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Date:   Tue, 2 Nov 2021 10:26:29 -0700
From:   Kees Cook <>
To:     Peter Zijlstra <>
Cc:     Ard Biesheuvel <>,
        Mark Rutland <>,
        Sami Tolvanen <>,
        X86 ML <>, Josh Poimboeuf <>,
        Nathan Chancellor <>,
        Nick Desaulniers <>,
        Sedat Dilek <>,
        Steven Rostedt <>,,
        Linux Kernel Mailing List <>,
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

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

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