[wasm] Simplify bounds check

We really just need one check instead of three. This also unifies the
error message to be the same on 32 and 64 bit systems.

Drive-by: Fix potential overflow in {validate_size}.

R=titzer@chromium.org

Bug: chromium:794353
Change-Id: I63c1f5ef53c1f245b9e82bcbf86a5d9ac0d2725e
Reviewed-on: https://chromium-review.googlesource.com/824082Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50071}

src/wasm/decoder.h

@@ -45,16 +45,20 @@ class Decoder {
   enum TraceFlag : bool { kTrace = true, kNoTrace = false };
 
   Decoder(const byte* start, const byte* end, uint32_t buffer_offset = 0)
-      : start_(start), pc_(start), end_(end), buffer_offset_(buffer_offset) {}
+      : Decoder(start, start, end, buffer_offset) {}
   Decoder(const byte* start, const byte* pc, const byte* end,
           uint32_t buffer_offset = 0)
-      : start_(start), pc_(pc), end_(end), buffer_offset_(buffer_offset) {}
+      : start_(start), pc_(pc), end_(end), buffer_offset_(buffer_offset) {
+    DCHECK_LE(start, pc);
+    DCHECK_LE(pc, end);
+    DCHECK_EQ(static_cast<uint32_t>(end - start), end - start);
+  }
 
   virtual ~Decoder() {}
 
   inline bool validate_size(const byte* pc, uint32_t length, const char* msg) {
     DCHECK_LE(start_, pc);
-    if (V8_UNLIKELY(pc + length > end_)) {
+    if (V8_UNLIKELY(length > static_cast<uint32_t>(end_ - pc))) {
       error(pc, msg);
       return false;
     }
@@ -161,16 +165,11 @@ class Decoder {
 
   // Check that at least {size} bytes exist between {pc_} and {end_}.
   bool checkAvailable(uint32_t size) {
-    uintptr_t pc_overflow_value = std::numeric_limits<uintptr_t>::max() - size;
-    if ((uintptr_t)pc_ > pc_overflow_value) {
-      errorf(pc_, "reading %u bytes would underflow/overflow", size);
-      return false;
-    } else if (pc_ < start_ || end_ < (pc_ + size)) {
+    if (V8_UNLIKELY(size > static_cast<uint32_t>(end_ - pc_))) {
       errorf(pc_, "expected %u bytes, fell off end", size);
       return false;
-    } else {
-      return true;
     }
+    return true;
   }
 
   void error(const char* msg) { errorf(pc_, "%s", msg); }
@@ -227,6 +226,8 @@ class Decoder {
 
   // Resets the boundaries of this decoder.
   void Reset(const byte* start, const byte* end, uint32_t buffer_offset = 0) {
+    DCHECK_LE(start, end);
+    DCHECK_EQ(static_cast<uint32_t>(end - start), end - start);
     start_ = start;
     pc_ = start;
     end_ = end;