Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option

From: Nick Desaulniers <ndesaulniers@google.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux@armlinux.org.uk,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Stefan Agner <stefan@agner.ch>,
	Nicolas Pitre <nicolas.pitre@linaro.org>,
	Peter Smith <peter.smith@linaro.org>
Subject: Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option
Date: Wed, 5 Dec 2018 10:21:51 -0800	[thread overview]
Message-ID: <CAKwvOdno9AdzNvMVmzqV7y+_u_z7VgfbRvKm73rRZ+iUuraYtw@mail.gmail.com> (raw)
In-Reply-To: <CAKv+Gu9Nq51Fj_14MYS6MEm0VPHOJB1CBFkCHmyC8Wt0=NWpRg@mail.gmail.com>

On Wed, Dec 5, 2018 at 12:10 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> (+ Arnd)
>
> On Wed, 5 Dec 2018 at 09:06, Nathan Chancellor <natechancellor@gmail.com> wrote:
> >
> > On Wed, Dec 05, 2018 at 08:37:05AM +0100, Ard Biesheuvel wrote:
> > > On Wed, 5 Dec 2018 at 02:42, Nathan Chancellor <natechancellor@gmail.com> wrote:
> > > >
> > > > This flag is not supported by lld:
> > > >
> > > >     ld.lld: error: unknown argument: --pic-veneer
> > > >
> > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > >
> > > Hi Nate,
> > >
> > > Does this mean ld.lld is guaranteed to produce position independent
> > > veneers if you build kernels that are bigger than the typical range of
> > > a relative branch?

(+ Peter) who might be able to answer this.

> > >
> >
> > Hi Ard,
> >
> > Honestly, I'm not quite sure. I saw your commit that introduced this
> > flag and I wasn't quite sure what to make of it for lld. What
> > configuration would I use to verify and what would I check for?
> >
>
> Try building allyesconfig, and check the resulting binary for veneers
> (which have 'veneer' in the symbol name, at least when ld.bfd emits
> them). These veneers should not take the [virtual] address of the
> branch target directly, but take a PC relative offset (as in the
> example in the commit log of that patch you are referring to)

Not sure clang can compile an arm32 allyesconfig (haven't tried
personally, yet, TODO), but we could try gcc+lld.

>
> > Additionally, I have filed an LLVM bug for the lld developers to
> > check and see if this is a flag they should support:
> >
> > https://bugs.llvm.org/show_bug.cgi?id=39886
> >
> > Thanks for the quick reply,
> > Nathan
> >
> > > > ---
> > > >  arch/arm/Makefile | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> > > > index e2a0baf36766..4fab2aa29570 100644
> > > > --- a/arch/arm/Makefile
> > > > +++ b/arch/arm/Makefile
> > > > @@ -10,7 +10,7 @@
> > > >  #
> > > >  # Copyright (C) 1995-2001 by Russell King
> > > >
> > > > -LDFLAGS_vmlinux        := --no-undefined -X --pic-veneer
> > > > +LDFLAGS_vmlinux        := --no-undefined -X $(call ld-option,--pic-veneer)
> > > >  ifeq ($(CONFIG_CPU_ENDIAN_BE8),y)
> > > >  LDFLAGS_vmlinux        += --be8
> > > >  KBUILD_LDFLAGS_MODULE  += --be8
> > > > --
> > > > 2.20.0.rc1
> > > >

-- 
Thanks,
~Nick Desaulniers