[PATCH v4 2/2] powerpc/64: Option to use ELF V2 ABI for big-endian kernels

Nicholas Piggin
2021-06-11T09:41:26Z

Provide an option to build big-endian kernels using the ELFv2 ABI. This
works on GCC only so far, although it is rumored to work with clang
that's not been tested yet. A new module version check ensures the
module ELF ABI level matches the kernel build.

This can give big-endian kernels some useful advantages of the ELFv2 ABI
(e.g., less stack usage, -mprofile-kernel, better compatibility with eBPF
tools).

BE+ELFv2 is not officially supported by the GNU toolchain, but it works
fine in testing and has been used by some userspace for some time (e.g.,
Void Linux).

Tested-by: Michal Suchánek <msuchanek@suse.de>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
arch/powerpc/Kconfig | 22 ++++++++++++++++++++++
arch/powerpc/Makefile | 18 ++++++++++++------
arch/powerpc/boot/Makefile | 4 +++-
arch/powerpc/include/asm/module.h | 24 ++++++++++++++++++++++++
arch/powerpc/kernel/vdso64/Makefile | 13 +++++++++++++
drivers/crypto/vmx/Makefile | 8 ++++++--
drivers/crypto/vmx/ppc-xlate.pl | 10 ++++++----
7 files changed, 86 insertions(+), 13 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 088dd2afcfe4..093f973a28b9 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -163,6 +163,7 @@ config PPC
select ARCH_WEAK_RELEASE_ACQUIRE
select BINFMT_ELF
select BUILDTIME_TABLE_SORT
+ select PPC64_BUILD_ELF_V2_ABI if PPC64 && CPU_LITTLE_ENDIAN
select CLONE_BACKWARDS
select DCACHE_WORD_ACCESS if PPC64 && CPU_LITTLE_ENDIAN
select DMA_OPS_BYPASS if PPC64
@@ -561,6 +562,27 @@ config KEXEC_FILE
config ARCH_HAS_KEXEC_PURGATORY
def_bool KEXEC_FILE

+config PPC64_BUILD_ELF_V2_ABI
+ bool
+
+config PPC64_BUILD_BIG_ENDIAN_ELF_V2_ABI
+ bool "Build big-endian kernel using ELF ABI V2 (EXPERIMENTAL)"
+ depends on PPC64 && CPU_BIG_ENDIAN && EXPERT
+ depends on CC_IS_GCC && LD_VERSION >= 22400
+ default n
+ select PPC64_BUILD_ELF_V2_ABI
+ help
+ This builds the kernel image using the "Power Architecture 64-Bit ELF
+ V2 ABI Specification", which has a reduced stack overhead and faster
+ function calls. This internal kernel ABI option does not affect
+ userspace compatibility.
+
+ The V2 ABI is standard for 64-bit little-endian, but for big-endian
+ it is less well tested by kernel and toolchain. However some distros
+ build userspace this way, and it can produce a functioning kernel.
+
+ This requires GCC and binutils 2.24 or newer.
+
config RELOCATABLE
bool "Build a relocatable kernel"
depends on PPC64 || (FLATMEM && (44x || FSL_BOOKE))
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 3212d076ac6a..b90b5cb799aa 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -91,10 +91,14 @@ endif

ifdef CONFIG_PPC64
ifndef CONFIG_CC_IS_CLANG
-cflags-$(CONFIG_CPU_BIG_ENDIAN) += $(call cc-option,-mabi=elfv1)
-cflags-$(CONFIG_CPU_BIG_ENDIAN) += $(call cc-option,-mcall-aixdesc)
-aflags-$(CONFIG_CPU_BIG_ENDIAN) += $(call cc-option,-mabi=elfv1)
-aflags-$(CONFIG_CPU_LITTLE_ENDIAN) += -mabi=elfv2
+ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
+cflags-y += $(call cc-option,-mabi=elfv2)
+aflags-y += $(call cc-option,-mabi=elfv2)
+else
+cflags-y += $(call cc-option,-mabi=elfv1)
+cflags-y += $(call cc-option,-mcall-aixdesc)
+aflags-y += $(call cc-option,-mabi=elfv1)
+endif
endif
endif

