> If you can already make the process you're attacking run arbitrary syscalls instead of use glibc, then you've already won and no amount of protection will help

You're basically arguing that in a post-spectre world, native processes can fundamentally never be a security boundary again, right?

I'm wondering if this is necessarily true. For the concrete example at hand, Linux could offer some opt-in mechanism, e.g. an argument to exec(), that restricts syscalls to glibc only. A sandboxing mechanism could then require all executed processes to go through glibc and instantiate them only using that option.