linux-kernel - Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations

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Date:   Tue, 30 Jun 2020 13:54:08 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Fangrui Song <maskray@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        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 ML <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime
 relocations

On Tue, Jun 30, 2020 at 06:26:43PM +0200, Ard Biesheuvel wrote:
> On Tue, 30 Jun 2020 at 01:34, Fangrui Song <maskray@...gle.com> wrote:
> 
> > If (1) is not satisfied, the created .got.plt is just served as an anchor for
> > things that want to reference (the distance from GOT base to some point). The
> > linker will still reserve 3 words but the words are likely not needed.
> >
> > I don't think there is a specific need for another option to teach the linker
> > (GNU ld or LLD) that this is a kernel link.  For -ffreestanding builds, cc
> > -static (ld -no-pie))/-static-pie (-pie) already work quite well.
> 
> You mean 'ld -static -pie' right? That seems to work. Is that a recent
> invention?

gcc -static-pie is fairly recent [0], but it just influences how the
linker is invoked AFAIK (at least for gcc) -- in addition to passing
some linker flags, it will change what startup files get linked in (for
non-freestanding). It does not even imply -fPIE to the compiler, which
is confusing as hell. It _would_ be nice if this also told the compiler
that all symbols (perhaps unless explicitly marked) will be resolved at
static link time, so there is no need to use the GOT or PLT for globals.

As it stands, the executable can still have relocations, GOT and PLT, it
just needs to have startup code to handle them (provided by libc
typically) instead of relying on an external dynamic linker.

I don't think it's really relevant for the kernel build -- all we get is
ld -static --no-dynamic-linker, all -static does is prevent searching
shared libraries, and we already pass --no-dynamic-linker if it's
supported.

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81498