linux-hardening - Re: [RFC PATCH 16/21] objtool: Add support for CONFIG_CFI

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Date:   Fri, 29 Apr 2022 18:00:03 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 16/21] objtool: Add support for CONFIG_CFI_CLANG

On Fri, Apr 29, 2022 at 4:30 PM Peter Zijlstra <peterz@...radead.org> wrote:
> Urgh, what does this do for decode speed? This is a hash-lookup for
> every single instruction.

Two actually, since .kcfi_traps only contains compiler-emitted
locations and we also have to check for manual type annotations. I
haven't measured performance yet, but I also didn't notice a
significant impact here.

> Is that kcfi location array sorted by the compiler? Because then you can
> keep a running iterator and replace the whole lookup with a simple
> equality comparison.

The compiler generates a separate .kcfi_types section for each text
section and the entries are emitted in order, so this should be
doable.

Sami