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Date:   Wed, 31 Aug 2022 15:04:20 -0700
From:   Kees Cook <>
To:     Sami Tolvanen <>
Cc:, Josh Poimboeuf <>,
        Peter Zijlstra <>,,
        Catalin Marinas <>,
        Will Deacon <>,
        Mark Rutland <>,
        Nathan Chancellor <>,
        Nick Desaulniers <>,
        Joao Moreira <>,
        Sedat Dilek <>,
        Steven Rostedt <>,,,
Subject: Re: [PATCH v4 00/21] KCFI support

On Tue, Aug 30, 2022 at 04:31:08PM -0700, Sami Tolvanen wrote:
> KCFI is a forward-edge control-flow integrity scheme in the upcoming
> Clang 16 release, which is more suitable for kernel use than the
> existing CFI scheme used by CONFIG_CFI_CLANG. KCFI doesn't require
> LTO, doesn't alter function references to point to a jump table, and
> won't break function address equality.
> This series replaces the current arm64 CFI implementation with KCFI
> and adds support for x86_64.
> KCFI requires assembly functions that are indirectly called from C
> code to be annotated with type identifiers. As type information is
> only available in C, the compiler emits expected type identifiers
> into the symbol table, so they can be referenced from assembly
> without having to hardcode type hashes. Patch 6 adds helper macros
> for annotating functions, and patches 9 and 19 add annotations.
> In case of a type mismatch, KCFI always traps. To support error
> handling, the compiler generates a .kcfi_traps section for x86_64,
> which contains the locations of each trap, and for arm64, encodes
> the necessary register information to the ESR. Patches 10 and 21 add
> arch-specific error handlers.
> To test this series, you'll a ToT Clang toolchain. The series is
> also available in GitHub:

This happily runs through all my initial testing hoops. :)

Tested-by: Kees Cook <>

Kees Cook

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