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

From: Nicholas Piggin <[email-masked]>
Date: Fri, 11 Jun 2021 19:39:59 +1000

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 <[email-masked]>
Reviewed-by: Segher Boessenkool <[email-masked]>
Signed-off-by: Nicholas Piggin <[email-masked]>
---
 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