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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 12 Feb 2020 09:36:23 -0800
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Subject: Re: [PATCH v7 00/11] add support for Clang's Shadow Call Stack
To:     James Morse <james.morse@arm.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
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On Tue, Feb 11, 2020 at 5:57 AM James Morse <james.morse@arm.com> wrote:
> I found I had to add:
> | KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
>
> to drivers/firmware/efi/libstub/Makefile, to get this going.

Ah, good catch!

> I don't think there is much point supporting SCS for the EFIstub, its already isolated
> from the rest of the kernel's C code by the __efistub symbol prefix machinery, and trying
> to use it would expose us to buggy firmware at a point we can't handle it!

Yes, fully agreed.

> I can send a patch if its easier for you,

It's not a problem, I will include a patch for this in v8.

Sami