fix(libfuzzer): add cstddef for size_t

[kolbma]

Add missing cstddef header for size_t.
Fixes https://github.com/rust-fuzz/libfuzzer/issues/87
libfuzzer code is then up-to-date with llvm branch release/13.x up to 2022/01/20.
Upstream fix is with https://github.com/llvm/llvm-project/commit/60e32a1f34e9ea60155a98bbe6ee5ec2a383efa3

[fitzgen]

This PR doesn't seem to have everything from 60e32a1. When I run the update script locally, I get this larger diff:

diff --git a/libfuzzer/FuzzerInterceptors.cpp b/libfuzzer/FuzzerInterceptors.cpp
index b877986..d5b0a42 100644
--- a/libfuzzer/FuzzerInterceptors.cpp
+++ b/libfuzzer/FuzzerInterceptors.cpp
@@ -20,16 +20,17 @@
 #define FUNC_TYPE(x) x##_type
 #define DEFINE_REAL(ret_type, func, ...)                                       \
   typedef ret_type (*FUNC_TYPE(func))(__VA_ARGS__);                            \
   namespace __interception {                                                   \
   FUNC_TYPE(func) PTR_TO_REAL(func);                                           \
   }

 #include <cassert>
+#include <cstddef> // for size_t
 #include <cstdint>
 #include <dlfcn.h> // for dlsym()

 static void *getFuncAddr(const char *name, uintptr_t wrapper_addr) {
   void *addr = dlsym(RTLD_NEXT, name);
   if (!addr) {
     // If the lookup using RTLD_NEXT failed, the sanitizer runtime library is
     // later in the library search order than the DSO that we are trying to
diff --git a/libfuzzer/dataflow/DataFlow.cpp b/libfuzzer/dataflow/DataFlow.cpp
index 78b3f9a..3af48e2 100644
--- a/libfuzzer/dataflow/DataFlow.cpp
+++ b/libfuzzer/dataflow/DataFlow.cpp
@@ -12,21 +12,19 @@
 //
 // It executes the fuzz target on the given input while monitoring the
 // data flow for every instrumented comparison instruction.
 //
 // The output shows which functions depend on which bytes of the input,
 // and also provides basic-block coverage for every input.
 //
 // Build:
-//   1. Compile this file (DataFlow.cpp) with -fsanitize=dataflow -mllvm
-//       -dfsan-fast-16-labels and -O2.
+//   1. Compile this file (DataFlow.cpp) with -fsanitize=dataflow and -O2.
 //   2. Compile DataFlowCallbacks.cpp with -O2 -fPIC.
 //   3. Build the fuzz target with -g -fsanitize=dataflow
-//       -mllvm -dfsan-fast-16-labels
 //       -fsanitize-coverage=trace-pc-guard,pc-table,bb,trace-cmp
 //   4. Link those together with -fsanitize=dataflow
 //
 //  -fsanitize-coverage=trace-cmp inserts callbacks around every comparison
 //  instruction, DFSan modifies the calls to pass the data flow labels.
 //  The callbacks update the data flow label for the current function.
 //  See e.g. __dfsw___sanitizer_cov_trace_cmp1 below.
 //
@@ -77,17 +75,17 @@ CallbackData __dft;
 static size_t InputLen;
 static size_t NumIterations;
 static dfsan_label **FuncLabelsPerIter;  // NumIterations x NumFuncs;

 static inline bool BlockIsEntry(size_t BlockIdx) {
   return __dft.PCsBeg[BlockIdx * 2 + 1] & PCFLAG_FUNC_ENTRY;
 }

-const int kNumLabels = 16;
+const int kNumLabels = 8;

 // Prints all instrumented functions.
 static int PrintFunctions() {
   // We don't have the symbolizer integrated with dfsan yet.
   // So use backtrace_symbols_fd and pipe it through llvm-symbolizer.
   // TODO(kcc): this is pretty ugly and may break in lots of ways.
   //      We'll need to make a proper in-process symbolizer work with DFSan.
   FILE *Pipe = popen("sed 's/(+/ /g; s/).*//g' "

[kolbma]

Not sure what you have compared. The linked commit is only this one added line.
I've compared branch release/13.x. Maybe there is more in main branch. Not sure if I'd want to have main branch.

[fitzgen]

I'm comparing updating all of libfuzzer to that commit. We don't generally cherry pick commits, we just update our whole vendored copy of libfuzzer.

Anyways, closing this in favor of #89.