Fuzzer: Do not emit huge and possibly non-validating tables

src/tools/fuzzing/fuzzing.cpp

@@ -513,6 +513,23 @@ void TranslateToFuzzReader::finalizeTable() {
  }
  table->initial = std::max(table->initial, maxOffset);
  });
+
+ // The code above raises table->initial to a size large enough to accomodate
+ // all of its segments, with the intention of avoiding a trap during
+ // startup. However a single segment of (say) size 4GB would have a table of
+ // that size, which will use a lot of memory and execute very slowly, so we
+ // prefer in the fuzzer to trap on such a thing. To achieve that, set a
+ // reasonable limit for the maximum table size.
+ //
+ // This also avoids an issue that arises from table->initial being an
+ // Address (64 bits) but Table::kMaxSize being an Index (32 bits), as a
+ // result of which we need to clamp to Table::kMaxSize as well in order for
+ // the module to validate (but since we are clamping to a smaller value,
+ // there is no need).
+ const Address ReasonableMaxTableSize = 10000;
+ table->initial = std::min(table->initial, ReasonableMaxTableSize);
+ assert(ReasonableMaxTableSize <= Table::kMaxSize);
+
  table->max = oneIn(2) ? Address(Table::kUnlimitedSize) : table->initial;
  // Avoid an imported table (which the fuzz harness would need to handle).
  table->module = table->base = Name();