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Date:   Thu, 14 Oct 2021 19:51:11 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Sami Tolvanen" <samitolvanen@google.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
Cc:     "Kees Cook" <keescook@chromium.org>,
        "Josh Poimboeuf" <jpoimboe@redhat.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Nathan Chancellor" <nathan@kernel.org>,
        "Nick Desaulniers" <ndesaulniers@google.com>,
        "Sedat Dilek" <sedat.dilek@gmail.com>,
        "Steven Rostedt" <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
Subject: Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C
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On Wed, Oct 13, 2021, at 11:16 AM, Sami Tolvanen wrote:
> The kernel has several assembly functions, which are not directly
> callable from C but need to be referred to from C code. This change ad=
ds
> the DECLARE_NOT_CALLED_FROM_C macro, which allows us to declare these
> symbols using an opaque type, which makes misuse harder, and avoids the
> need to annotate references to the functions for Clang's Control-Flow
> Integrity (CFI).
>
> Suggested-by: Andy Lutomirski <luto@amacapital.net>
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> ---
>  include/linux/linkage.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/include/linux/linkage.h b/include/linux/linkage.h
> index dbf8506decca..f982d5f550ac 100644
> --- a/include/linux/linkage.h
> +++ b/include/linux/linkage.h
> @@ -48,6 +48,19 @@
>  #define __PAGE_ALIGNED_DATA	.section ".data..page_aligned", "aw"
>  #define __PAGE_ALIGNED_BSS	.section ".bss..page_aligned", "aw"
>=20
> +/*
> + * Declares a function not callable from C using an opaque type. Defi=
ned as
> + * an array to allow the address of the symbol to be taken without '&=
'.
> + */

I=E2=80=99m not convinced that taking the address without using & is a l=
audable goal.  The magical arrays-are-pointers-too behavior of C is a mi=
stake, 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=E2=80=99t that they=E2=80=99r=
e not called from C.  The relevant thing is that they are just and not o=
bjects 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 m=
emory or if they=E2=80=99re in a funny section, dereferencing them may f=
ail.)

So I think we should use incomplete structs, which can=E2=80=99t be dere=
ferenced and will therefore be less error prone.