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

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Message-ID: <20200629165210.GC900899@rani.riverdale.lan>
Date:   Mon, 29 Jun 2020 12:52:10 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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 09:06:07AM -0700, Kees Cook wrote:
> 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

It doesn't actually reduce the size? mkpiggy.c _could_ I think be
replaced with some carefully written shell code, but this doesn't do
anything like that.