[trace-reader] Avoid UBSAN offsetting nullptr

```
../../zircon/system/ulib/trace-reader/reader.cc:766:16: runtime error: applying non-zero offset 8 to null pointer
   #0    0x0000425cf320f4bc in trace::Chunk::ReadString(trace::Chunk*, size_t, std::__2::string_view*) ../../zircon/system/ulib/trace-reader/reader.cc:766 <<application>>+0x1044bc
   #1.2  0x00004377ab0103c0 in ubsan_GetStackTrace() compiler-rt/lib/ubsan/ubsan_diag.cpp:43 <libclang_rt.asan.so>+0x363c0
   #1.1  0x00004377ab0103c0 in MaybePrintStackTrace() compiler-rt/lib/ubsan/ubsan_diag.cpp:53 <libclang_rt.asan.so>+0x363c0
   #1    0x00004377ab0103c0 in ~ScopedReport() compiler-rt/lib/ubsan/ubsan_diag.cpp:389 <libclang_rt.asan.so>+0x363c0
   #2    0x00004377ab0137c8 in handlePointerOverflowImpl() compiler-rt/lib/ubsan/ubsan_handlers.cpp:809 <libclang_rt.asan.so>+0x397c8
   #3    0x00004377ab01343c in compiler-rt/lib/ubsan/ubsan_handlers.cpp:815 <libclang_rt.asan.so>+0x3943c
   #4    0x0000425cf320f4bc in trace::Chunk::ReadString(trace::Chunk*, size_t, std::__2::string_view*) ../../zircon/system/ulib/trace-reader/reader.cc:766 <<application>>+0x1044bc
   #5    0x0000425cf316b31c in trace::$anon::TraceReader_EmptyChunk_Class::TestBody(trace::$anon::TraceReader_EmptyChunk_Class*) ../../zircon/system/ulib/trace-reader/test/reader_tests.cc:39 <<application>>+0x6031c
   #6    0x0000425cf322b564 in zxtest::Test::Run(zxtest::Test*) ../../zircon/system/ulib/zxtest/test.cc:18 <<application>>+0x120564
   #7    0x0000425cf3229d6c in zxtest::TestCase::Run(zxtest::TestCase*, zxtest::LifecycleObserver*, zxtest::internal::TestDriver*) ../../zircon/system/ulib/zxtest/test-case.cc:106 <<application>>+0x11ed6c
   #8    0x0000425cf321e8f0 in zxtest::Runner::Run(zxtest::Runner*, const zxtest::Runner::Options&) ../../zircon/system/ulib/zxtest/runner.cc:121 <<application>>+0x1138f0
   #9    0x0000425cf32204e0 in zxtest::RunAllTests(int, char**) ../../zircon/system/ulib/zxtest/runner.cc:217 <<application>>+0x1154e0
   #10   0x0000425cf322b648 in main(int, char**) ../../zircon/system/ulib/zxtest/zxtest-main.cc:14 <<application>>+0x120648
   #11   0x0000827600fbc654 in start_main(const start_params*) ../../zircon/third_party/ulib/musl/src/env/__libc_start_main.c:139 <libc.so>+0xcc654
   #12   0x0000827600fbcf24 in __libc_start_main(zx_handle_t, int (*)(int, char**, char**)) ../../zircon/third_party/ulib/musl/src/env/__libc_start_main.c:256 <libc.so>+0xccf24
   #13   0x0000425cf320a510 in _start(zx_handle_t) ../../zircon/system/ulib/c/Scrt1.cc:7 <<application>>+0xff510
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior    #0    0x0000000000000000 is not covered by any module
```

It is impossible to use empty chunks without triggering UB. Delete the
default constructor and change surrounding APIs to avoid the need.