@@ -142,15 +146,17 @@ endif

CFLAGS-$(CONFIG_PPC64) := $(call cc-option,-mtraceback=no)
ifndef CONFIG_CC_IS_CLANG
-ifdef CONFIG_CPU_LITTLE_ENDIAN
-CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mabi=elfv2,$(call cc-option,-mcall-aixdesc))
+ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
+CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mabi=elfv2)
AFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mabi=elfv2)
else
+# Keep these in synch with arch/powerpc/kernel/vdso64/Makefile
CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mabi=elfv1)
CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mcall-aixdesc)
AFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mabi=elfv1)
endif
endif
+
CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mcmodel=medium,$(call cc-option,-mminimal-toc))
CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mno-pointers-to-nested-functions)

diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index 2b8da923ceca..be84a72f8258 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -40,6 +40,9 @@ BOOTCFLAGS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \

ifdef CONFIG_PPC64_BOOT_WRAPPER
BOOTCFLAGS += -m64
+ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
+BOOTCFLAGS += $(call cc-option,-mabi=elfv2)
+endif
else
BOOTCFLAGS += -m32
endif
@@ -50,7 +53,6 @@ ifdef CONFIG_CPU_BIG_ENDIAN
BOOTCFLAGS += -mbig-endian
else
BOOTCFLAGS += -mlittle-endian
-BOOTCFLAGS += $(call cc-option,-mabi=elfv2)
endif

BOOTAFLAGS := -D__ASSEMBLY__ $(BOOTCFLAGS) -nostdinc
diff --git a/arch/powerpc/include/asm/module.h b/arch/powerpc/include/asm/module.h
index 857d9ff24295..043e11068ff4 100644
--- a/arch/powerpc/include/asm/module.h
+++ b/arch/powerpc/include/asm/module.h
@@ -52,6 +52,30 @@ struct mod_arch_specific {
unsigned int num_bugs;
};

+/*
+ * Check kernel module ELF header architecture specific compatibility.
+ */
+static inline bool elf_check_module_arch(Elf_Ehdr *hdr)
+{
+ if (!elf_check_arch(hdr))
+ return false;
+
+ if (IS_ENABLED(CONFIG_PPC64)) {
+ unsigned long abi_level = hdr->e_flags & 0x3;
+
+ if (IS_ENABLED(CONFIG_PPC64_BUILD_ELF_V2_ABI)) {
+ if (abi_level != 2)
+ return false;
+ } else {
+ if (abi_level >= 2)
+ return false;
+ }
+ }
+
+ return true;
+}
+#define elf_check_module_arch elf_check_module_arch
+
/*
* Select ELF headers.
* Make empty section for module_frob_arch_sections to expand.
diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
index 2813e3f98db6..d783c07e558f 100644
--- a/arch/powerpc/kernel/vdso64/Makefile
+++ b/arch/powerpc/kernel/vdso64/Makefile
@@ -25,6 +25,19 @@ KCOV_INSTRUMENT := n
UBSAN_SANITIZE := n
KASAN_SANITIZE := n

