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Date:   Sun, 31 Oct 2021 21:09:46 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <x86@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        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] static_call,x86: Robustify trampoline patching
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On Sun, Oct 31, 2021 at 05:44:04PM +0100, Ard Biesheuvel wrote:

> > Is is also a terriblly gross hack. I really want the clang-cfi stuff to
> > improve, not add layers of hacks on top of it.
> 
> I'm just as annoyed as you are about the apparent need for this.
> However, emitting an alias at build time is far better IMHO than
> adding a magic byte sequence and having to check it at runtime.

Oh, I'm keeping that magic sequence :-) That's hardening in general, and
I don't want to ever want to debug a wrong poke like that again.

Adding an extra label fixes this thing, but there's still the other
cases where we need/want/desire a *real* function pointer.

I'm very close to saying that anything that mucks up function pointers
like this is a complete non-starter. Let's start re-start this whole CFI
endeavour from the start.