cc: Use CheckedNumeric for resource size calculations.

cc/resources/resource.h

Index: cc/resources/resource.h
diff --git a/cc/resources/resource.h b/cc/resources/resource.h
index 0e3c3df688387b9ee927f150737bc8dd37a7a6b7..676ca6dd0f46df6de58681b654f87e5d02a3dd59 100644
--- a/cc/resources/resource.h
+++ b/cc/resources/resource.h
@@ -5,6 +5,7 @@
 #ifndef CC_RESOURCES_RESOURCE_H_
 #define CC_RESOURCES_RESOURCE_H_
 
+#include "base/numerics/safe_math.h"
 #include "cc/base/cc_export.h"
 #include "cc/resources/resource_provider.h"
 #include "ui/gfx/geometry/size.h"
@@ -22,14 +23,46 @@ class CC_EXPORT Resource {
   ResourceId id() const { return id_; }
   gfx::Size size() const { return size_; }
   ResourceFormat format() const { return format_; }
-  size_t bytes() const;
 
-  inline static size_t MemorySizeBytes(const gfx::Size& size,
+  // Return true if the call to UncheckedMemorySizeBytes would return a value
+  // that fits in a size_t.
+  static bool VerifySizeInBytes(const gfx::Size& size,
+                                ResourceFormat format,
+                                size_t* bytes) {
+    if (size.IsEmpty()) {
+      if (bytes)
+        *bytes = 0;
+      return true;
+    }
+
+    base::CheckedNumeric<size_t> checked_value = BitsPerPixel(format);
+    checked_value *= size.width();
+    checked_value *= size.height();
+    if (!checked_value.IsValid())
+      return false;
+    size_t value = checked_value.ValueOrDie();
+    if ((value % 8) != 0)
+      return false;
+    if (bytes)
+      *bytes = value / 8;

[danakj, 2015/06/04 22:43:49]

Eh, you're adding this optional pointer thing just to avoid duplicating like 3
lines of code over to CheckedMemorySizeBytes?

If it's the %8 stuff you're worried about, DCHECK(VerifySizeInBytes) there too?
CheckedMemorySizeBytes should just be DCHECK(..whatever you want..) and then
computing CheckedNumeric and return ValueOrDie. Else this is so many branches
just to compute a number.

[vmpstr, 2015/06/04 23:17:30]

Done.

+    return true;
+  }
+
+  static size_t CheckedMemorySizeBytes(const gfx::Size& size,
                                        ResourceFormat format) {
-    DCHECK_EQ(0, (BitsPerPixel(format) * size.width() * size.height()) % 8);
-    // TODO(vmpstr): Make this function overflow safe. crbug.com/495867
-    return static_cast<size_t>(
-        (BitsPerPixel(format) * size.width() * size.height()) / 8);
+    size_t bytes = 0;
+    bool is_valid = VerifySizeInBytes(size, format, &bytes);
+    CHECK(is_valid);
+    return bytes;
+  }
+
+  inline static size_t UncheckedMemorySizeBytes(const gfx::Size& size,
+                                                ResourceFormat format) {
+    if (size.IsEmpty())
+      return 0;
+    DCHECK(VerifySizeInBytes(size, format, nullptr));
+    return static_cast<size_t>(BitsPerPixel(format)) * size.width() *
+           size.height() / 8;
   }
 
  protected: