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

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Message-ID: <202110131200.1CAF319F@keescook>
Date:   Wed, 13 Oct 2021 12:00:38 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <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@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C

On Wed, Oct 13, 2021 at 11:16:46AM -0700, 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 adds
> 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@...capital.net>
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

I like this; I have a sense CFI won't stay the only user of this
annotation.

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook