linux-kernel - Re: [PATCH 05/15] x86: Implement function

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Message-ID: <CALCETrVTtKqD6fonUmT_qr0HJ0X9TWzLGq-wpm+A7XKyjn3W5g@mail.gmail.com>
Date:   Fri, 16 Apr 2021 15:06:17 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, X86 ML <x86@...nel.org>,
        Kees Cook <keescook@...omium.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>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi

On Fri, Apr 16, 2021 at 3:03 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > __nocfi only disables CFI checking in a function, the compiler still
> > changes function addresses to point to the CFI jump table, which is
> > why we need function_nocfi().
>
> So call it __func_addr() or get_function_addr() or so, so that at least
> it is clear what this does.
>

This seems backwards to me.  If I do:

extern void foo(some signature);

then I would, perhaps naively, expect foo to be the actual symbol that
gets called and for the ABI to be changed to do the CFI checks.  The
foo symbol would point to whatever magic is needed.  I assume I'm
missing something.