linux-kernel - Re: [PATCH v2] arm64/alternatives: move length validation inside the subsection

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Date:   Thu, 30 Jul 2020 10:34:24 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] arm64/alternatives: move length validation inside the subsection

On Thu, Jul 30, 2020 at 10:24 AM Catalin Marinas
<catalin.marinas@....com> wrote:
>
> On Thu, Jul 30, 2020 at 08:37:01AM -0700, Sami Tolvanen wrote:
> > Commit f7b93d42945c ("arm64/alternatives: use subsections for replacement
> > sequences") breaks LLVM's integrated assembler, because due to its
> > one-pass design, it cannot compute instruction sequence lengths before the
> > layout for the subsection has been finalized. This change fixes the build
> > by moving the .org directives inside the subsection, so they are processed
> > after the subsection layout is known.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1078
> > Fixes: f7b93d42945c ("arm64/alternatives: use subsections for replacement sequences")
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> > v1 -> v2:
> > - Added the missing Fixes tag and dropped CC: stable@.
> >
> > ---
> >  arch/arm64/include/asm/alternative.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
> > index 12f0eb56a1cc..619db9b4c9d5 100644
> > --- a/arch/arm64/include/asm/alternative.h
> > +++ b/arch/arm64/include/asm/alternative.h
> > @@ -77,9 +77,9 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
> >       "663:\n\t"                                                      \
> >       newinstr "\n"                                                   \
> >       "664:\n\t"                                                      \
> > -     ".previous\n\t"                                                 \
> >       ".org   . - (664b-663b) + (662b-661b)\n\t"                      \
> > -     ".org   . - (662b-661b) + (664b-663b)\n"                        \
> > +     ".org   . - (662b-661b) + (664b-663b)\n\t"                      \
> > +     ".previous\n"                                                   \
> >       ".endif\n"
>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
>
> There are a few instances of the .org test outside the subsection,
> though using in .S files. Are those ok?

Yes, this only appears to be a problem when used in inline assembly.

Sami