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From:   Jian Cai <jiancai@google.com>
Date:   Tue, 23 Mar 2021 15:39:50 -0700
Message-ID: <CA+SOCLJTSHs9CZc+h0bWz=k5UUp5zLSLFwLyVGdr1v7O3VUOew@mail.gmail.com>
Subject: Re: [PATCH v5] ARM: Implement SLS mitigation
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Manoj Gupta <manojgupta@google.com>,
        Luis Lozano <llozano@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <nathan@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Will Deacon <will@kernel.org>,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        =?UTF-8?Q?Andreas_F=C3=A4rber?= <afaerber@suse.de>,
        Ard Biesheuvel <ardb@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        David Brazdil <dbrazdil@google.com>,
        James Morse <james.morse@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Thanks for the suggestion. I've sent an inquiry to the author of
-mharden-sls* in GCC and hopefully that would shed some more light. We
do get warnings for oraphon sections when using lld. The other linkers
do not seem to provide such warnings, although the boot failure also
does not seem to happen with them.

On Mon, Mar 22, 2021 at 4:45 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> On Wed, Mar 10, 2021 at 5:43 AM Jian Cai <jiancai@google.com> wrote:
> > On Sat, Mar 6, 2021 at 4:25 AM Linus Walleij <linus.walleij@linaro.org> wrote:
> > > On Fri, Mar 5, 2021 at 12:23 AM Jian Cai <jiancai@google.com> wrote:
> > > > On Wed, Mar 3, 2021 at 7:04 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> > > > I think gcc also has these options.
> > > > https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
> > >
> > > And how does that work with this part of your patch:
> > >
> > > +#define SLS_TEXT                                       \
> > > +       ALIGN_FUNCTION();                              \
> > > +       *(.text.__llvm_slsblr_thunk_*)
> > >
> > > This does not look compiler agnostic?
> >
> > You are right, GCC does generate different oraphan section names. I
> > will address it in the next version of the patch. Also it seems only
> > arm64 gcc supports -mharden-sls=* at this moment, arm32 gcc does not
> > support it yet. I don't know if there is any plan to implement it for
> > 32-bit gcc, but should we patch arm32 linker script preemptively,
> > assuming the sections will be named with the same pattern like how
> > clang does so the kernel would not fail to boot when the flag is
> > implemented?
>
> I think the best thing is to have something like this:
> Implement a macro such as this in
> include/linux/compiler-clang.h
>
> #define SLS_TEXT_SECTION *(.text.__llvm_slsblr_thunk_*)
>
> then the corresponding in include/linux/compiler-gcc.h
> but here also add a
>
> #define SLS_TEXT_SECTION #error "no compiler support"
>
> if the compiler version does not have this.
>
> I don't know the exact best approach sadly, as the patch
> looks now it seems a bit fragile, I wonder if you get linker
> warnings when this section is unused?
>
> Yours,
> Linus Walleij