[wasm] Interpreter: Don't pause on invalid position

src/wasm/wasm-interpreter.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/wasm/wasm-interpreter.h"
 
 #include "src/utils.h"
 #include "src/wasm/decoder.h"
 #include "src/wasm/function-body-decoder.h"
 #include "src/wasm/wasm-external-refs.h"
@@ skipping 1161 matching lines @@
     size_t pop_count = stack_.size() - dest - arity;
     for (size_t i = 0; i < arity; i++) {
       stack_[dest + i] = stack_[dest + pop_count + i];
     }
     stack_.resize(stack_.size() - pop_count);
   }
 
   void Execute(InterpreterCode* code, pc_t pc, int max) {
     Decoder decoder(code->start, code->end);
     pc_t limit = code->end - code->start;
-    while (--max >= 0) {
+    while (pc >= limit || --max >= 0) {

[titzer, 2017/01/25 12:25:06]

Hmm, that's weird. Why are we checking that the pc is out of bounds? (>=)?

[Clemens Hammacher, 2017/01/25 12:33:33]

To run into the if 4 lines below. I can also make it "pc == limit", if that's
more readable.
My first attempt was to move the "pc >= limit" check into the kExprEnd case, but
it also happens after a kExprBr or BrIf.

 #define PAUSE_IF_BREAK_FLAG(flag) \
   if (V8_UNLIKELY(break_flags_ & WasmInterpreter::BreakFlag::flag)) max = 0;
 
       if (pc >= limit) {
+        DCHECK_EQ(pc, limit);
         // Fell off end of code; do an implicit return.
         TRACE("@%-3zu: ImplicitReturn\n", pc);
         if (!DoReturn(&code, &pc, &limit, code->function->sig->return_count()))
           return;
         decoder.Reset(code->start, code->end);
         PAUSE_IF_BREAK_FLAG(AfterReturn);
         continue;
       }
 
       const char* skip = "        ";
@@ skipping 725 matching lines @@
   return none;
 }
 
 void InterpretedFrame::SetLocalVal(int index, WasmVal val) { UNIMPLEMENTED(); }
 
 void InterpretedFrame::SetExprVal(int pc, WasmVal val) { UNIMPLEMENTED(); }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8