Fixed: 41892
Change-Id: Ia340d4e99d4f709d4f58dd85a7bc241918115e7e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/574302
Commit-Queue: Auto-Submit <[email protected]>
Fuchsia-Auto-Submit: Tamir Duberstein <[email protected]>
Reviewed-by: Fadi Meawad <[email protected]>

zircon/system/ulib/trace-reader/BUILD.gn

@@ -31,10 +31,6 @@ zx_library("trace-reader") {
   ]
   deps = [ "//zircon/public/lib/fbl" ]
 
-  # TODO(fxbug.dev/41892): UBSan has found an instance of undefined behavior in this target.
-  # Disable UBSan for this target temporarily until it is migrated into CI/CQ.
-  configs += [ "//build/config:temporarily_disable_ubsan_do_not_use" ]
-
   # TODO(fxbug.dev/58162): delete the below and fix compiler warnings
   configs += [ "//build/config:Wno-conversion" ]
 }

zircon/system/ulib/trace-reader/include/trace-reader/reader.h

@@ -8,6 +8,7 @@
 #include <zircon/assert.h>
 
 #include <memory>
+#include <optional>
 #include <string_view>
 #include <utility>
 
@@ -47,7 +48,7 @@ class TraceReader {
   // Reads as many records as possible from the chunk, invoking the
   // record consumer for each one.  Returns true if the stream could possibly
   // contain more records if the chunk were extended with new data.
-  // Returns false if the trace stream is unrecoverably corrupt and no
+  // Returns false if the trace stream is irrecoverably corrupt and no
   // further decoding is possible.  May be called repeatedly with new
   // chunks as they become available to resume decoding.
   bool ReadRecords(Chunk& chunk);
@@ -86,7 +87,7 @@ class TraceReader {
 
   void SetCurrentProvider(ProviderId id);
   void RegisterProvider(ProviderId id, fbl::String name);
-  void RegisterString(trace_string_index_t index, fbl::String string);
+  void RegisterString(trace_string_index_t index, const fbl::String& string);
   void RegisterThread(trace_thread_index_t index, const ProcessThread& process_thread);
 
   bool DecodeStringRef(Chunk& chunk, trace_encoded_string_ref_t string_ref,
@@ -127,7 +128,7 @@ class TraceReader {
     ProviderId id;
     fbl::String name;
 
-    // TODO(fxbug.dev/30999): It would be more efficient to use something like
+    // TODO(https://fxbug.dev/30999): It would be more efficient to use something like
     // std::unordered_map<> here.  In particular, the table entries are
     // small enough that it doesn't make sense to heap allocate them
     // individually.
@@ -149,8 +150,7 @@ class TraceReader {
 // region of memory. The main use-case of this class is input to |TraceReader|.
 class Chunk final {
  public:
-  Chunk();
-  explicit Chunk(const uint64_t* begin, size_t num_words);
+  Chunk(const uint64_t* begin, size_t num_words);
 
   uint64_t current_byte_offset() const {
     return (reinterpret_cast<const uint8_t*>(current_) - reinterpret_cast<const uint8_t*>(begin_));
@@ -160,12 +160,12 @@ class Chunk final {
   // Reads from the chunk, maintaining proper alignment.
   // Returns true on success, false if the chunk has insufficient remaining
   // words to satisfy the request.
-  bool ReadUint64(uint64_t* out_value);
-  bool ReadInt64(int64_t* out_value);
-  bool ReadDouble(double* out_value);
-  bool ReadString(size_t length, std::string_view* out_string);
-  bool ReadChunk(size_t num_words, Chunk* out_chunk);
-  bool ReadInPlace(size_t num_words, const void** out_ptr);
+  std::optional<uint64_t> ReadUint64();
+  std::optional<int64_t> ReadInt64();
+  std::optional<double> ReadDouble();
+  std::optional<std::string_view> ReadString(size_t length);
+  std::optional<Chunk> ReadChunk(size_t num_words);
+  std::optional<void const*> ReadInPlace(size_t num_words);
 
  private:
   const uint64_t* begin_;