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

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

[Matthias Kaehlcke]

From: Greg Hackmann <ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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
-- 
2.13.0.rc2.291.g57267f2277-goog

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

[Greg Hackmann]

On 05/09/2017 12:36 PM, Matthias Kaehlcke wrote:
> From: Greg Hackmann <ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>
> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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.

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

[Matthias Kaehlcke]

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.

Thanks

Matthias

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

[Ard Biesheuvel]

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.

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

[Matthias Kaehlcke]

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

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

[Ard Biesheuvel]

> On 10 May 2017, at 20:38, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>>>> 
>>>>> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>>>> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
>>>>> Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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?

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

[Matthias Kaehlcke]

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

> 
> 
> > On 10 May 2017, at 20:38, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> >>>>> 
> >>>>> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> >>>>> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >>>>> Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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?

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

[Ard Biesheuvel]

(adding Arnd and Will to cc, who are likely to have an opinion as to
which GCC is the oldest we need to support for arm64)

On 10 May 2017 at 20:47, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
> El Wed, May 10, 2017 at 09:05:28PM +0200 Ard Biesheuvel ha dit:
>
>>
>>
>> > On 10 May 2017, at 20:38, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> >>>>>
>> >>>>> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> >>>>> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
>> >>>>> Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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?

Yes. Up until now, we have been relying on the position independent
nature of small model code, but it would be better to specify it
explicitly, so if -fpie gives us mostly identical code and does not
need visibility hacks, I would prefer to add it for all compilers and
not have an exception only for Clang. Note that the same applies to
the entire kernel when built in KASLR mode, so it would also be good
to know our options here.

Arnd, Will, what is the oldest GCC version we claim to support for arm64?

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

[Matthias Kaehlcke]

El Thu, May 11, 2017 at 02:51:48PM +0100 Ard Biesheuvel ha dit:

> (adding Arnd and Will to cc, who are likely to have an opinion as to
> which GCC is the oldest we need to support for arm64)
> 
> On 10 May 2017 at 20:47, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
> > El Wed, May 10, 2017 at 09:05:28PM +0200 Ard Biesheuvel ha dit:
> >
> >>
> >>
> >> > On 10 May 2017, at 20:38, Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> >> >>>>>
> >> >>>>> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> >> >>>>> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >> >>>>> Signed-off-by: Matthias Kaehlcke <mka-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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?
> 
> Yes. Up until now, we have been relying on the position independent
> nature of small model code, but it would be better to specify it
> explicitly, so if -fpie gives us mostly identical code and does not
> need visibility hacks, I would prefer to add it for all compilers and
> not have an exception only for Clang. Note that the same applies to
> the entire kernel when built in KASLR mode, so it would also be good
> to know our options here.

Thanks, makes perfect sense.

> Arnd, Will, what is the oldest GCC version we claim to support for arm64?

Any comments on this Arnd/Will?

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

[Greg Hackmann]

On 05/11/2017 06:51 AM, Ard Biesheuvel wrote:
[snip]
>>>>> 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?
> 
> Yes. Up until now, we have been relying on the position independent
> nature of small model code, but it would be better to specify it
> explicitly, so if -fpie gives us mostly identical code and does not
> need visibility hacks, I would prefer to add it for all compilers and
> not have an exception only for Clang. Note that the same applies to
> the entire kernel when built in KASLR mode, so it would also be good
> to know our options here.
> 
> Arnd, Will, what is the oldest GCC version we claim to support for arm64?
> 

Unfortunately, after looking into this a bit more, -fpie by itself 
doesn't force clang to disable symbol preeemption.  For example when 
building the EFI stub from 4.9 with clang, -fpie gives me a stub that 
crashes with a synchronous exception inside handle_kernel_image().  The 
faulting instruction is a read from __nokaslr that still goes through 
the GOT.

Right now you'll get a usable EFI stub with -fpie anyway, since 
60f38de7a8d4 ("efi/libstub: Unify command line param parsing") masked 
the problem when it moved __nokaslr behind a helper function.  But AIUI 
there's nothing really preventing a similar problem in the future.

You *can* force clang to disable symbol preemption using "-fpie 
-mpie-copy-relocations".  That said, I don't know enough about EFI to 
say whether this is actually appropriate for building the EFI stub.

