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

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Date:   Wed, 5 Dec 2018 11:36:06 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stefan Agner <stefan@...er.ch>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option

On Wed, Dec 05, 2018 at 09:09:56AM +0100, Ard Biesheuvel wrote:
> (+ Arnd)
> 
> On Wed, 5 Dec 2018 at 09:06, Nathan Chancellor <natechancellor@...il.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@...il.com> wrote:
> > > >
> > > > This flag is not supported by lld:
> > > >
> > > >     ld.lld: error: unknown argument: --pic-veneer
> > > >
> > > > Signed-off-by: Nathan Chancellor <natechancellor@...il.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)
> 

Alright, compiling with allyesconfig is a little rough at the moment
(bug reports I will file in due time) but I was able to do it. Here's
the disassembly specifically for the functions you had in your commit,
my assembly knowledge is pretty much non-existent unfortunately so I am
not sure what to make of it (it doesn't look like there is a virtual
address for pc in that mix?). I am happy to provide any more information
that is needed.

c03030cc <__enable_mmu>:
c03030cc:	e3c00002 	bic	r0, r0, #2
c03030d0:	e3c00b02 	bic	r0, r0, #2048	; 0x800
c03030d4:	e3c00a01 	bic	r0, r0, #4096	; 0x1000
c03030d8:	e3a05051 	mov	r5, #81	; 0x51
c03030dc:	ee035f10 	mcr	15, 0, r5, cr3, cr0, {0}
c03030e0:	ee024f10 	mcr	15, 0, r4, cr2, cr0, {0}
c03030e4:	eafff3c5 	b	c0300000 <__turn_mmu_on>
c03030e8:	e320f000 	nop	{0}
c03030ec:	e320f000 	nop	{0}
c03030f0:	e320f000 	nop	{0}
c03030f4:	e320f000 	nop	{0}
c03030f8:	e320f000 	nop	{0}
c03030fc:	e320f000 	nop	{0}

c0300000 <__turn_mmu_on>:
c0300000:	e1a00000 	nop			; (mov r0, r0)
c0300004:	ee070f95 	mcr	15, 0, r0, cr7, cr5, {4}
c0300008:	ee010f10 	mcr	15, 0, r0, cr1, cr0, {0}
c030000c:	ee103f10 	mrc	15, 0, r3, cr0, cr0, {0}
c0300010:	ee070f95 	mcr	15, 0, r0, cr7, cr5, {4}
c0300014:	e1a03003 	mov	r3, r3
c0300018:	e1a0300d 	mov	r3, sp
c030001c:	e1a0f003 	mov	pc, r3

Thanks,
Nathan

> > 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
> > > >