linux-kernel - Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM

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Date:   Fri, 15 Oct 2021 17:55:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C

On Thu, Oct 14 2021 at 19:51, Andy Lutomirski wrote:
> On Wed, Oct 13, 2021, at 11:16 AM, Sami Tolvanen wrote:
>> 
>> +/*
>> + * Declares a function not callable from C using an opaque type. Defined as
>> + * an array to allow the address of the symbol to be taken without '&'.
>> + */
> I’m not convinced that taking the address without using & is a
> laudable goal.  The magical arrays-are-pointers-too behavior of C is a
> mistake, not a delightful simplification.

>> +#ifndef DECLARE_NOT_CALLED_FROM_C
>> +#define DECLARE_NOT_CALLED_FROM_C(sym) \
>> +	extern const u8 sym[]
>> +#endif
>

> The relevant property of these symbols isn’t that they’re not called
> from C.  The relevant thing is that they are just and not objects of a
> type that the programmer cares to tell the compiler about. (Or that
> the compiler understands, for that matter. On a system with XO memory
> or if they’re in a funny section, dereferencing them may fail.)

I agree.

> So I think we should use incomplete structs, which can’t be
> dereferenced and will therefore be less error prone.

While being late to that bike shed painting party, I really have to ask
the question _why_ can't the compiler provide an annotation for these
kind of things which:

    1) Make the build fail when invoked directly

    2) Tell CFI that this is _NOT_ something it can understand

-void clear_page_erms(void *page);
+void __bikeshedme clear_page_erms(void *page);

That still tells me:

    1) This is a function
    
    2) It has a regular argument which is expected to be in RDI

which even allows to do analyis of e.g. the alternative call which
invokes that function.

DECLARE_NOT_CALLED_FROM_C(clear_page_erms);

loses these properties and IMO it's a tasteless hack.

Thanks,

        tglx