+# Always build vdso64 with ELFv1 ABI for BE kernels
+ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
+ifdef CONFIG_CPU_BIG_ENDIAN
+KBUILD_CFLAGS := $(filter-out -mabi=elfv2,$(KBUILD_CFLAGS))
+KBUILD_AFLAGS := $(filter-out -mabi=elfv2,$(KBUILD_AFLAGS))
+
+# These are derived from arch/powerpc/Makefile
+KBUILD_CFLAGS += $(call cc-option,-mabi=elfv1)
+KBUILD_CFLAGS += $(call cc-option,-mcall-aixdesc)
+KBUILD_AFLAGS += $(call cc-option,-mabi=elfv1)
+endif
+endif
+
ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
-Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
asflags-y := -D__VDSO64__ -s
diff --git a/drivers/crypto/vmx/Makefile b/drivers/crypto/vmx/Makefile
index 709670d2b553..d9ccf9fc3483 100644
--- a/drivers/crypto/vmx/Makefile
+++ b/drivers/crypto/vmx/Makefile
@@ -5,18 +5,22 @@ vmx-crypto-objs := vmx.o aesp8-ppc.o ghashp8-ppc.o aes.o aes_cbc.o aes_ctr.o aes
ifeq ($(CONFIG_CPU_LITTLE_ENDIAN),y)
override flavour := linux-ppc64le
else
+ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
+override flavour := linux-ppc64-elfv2
+else
override flavour := linux-ppc64
endif
+endif

quiet_cmd_perl = PERL $@
cmd_perl = $(PERL) $(<) $(flavour) > $(@)

targets += aesp8-ppc.S ghashp8-ppc.S

-$(obj)/aesp8-ppc.S: $(src)/aesp8-ppc.pl FORCE
+$(obj)/aesp8-ppc.S: $(src)/aesp8-ppc.pl $(src)/ppc-xlate.pl FORCE
$(call if_changed,perl)

-$(obj)/ghashp8-ppc.S: $(src)/ghashp8-ppc.pl FORCE
+$(obj)/ghashp8-ppc.S: $(src)/ghashp8-ppc.pl $(src)/ppc-xlate.pl FORCE
$(call if_changed,perl)

clean-files := aesp8-ppc.S ghashp8-ppc.S
diff --git a/drivers/crypto/vmx/ppc-xlate.pl b/drivers/crypto/vmx/ppc-xlate.pl
index 36db2ef09e5b..b583898c11ae 100644
--- a/drivers/crypto/vmx/ppc-xlate.pl
+++ b/drivers/crypto/vmx/ppc-xlate.pl
@@ -9,6 +9,8 @@ open STDOUT,">$output" || die "can't open $output: $!";

my %GLOBALS;
my $dotinlocallabels=($flavour=~/linux/)?1:0;
+my $elfv2abi=(($flavour =~ /linux-ppc64le/) or ($flavour =~ /linux-ppc64-elfv2/))?1:0;
+my $dotfunctions=($elfv2abi=~1)?0:1;

################################################################
# directives which need special treatment on different platforms
@@ -40,7 +42,7 @@ my $globl = sub {
};
my $text = sub {
my $ret = ($flavour =~ /aix/) ? ".csect\t.text[PR],7" : ".text";
- $ret = ".abiversion 2\n".$ret if ($flavour =~ /linux.*64le/);
+ $ret = ".abiversion 2\n".$ret if ($elfv2abi);
$ret;
};
my $machine = sub {
@@ -56,8 +58,8 @@ my $size = sub {
if ($flavour =~ /linux/)
{ shift;
my $name = shift; $name =~ s|^[\.\_]||;
- my $ret = ".size $name,.-".($flavour=~/64$/?".":"").$name;
- $ret .= "\n.size .$name,.-.$name" if ($flavour=~/64$/);
+ my $ret = ".size $name,.-".($dotfunctions?".":"").$name;
+ $ret .= "\n.size .$name,.-.$name" if ($dotfunctions);
$ret;
}
else
@@ -142,7 +144,7 @@ my $vmr = sub {

# Some ABIs specify vrsave, special-purpose register #256, as reserved
# for system use.
-my $no_vrsave = ($flavour =~ /linux-ppc64le/);
+my $no_vrsave = ($elfv2abi);
my $mtspr = sub {
my ($f,$idx,$ra) = @_;
if ($idx == 256 && $no_vrsave) {
--
2.23.0