Fix silent truncations when extracting values from CheckedNumeric

components/webcrypto/algorithms/aes_cbc.cc

Index: components/webcrypto/algorithms/aes_cbc.cc
diff --git a/components/webcrypto/algorithms/aes_cbc.cc b/components/webcrypto/algorithms/aes_cbc.cc
index c9e941aa7ee92f636d0dc84e5e4e8c0853fb35aa..a31882964e9862ef1ddddd1b112f297c064302b6 100644
--- a/components/webcrypto/algorithms/aes_cbc.cc
+++ b/components/webcrypto/algorithms/aes_cbc.cc
@@ -55,7 +55,8 @@ Status AesCbcEncryptDecrypt(EncryptOrDecrypt cipher_operation,
   if (!output_max_len.IsValid())
     return Status::ErrorDataTooLarge();
 
-  const unsigned remainder = output_max_len.ValueOrDie() % AES_BLOCK_SIZE;
+  const unsigned remainder =
+      (output_max_len % AES_BLOCK_SIZE).template ValueOrDie<unsigned>();
   if (remainder != 0)

[Tom Sepez, 2016/11/29 20:06:26]

I'm not grokking this.  Can you explain?  May be too complicate for mere mortals
to use.

[jschuh, 2016/11/29 21:04:47]

It's the template disambiguator for dependent names. Without the template
keyword the compiler may assume it's getting a less-than token rather than a
dependent template parameter. MSVS doesn't need it, but clang and GCC both do.
However, I assume at some point in the future this will be fixed (like how
nested template brackets no longer get treated as right shift operations).

It's described in the last section of this page:
http://en.cppreference.com/w/cpp/language/dependent_name

[jschuh, 2016/11/30 23:38:52]

I added some helpers and am using those at the call sites.

     output_max_len += AES_BLOCK_SIZE - remainder;
   if (!output_max_len.IsValid())
@@ -71,7 +72,7 @@ Status AesCbcEncryptDecrypt(EncryptOrDecrypt cipher_operation,
     return Status::OperationError();
   }
 
-  buffer->resize(output_max_len.ValueOrDie());
+  buffer->resize(output_max_len.template ValueOrDie<size_t>());
 
   int output_len = 0;
   if (!EVP_CipherUpdate(context.get(), buffer->data(), &output_len,