Add support for GNU gold

These patches add macros for detecting the linker name and version, and apply fixes and workarounds needed to link the arm64 kernel with GNU gold.

Sami Tolvanen (7):
kbuild: add ld-name macro and support for GNU gold
kbuild: add ld-if-name-version and linker-specific macros
kbuild: fix LD_DEAD_CODE_DATA_ELIMINATION with GNU gold
arm64: fix -m for GNU gold
arm64: keep .altinstructions and .altinstr_replacement
arm64: explicitly pass --no-fix-cortex-a53-843419 to GNU gold
arm64: add a workaround for GNU gold with ARM64_MODULE_PLTS

 Makefile                          |  5 +++++
 arch/arm64/Makefile               | 17 +++++++++++++++++
 arch/arm64/kernel/module.lds      |  4 ++--
 arch/arm64/kernel/vmlinux.lds.S   |  4 ++--
 include/asm-generic/vmlinux.lds.h |  8 ++++----
 scripts/Kbuild.include            | 16 ++++++++++++++++
 6 files changed, 46 insertions(+), 8 deletions(-)

2.15.0.531.g2ccb3012c9-goog

GNU gold may require different flags than GNU ld. Add a macro for detecting the linker and conditionally add gold specific flags from LDFLAGS_GOLD.

---
 Makefile               | 5 +++++
 scripts/Kbuild.include | 4 ++++
 2 files changed, 9 insertions(+)

index f761bf475ba5..a16c98d6e3ba 100644
@@ -814,6 +814,11 @@ include scripts/Makefile.kasan
 include scripts/Makefile.extrawarn
 include scripts/Makefile.ubsan
 
+# Add any flags specific to ld.gold
+ifeq ($(ld-name),gold)
+LDFLAGS		+= $(LDFLAGS_GOLD)
+endif
+
 # Add any arch overrides and user supplied CPPFLAGS, AFLAGS and CFLAGS as the
 # last assignments
 KBUILD_CPPFLAGS += $(ARCH_CPPFLAGS) $(KCPPFLAGS)

index b6d7d347b203..a7c7843c2cf1 100644
@@ -277,6 +277,10 @@ ld-option = $(call try-run-cached,\
 # Important: no spaces around options
 ar-option = $(call try-run-cached, $(AR) rc$(1) "$$TMP",$(1),$(2))
 
+# ld-name
+# Expands to either bfd or gold
+ld-name = $(shell $(LD) -v 2>&1 | grep -q "GNU gold" && echo gold || echo bfd)
+
 # ld-version
 # Note this is mainly for HJ Lu's 3 number binutil versions
 ld-version = $(call shell-cached,$(LD) --version | $(srctree)/scripts/ld-version.sh)

2.15.0.531.g2ccb3012c9-goog

Right, but you're still only ever using one linker per build, correct?
Can we get away without 2 distinct flags?

Correct. LDFLAGS_GOLD makes it move convenient to add gold specific flags without explicit $(ld-name) checks everywhere, but I'm fine with removing it in v2.

Sami

Add macros for testing both linker name and version.

---
 scripts/Kbuild.include | 12 ++++++++++++
 1 file changed, 12 insertions(+)

index a7c7843c2cf1..5d286d69e8dc 100644
@@ -289,6 +289,18 @@ ld-version = $(call shell-cached,$(LD) --version | $(srctree)/scripts/ld-version
 # Usage:  $(call ld-ifversion, -ge, 22252, y)
 ld-ifversion = $(shell [ $(ld-version) $(1) $(2) ] && echo $(3) || echo $(4))
 
+# ld-if-name-version
+# Usage:  $(call ld-if-name-version, gold, -ge, 112000000, y)
+ld-if-name-version = $(shell [ $(ld-name) == $(1) ] && [ $(ld-version) $(2) $(3) ] && echo $(4) || echo $(5))
+
+# bfd-ifversion
+# Usage:  $(call bfd-ifversion, -ge, 227000000, y)
+bfd-ifversion = $(call ld-if-name-version, bfd, $(1), $(2), $(3), $(4))
+
+# gold-ifversion
+# Usage:  $(call gold-ifversion, -ge, 112000000, y)
+gold-ifversion = $(call ld-if-name-version, gold, $(1), $(2), $(3), $(4))
+
 ######
 
 ###

2.15.0.531.g2ccb3012c9-goog

Don't remove .head.text or .exitcall.exit when linking with --gc-sections, and include .init.text.* in .init.text and .init.rodata.* in .init.rodata.

---
 include/asm-generic/vmlinux.lds.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index ee8b707d9fa9..eb4199c12bd2 100644
@@ -515,7 +515,7 @@
 		VMLINUX_SYMBOL(__softirqentry_text_end) = .;
 
 /* Section used for early init (in .S files) */
-#define HEAD_TEXT  *(.head.text)
+#define HEAD_TEXT  KEEP(*(.head.text))
 
 #define HEAD_TEXT_SECTION							\
 	.head.text : AT(ADDR(.head.text) - LOAD_OFFSET) {		\

@@ -560,7 +560,7 @@
 	MEM_DISCARD(init.data)						\
 	KERNEL_CTORS()							\
 	MCOUNT_REC()							\
-	*(.init.rodata)							\
+	*(.init.rodata .init.rodata.*)					\
 	FTRACE_EVENTS()							\
 	TRACE_SYSCALLS()						\
 	KPROBE_BLACKLIST()						\

@@ -579,7 +579,7 @@
 	EARLYCON_TABLE()
 
 #define INIT_TEXT							\
-	*(.init.text)							\
+	*(.init.text .init.text.*)					\
 	*(.text.startup)						\
 	MEM_DISCARD(init.text)
 

@@ -596,7 +596,7 @@
 	MEM_DISCARD(exit.text)
 
 #define EXIT_CALL							\
-	*(.exitcall.exit)
+	KEEP(*(.exitcall.exit))
 
 /*
  * bss (Block Started by Symbol) - uninitialized data

2.15.0.531.g2ccb3012c9-goog

Fine by me, if you consider my other comments.