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

[Ard Biesheuvel]

On 18 May 2017 at 00:24, Greg Hackmann <ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> On 05/11/2017 06:51 AM, Ard Biesheuvel wrote:
> [snip]
>>>>>>
>>>>>> 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?
>>
>>
>> Yes. Up until now, we have been relying on the position independent
>> nature of small model code, but it would be better to specify it
>> explicitly, so if -fpie gives us mostly identical code and does not
>> need visibility hacks, I would prefer to add it for all compilers and
>> not have an exception only for Clang. Note that the same applies to
>> the entire kernel when built in KASLR mode, so it would also be good
>> to know our options here.
>>
>> Arnd, Will, what is the oldest GCC version we claim to support for arm64?
>>
>
> Unfortunately, after looking into this a bit more, -fpie by itself doesn't
> force clang to disable symbol preeemption.  For example when building the
> EFI stub from 4.9 with clang, -fpie gives me a stub that crashes with a
> synchronous exception inside handle_kernel_image().  The faulting
> instruction is a read from __nokaslr that still goes through the GOT.
>
> Right now you'll get a usable EFI stub with -fpie anyway, since 60f38de7a8d4
> ("efi/libstub: Unify command line param parsing") masked the problem when it
> moved __nokaslr behind a helper function.  But AIUI there's nothing really
> preventing a similar problem in the future.
>
> You *can* force clang to disable symbol preemption using "-fpie
> -mpie-copy-relocations".  That said, I don't know enough about EFI to say
> whether this is actually appropriate for building the EFI stub.

Thanks for digging into this. It is really quite unfortunate that it
is so difficult to force Clang (or GCC for that matter) to generate
relative references without the compiler assuming that you are
building a shared library. Perhaps we need a stronger version of
-fvisibility=hidden, i.e., one that applies to extern declarations as
well.

For the stub, we could simply replace all remaining extern symbol
references (if any) with accessor functions, such as the one I added
for __nokaslr (which is actually needed for x86 as well, for different
reasons). Let me look into this.

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

[Matthias Kaehlcke]

El Thu, May 18, 2017 at 08:41:26AM +0100 Ard Biesheuvel ha dit:

> On 18 May 2017 at 00:24, Greg Hackmann <ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> > On 05/11/2017 06:51 AM, Ard Biesheuvel wrote:
> > [snip]
> >>>>>>
> >>>>>> 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?
> >>
> >>
> >> Yes. Up until now, we have been relying on the position independent
> >> nature of small model code, but it would be better to specify it
> >> explicitly, so if -fpie gives us mostly identical code and does not
> >> need visibility hacks, I would prefer to add it for all compilers and
> >> not have an exception only for Clang. Note that the same applies to
> >> the entire kernel when built in KASLR mode, so it would also be good
> >> to know our options here.
> >>
> >> Arnd, Will, what is the oldest GCC version we claim to support for arm64?
> >>
> >
> > Unfortunately, after looking into this a bit more, -fpie by itself doesn't
> > force clang to disable symbol preeemption.  For example when building the
> > EFI stub from 4.9 with clang, -fpie gives me a stub that crashes with a
> > synchronous exception inside handle_kernel_image().  The faulting
> > instruction is a read from __nokaslr that still goes through the GOT.
> >
> > Right now you'll get a usable EFI stub with -fpie anyway, since 60f38de7a8d4
> > ("efi/libstub: Unify command line param parsing") masked the problem when it
> > moved __nokaslr behind a helper function.  But AIUI there's nothing really
> > preventing a similar problem in the future.
> >
> > You *can* force clang to disable symbol preemption using "-fpie
> > -mpie-copy-relocations".  That said, I don't know enough about EFI to say
> > whether this is actually appropriate for building the EFI stub.

Thanks for the investigation, Greg.

> Thanks for digging into this. It is really quite unfortunate that it
> is so difficult to force Clang (or GCC for that matter) to generate
> relative references without the compiler assuming that you are
> building a shared library. Perhaps we need a stronger version of
> -fvisibility=hidden, i.e., one that applies to extern declarations as
> well.
> 
> For the stub, we could simply replace all remaining extern symbol
> references (if any) with accessor functions, such as the one I added
> for __nokaslr (which is actually needed for x86 as well, for different
> reasons). Let me look into this.

I saw you sent out a bunch of patches, thanks, your help is greatly
appreciated.