Date:   Wed, 10 May 2017 12:47:34 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Greg Hackmann <ghackmann@...gle.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Grant Grundler <grundler@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Bernhard Rosenkränzer 
        <Bernhard.Rosenkranzer@...aro.org>
Subject: Re: [PATCH] efi/libstub: Indicate clang the relocation mode for arm64

El Wed, May 10, 2017 at 09:05:28PM +0200 Ard Biesheuvel ha dit:

> 
> 
> > On 10 May 2017, at 20:38, Matthias Kaehlcke <mka@...omium.org> wrote:
> > 
> > Hoi Ard,
> > 
> > El Wed, May 10, 2017 at 08:51:44AM +0100 Ard Biesheuvel ha dit:
> > 
> >> On 9 May 2017 at 22:49, Matthias Kaehlcke <mka@...omium.org> wrote:
> >>> El Tue, May 09, 2017 at 01:50:36PM -0700 Greg Hackmann ha dit:
> >>> 
> >>>> On 05/09/2017 12:36 PM, Matthias Kaehlcke wrote:
> >>>>> From: Greg Hackmann <ghackmann@...gle.com>
> >>>>> 
> >>>>> Without any extra guidance, clang will generate libstub with either
> >>>>> absolute or relative ELF relocations. Use the right combination of
> >>>>> -fpic and -fno-pic on different files to avoid this.
> >>>>> 
> >>>>> Signed-off-by: Greg Hackmann <ghackmann@...gle.com>
> >>>>> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@...aro.org>
> >>>>> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> >>>>> ---
> >>>>> drivers/firmware/efi/libstub/Makefile | 6 ++++++
> >>>>> 1 file changed, 6 insertions(+)
> >>>>> 
> >>>>> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> >>>>> index f7425960f6a5..ccbaaf4d8650 100644
> >>>>> --- a/drivers/firmware/efi/libstub/Makefile
> >>>>> +++ b/drivers/firmware/efi/libstub/Makefile
> >>>>> @@ -11,6 +11,9 @@ cflags-$(CONFIG_X86)               += -m$(BITS) -D__KERNEL__ -O2 \
> >>>>>                               -mno-mmx -mno-sse
> >>>>> 
> >>>>> cflags-$(CONFIG_ARM64)              := $(subst -pg,,$(KBUILD_CFLAGS))
> >>>>> +ifeq ($(cc-name),clang)
> >>>>> +cflags-$(CONFIG_ARM64)              += -fpic
> >>>>> +endif
> >>>>> cflags-$(CONFIG_ARM)                := $(subst -pg,,$(KBUILD_CFLAGS)) \
> >>>>>                               -fno-builtin -fpic -mno-single-pic-base
> >>>>> 
> >>>>> @@ -38,6 +41,9 @@ $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
> >>>>> 
> >>>>> lib-$(CONFIG_EFI_ARMSTUB)   += arm-stub.o fdt.o string.o random.o \
> >>>>>                               $(patsubst %.c,lib-%.o,$(arm-deps))
> >>>>> +ifeq ($(cc-name),clang)
> >>>>> +CFLAGS_arm64-stub.o            += -fno-pic
> >>>>> +endif
> >>>>> 
> >>>>> lib-$(CONFIG_ARM)           += arm32-stub.o
> >>>>> lib-$(CONFIG_ARM64)         += arm64-stub.o
> >>>>> 
> >>>> 
> >>>> NAK.
> >>>> 
> >>>> This patch was labeled "HACK:" in our experimental tree.  There's no
> >>>> rhyme or reason to why this combination of -f[no-]pic flags
> >>>> generates code without problematic relocations.  It's inherently
> >>>> fragile, and was only intended as a temporary workaround until I (or
> >>>> someone more familiar with EFI) got a chance to revisit the problem.
> >>>> 
> >>>> Unless the gcc CFLAGS are also an artifact of "mess with -f[no-]pic
> >>>> until the compiler generates what you want", this doesn't belong
> >>>> upstream.
> >>> 
> >>> Sorry, I didn't realize it is that bad of a hack. Unfortunately I'm
> >>> not very familiar with EFI either.
> >>> 
> >>> I saw Ard did some work in this code related with relocation, maybe he
> >>> can provide a pointer towards a better solution.
> >>> 
> >> 
> >> This is a known issue. The problem is that generic AArch64 small model
> >> code is mostly position independent already, due to its use of
> >> adrp/add pairs to generate symbol references with a +/- 4 GB range.
> >> Building the same code with -fpic will result in GOT entries to be
> >> generated, which carry absolute addresses, so this achieves the exact
> >> opposite of what we want.
> >> 
> >> The reason for the GOT entries is that GCC (and Clang, apparently)
> >> infer from the -fpic flag that you are building objects that will be
> >> linked into a shared library, to which ELF symbol preemption rules
> >> apply that stipulate that a symbol in the main executable supersedes a
> >> symbol under the same name in the shared library, and that the shared
> >> library should update all its internal references to the main
> >> executable's version of the symbol. The easiest way (but certainly not
> >> the only way) to achieve that is to indirect all internal symbol
> >> references via GOT entries, which can be made to refer to another
> >> symbol by updating a single value.
> >> 
> >> The workaround I used is to use hidden visibility, using a #pragma.
> >> (There is a -fvisibility=hidden command line option as well, but this
> >> is a weaker form that does not apply to extern declarations, only to
> >> definitions). So if you add
> >> 
> >> #pragma GCC visibility push(hidden)
> >> 
> >> at the beginning of arm64-stub.c (and perhaps to one or two other
> >> files that contain externally visible symbol declarations these days),
> >> you should be able to compile the entire EFI stub with -fpic. Note
> >> that making those externally visible symbols 'static' where possible
> >> would solve the problem as well, but this triggers another issue in
> >> the 32-bit ARM stub.
> >> 
> >> In my opinion, the correct fix would be to make -fpie (as opposed to
> >> -fpic) imply hidden visibility, given that PIE executables don't
> >> export symbols in the first place, and so the preemption rules do not
> >> apply. It is worth a try whether -fpie works as expected in this case
> >> on Clang, but the last time I tried it on GCC, it behaved exactly like
> >> -fpic.
> > 
> > Thanks a lot for the detailed description and your suggestions!
> > 
> > A clang build with -fpie for the EFI stub succeeds without complaints
> > about GOT entries. I will send out an updated patch (with -fpie only
> > for clang) later.
> > 
> 
> Good! I never liked the visibility hack, which is why I never upstreamed it.
> 
> Could you please check how recent GCC behaves?

I tried GCC v4.9.4 and v6.3.1, both build the EFI stub with -fpie
without errors.

Are you suggesting to use -fpie for both clang and GCC? Do you know
what the minimum required GCC version is for building an arm64 kernel?

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