[libFuzzer] Add dummy call of LLVMFuzzerTestOneInput to afl_driver.

[metzman]

Summary

Add dummy call of LLVMFuzzerTestOneInput to afl_driver before it starts
executing on actual inputs. Do this so that first time initialization
performed by LLVMFuzzerTestOneInput is not considered code covered by
a particular input.

[metzman]

• Don't do dummy execution when executing files one-by-one.

[kcc]

LGTM
Matt, please land

[Dor1s]

lib/fuzzer/afl/afl_driver.cpp

314

nit: I'd rather do:

uint8_t dummy_input[] = {0};
LLVMFuzzerTestOneInput(dummy_input, sizeof(dummy_input));

to avoid using "magic" numbers

lib/fuzzer/afl/afl_driver.cpp

 
 // Used to avoid repeating error checking boilerplate. If cond is false, a
 // fatal error has occured in the program. In this event print error_message
 // to stderr and abort(). Otherwise do nothing. Note that setting
 // AFL_DRIVER_STDERR_DUPLICATE_FILENAME may cause error_message to be appended
 // to the file as well, if the error occurs after the duplication is performed.
 #define CHECK_ERROR(cond, error_message)                                       \
   if (!(cond)) {                                                               \
-    fprintf(stderr, (error_message));                                          \
+    fprintf(stderr, "%s\n", (error_message));                                  \
     abort();                                                                   \
   }
 
 // libFuzzer interface is thin, so we don't include any libFuzzer headers.
 extern "C" {
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
 __attribute__((weak)) int LLVMFuzzerInitialize(int *argc, char ***argv);
 }
if (argc == 2 && argv[1][0] == '-')
       N = atoi(argv[1] + 1);
   else if(argc == 2 && (N = atoi(argv[1])) > 0)
       fprintf(stderr, "WARNING: using the deprecated call style `%s %d`\n",
               argv[0], N);
   else if (argc > 1)
     return ExecuteFilesOnyByOne(argc, argv);
 
   assert(N > 0);
+
+  // Call LLVMFuzzerTestOneInput here so that coverage caused by initialization
+  // on the first execution of LLVMFuzzerTestOneInput is ignored.
+  uint8_t dummy_input[1] = {0};
+  LLVMFuzzerTestOneInput(dummy_input, 1);
+
   time_t unit_time_secs;
   int num_runs = 0;
   while (__afl_persistent_loop(N)) {
     ssize_t n_read = read(0, AflInputBuf, kMaxAflInputSize);
     if (n_read > 0) {
       // Copy AflInputBuf into a separate buffer to let asan find buffer
       // overflows. Don't use unique_ptr/etc to avoid extra dependencies.
       uint8_t *copy = new uint8_t[n_read];