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Date:   Tue, 17 May 2022 10:57:50 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, x86@kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, llvm@lists.linux.dev
Subject: Re: [RFC PATCH v2 00/21] KCFI support
Message-ID: <YoNjjjb3ygIKnwfj@hirez.programming.kicks-ass.net>
References: <20220513202159.1550547-1-samitolvanen@google.com>
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On Fri, May 13, 2022 at 01:21:38PM -0700, Sami Tolvanen wrote:
> KCFI is a proposed forward-edge control-flow integrity scheme for
> Clang, 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. The latest LLVM patch is here:
> 
>   https://reviews.llvm.org/D119296
> 
> This RFC series replaces the current arm64 CFI implementation with
> KCFI and adds support for x86_64.

You have some weird behaviour vs weak functions (I so hate those)...

100: 0000000000000980     9 FUNC    LOCAL  DEFAULT    2 __cfi_free_initmem
233: 0000000000000989    35 FUNC    WEAK   DEFAULT    2 free_initmem

With the result that on the final link:

   179: 00000000000009b0     9 FUNC    LOCAL  DEFAULT    1 __cfi_free_initmem
  8689: 00000000000007f0     9 FUNC    LOCAL  DEFAULT   65 __cfi_free_initmem
173283: 00000000000007f9   198 FUNC    GLOBAL DEFAULT   65 free_initmem

This is getting me objtool issues (I'll fix them) but perhaps it's
something you can do something about as well.