linux-kernel - Re: [PATCH v3 6/7] x86/boot: Remove runtime relocations from head

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Message-ID: <202006290905.7531731A@keescook>
Date:   Mon, 29 Jun 2020 09:06:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux@...glegroups.com,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "H . J . Lu" <hjl@...rceware.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/7] x86/boot: Remove runtime relocations from
 head_{32,64}.S

On Mon, Jun 29, 2020 at 10:09:27AM -0400, Arvind Sankar wrote:
> The BFD linker generates runtime relocations for z_input_len and
> z_output_len, even though they are absolute symbols.
> 
> This is fixed for binutils-2.35 [1]. Work around this for earlier
> versions by defining two variables input_len and output_len in addition
> to the symbols, and use them via position-independent references.
> 
> This eliminates the last two runtime relocations in the head code and
> allows us to drop the -z noreloc-overflow flag to the linker.
> 
> [1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> Reviewed-by: Fangrui Song <maskray@...gle.com>
> ---
>  arch/x86/boot/compressed/Makefile  |  8 --------
>  arch/x86/boot/compressed/head_32.S | 17 ++++++++---------
>  arch/x86/boot/compressed/head_64.S |  4 ++--
>  arch/x86/boot/compressed/mkpiggy.c |  6 ++++++
>  4 files changed, 16 insertions(+), 19 deletions(-)

I continue to really enjoy the smaller code. Anything that makes
mkpiggy.c smaller is a win. :)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook