linux-kernel - Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug

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Message-ID: <202006291136.E6DF8242@keescook>
Date:   Mon, 29 Jun 2020 11:37:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Will Deacon <will@...nel.org>,
        Dave Martin <Dave.Martin@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Kiss <daniel.kiss@....com>
Subject: Re: [PATCH v4 05/17] ctype: Work around Clang
 -mbranch-protection=none bug

On Mon, Jun 29, 2020 at 11:02:51AM -0700, Nick Desaulniers wrote:
> This is definitely better than the empty function.  Though a patch is
> posted for fixing this in LLVM. Assuming that lands before this, we
> might not actually need this workaround?
> 
> arch/arm64/Kconfig
> 1625 config ARM64_BTI_KERNEL
> ...
> 1633   # https://reviews.llvm.org/rGb8ae3fdfa579dbf366b1bb1cbfdbf8c51db7fa55
> 1634   depends on !CC_IS_CLANG || CLANG_VERSION >= 100001
> 
> So if Daniel's patch lands AND is backported into the clang 10.0.1
> release, then we might not need to carry this workaround?  Either way,

True, though as I mentioned in the review, I don't think it's quite
right -- the warning getting removed is actually quite valuable.

> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks!

-- 
Kees Cook