABCDE00HIJ and
+# (Z)     FG
+#
+RUN: %cpp_compiler %S/CrossOverTest.cpp -o %t-CrossOverTest
+
+RUN: rm -rf %t-corpus
+RUN: mkdir %t-corpus
+RUN: echo -n ABCDE00HIJ > %t-corpus/A
+RUN: echo -n ZFG > %t-corpus/B
+
+
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus 2>&1 | FileCheck %s
+
+# Test the same thing but using -seed_inputs instead of passing the corpus dir.
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B 2>&1 | FileCheck %s
+
+CHECK: MS: 1 CrossOver-

compiler-rt/test/fuzzer/cross_over_insert.test

@@ -0,0 +1,20 @@
+# Tests CrossOver InsertPartOf.
+# We want to make sure that the test can find the input
+# ABCDEFGHIJ when given two other inputs in the seed corpus:
+#    ABCDE  HIJ and
+# (Z)     FG
+#
+RUN: %cpp_compiler %S/CrossOverTest.cpp -o %t-CrossOverTest
+
+RUN: rm -rf %t-corpus
+RUN: mkdir %t-corpus
+RUN: echo -n ABCDEHIJ > %t-corpus/A
+RUN: echo -n ZFG > %t-corpus/B
+
+
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus 2>&1 | FileCheck %s
+
+# Test the same thing but using -seed_inputs instead of passing the corpus dir.
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B 2>&1 | FileCheck %s
+
+CHECK: MS: 1 CrossOver-