The same way the underlying printf implementation does? This isn't wrapping printf(3), just the underlying write(2).

Are you trying to ask if it catches, e.g., format string vulnerabilities? I think the answer to that is: Native Client's aim is to be a safe x86 VM, so — hopefully. But to me personally, it seems unlikely that they've thought of everything.

The important part of the design - the bit that makes it achievable - is that it's not a general x86 VM. It only accepts a limited subset of valid x86 object code, a subset chosen to make validation a tractable problem. This requires a modified compiler be used.

The unlikelihood that the original design was perfect is probably why they had the "Native Client Security Contest" a few years ago - and indeed independent researchers found several flaws. Personally, I'm a lot happier with it now that they've fixed everything that Mark Dowd could find wrong with it ;)