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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 5 Dec 2018 10:40:30 -0800
Message-ID: <CAKwvOdn+qRhNX1x+pTiqGA=8ud5PxtiV61sPL5JTawQ6HAFtxw@mail.gmail.com>
Subject: Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        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>
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On Wed, Dec 5, 2018 at 10:36 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> 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@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)
> >
>
> 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.

Nathan,
Thanks for getting a build working!  I think Ard is looking for the
presence of symbols with `veneer` in the name.

Something like `llvm-objdump -t vmlinux | grep veneer` or `llvm-nm
vmlinux | grep veneer` should tell you if such symbols exist.

If they do, Ard was then looking for the disassembly of those labels
(like you provided for __enable_mmu/__turn_mmu_on).

>
> 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
> > > > >


-- 
Thanks,
~Nick Desaulniers