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

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Stefan Agner <stefan@agner.ch>,
	Nicolas Pitre <nicolas.pitre@linaro.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option
Date: Wed, 5 Dec 2018 09:09:56 +0100	[thread overview]
Message-ID: <CAKv+Gu9Nq51Fj_14MYS6MEm0VPHOJB1CBFkCHmyC8Wt0=NWpRg@mail.gmail.com> (raw)
In-Reply-To: <20181205080645.GA11936@flashbox>

(+ 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?
> >
>
> 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)

> 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
> > >