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Date:   Tue, 30 Apr 2019 13:05:49 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
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        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Jonathan Adams <jwadams@google.com>,
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        Paul Turner <pjt@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux-MM <linux-mm@kvack.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Apr 30, 2019 at 07:03:37AM +0200, Ingo Molnar wrote:
> > So the question IMHO isn't whether it's "valid C", because we already 
> > have the Linux kernel's own C syntax variant and are enforcing it with 
> > varying degrees of success.
> 
> I'm not getting into the whole 'safe' fight here; but you're under
> selling things. We don't have a C syntax, we have a full blown C
> lanugeage variant.
> 
> The 'Kernel C' that we write is very much not 'ANSI/ISO C' anymore in a
> fair number of places. And if I can get my way, we'll only diverge
> further from the standard.

Yeah, but I think it would be fair to say that random style variations 
aside, in the kernel we still allow about 95%+ of 'sensible C'.

> And this is quite separate from us using every GCC extention under the 
> sun; which of course also doesn't help. It mostly has to do with us 
> treating C as a portable assembler and the C people not wanting to 
> commit to sensible things because they think C is a high-level 
> language.

Indeed, and also because there's arguably somewhat of a "if the spec 
allows it then performance first, common-sense semantics second" mindset. 
Which is an understandable social dynamic, as compiler developers tend to 
distinguish themselves via the optimizations they've authored.

Anyway, the main point I tried to make is that I think we'd still be able 
to allow 95%+ of "sensible C" even if executed in a "safe runtime", and 
we'd still be able to build and run without such strong runtime type 
enforcement, i.e. get kernel code close to what we have today, minus a 
handful of optimizations and data structures. (But the performance costs 
even in that case are nonzero - I'm not sugarcoating it.)

( Plus even that isn't a fully secure solution with deterministic 
  outcomes, due to parallelism and data races. )

Thanks,

	Ingo