Re: [PATCH] efi/libstub: Indicate clang the relocation mode for arm64

[Matthias Kaehlcke <mka@chromium.org>]

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@chromium.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@google.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@google.com>
> >> >Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
> >> >Signed-off-by: Matthias Kaehlcke <mka@chromium.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.

Cheers

Matthias