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Date:   Tue, 2 Nov 2021 10:26:29 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        X86 ML <x86@kernel.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 v5 00/15] x86: Add support for Clang CFI
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On Thu, Oct 28, 2021 at 10:29:05PM +0200, Peter Zijlstra wrote:
> Now, since code (on x86) is variable length, there are no spare bits in
> the code address, but since static_call_key is aligned, we have spare
> bits. It is those bits we use to encode TAIL (Bit0) and INIT (Bit1).
> 
> If INIT, the address points to an __init section and we shouldn't try
> and touch if after those have been freed or bad stuff happens.
> 
> If TAIL, it's a tail-call and we get to write a jump instruction instead
> of a call instruction.

I think this is the part that I was missing: the information is about
the _address_, but it's stored in the _key_'s low bits (regardless of
the key's actual/masked key pointer).

> [...]
> Hope that clarifies things, instead of making it worse :-)

It does help, yes, thanks! I will need to read it again and go follow
along in the code, but yes, that helps explain it.

-- 
Kees Cook