GNU gold supports different emulations than bfd. Use aarch64_elf64_*_vec instead of aarch64linux.

---
 arch/arm64/Makefile | 8 ++++++++
 1 file changed, 8 insertions(+)

index b35788c909f1..82a0df6e865e 100644
@@ -64,14 +64,22 @@ KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
 AS		+= -EB
 LD		+= -EB
+ifeq ($(ld-name),gold)
+LDFLAGS		+= -maarch64_elf64_be_vec
+else
 LDFLAGS		+= -maarch64linuxb
+endif
 UTS_MACHINE	:= aarch64_be
 else
 KBUILD_CPPFLAGS	+= -mlittle-endian
 CHECKFLAGS	+= -D__AARCH64EL__
 AS		+= -EL
 LD		+= -EL
+ifeq ($(ld-name),gold)
+LDFLAGS		+= -maarch64_elf64_le_vec
+else
 LDFLAGS		+= -maarch64linux
+endif
 UTS_MACHINE	:= aarch64
 endif
 

2.15.0.531.g2ccb3012c9-goog

Make sure the linker doesn't remove .altinstructions or .altinstr_replacement when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled.

---
 arch/arm64/kernel/vmlinux.lds.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 7da3e5c366a0..15479995869c 100644
@@ -138,11 +138,11 @@ SECTIONS
 	. = ALIGN(4);
 	.altinstructions : {
 		__alt_instructions = .;
-		*(.altinstructions)
+		KEEP(*(.altinstructions))
 		__alt_instructions_end = .;
 	}
 	.altinstr_replacement : {
-		*(.altinstr_replacement)
+		KEEP(*(.altinstr_replacement))
 	}
 
 	. = ALIGN(PAGE_SIZE);

2.15.0.531.g2ccb3012c9-goog

This sounds like a bug in the original implementation of CONFIG_LD_DEAD_CODE_DATA_ELIMINATION? If so, this can likely get merged regardless of the rest of the patchset, ie. gold support.

Yes, also patch 3/7 probably (thanks for doing these, btw). The LD_DCDE option is actually been broken upstream thanks to lack of time (which is a known issue, this is why no arch enables it).

Basically it just needs a bit more effort to go through and ensure nothing is discarded incorrectly, and all new sections are placed into output sections, before each arch is enabled.

(Comparing readelf -S before/after is a way to spot bugs.)

So yes please if it's not too much trouble, could you remove the "gold" name from the generic patch and put it at the front of the series with this arm64 patch. Possibly then you could also do a 3rd patch to allow arm64 to select it if it's working with gcc?

Thanks,
Nick

Do you have a script that can be used to diff 2 readelf -S, look for missing references, and report an error? Might be easier to rerun that every so often and see what's missing.

I don't, I was just doing it by hand.

Thanks,
Nick

Sure, I'll do this in v2.

These patches only fix the issues I ran into with clang and gold when testing on a single device. I feel like more testing would be needed before enabling this by default for arm64.

Sami

Some versions of GNU gold are known to produce broken code with --fix-cortex-a53-843419 as explained in this bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=21491

If ARM64_ERRATUM_843419 is disabled and we're using GNU gold, pass --no-fix-cortex-a53-843419 to the linker to ensure the erratum fix is not used even if the linker is configured to enable it by default.

This change also adds a warning if the erratum fix is enabled and gold version <1.14 is used.

---
 arch/arm64/Makefile | 9 +++++++++
 1 file changed, 9 insertions(+)

index 82a0df6e865e..81a12713f7af 100644
@@ -26,10 +26,19 @@ ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
   ifeq ($(call ld-option, --fix-cortex-a53-843419),)
 $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)
   else
+    ifeq ($(call gold-ifversion, -lt, 114000000, y), y)
+$(warning This version of GNU gold may generate incorrect code with --fix-cortex-a53-843419;\
+	see https://sourceware.org/bugzilla/show_bug.cgi?id=21491)
+    endif
 LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
   endif
 endif
 
+ifeq ($(CONFIG_ARM64_ERRATUM_843419),)
+# https://sourceware.org/bugzilla/show_bug.cgi?id=21491
+LDFLAGS_GOLD	+= --no-fix-cortex-a53-843419
+endif
+
 KBUILD_DEFCONFIG := defconfig
 
 # Check for binutils support for specific extensions

2.15.0.531.g2ccb3012c9-goog

Sure. I'll clean this up in v2.

Sami

All current versions of GNU gold crash when linking kernel modules with ARM64_MODULE_PLTS due to a known bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=14592

To work around the problem, this change removes NOLOAD from .plt and .init.plt.

---
 arch/arm64/kernel/module.lds | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index f7c9781a9d48..eacb5c67f61e 100644
@@ -1,4 +1,4 @@
 SECTIONS {
-	.plt (NOLOAD) : { BYTE(0) }
-	.init.plt (NOLOAD) : { BYTE(0) }
+	.plt : { BYTE(0) }
+	.init.plt : { BYTE(0) }
 }

2.15.0.531.g2ccb3012c9-goog

These patches add macros for detecting the linker name and version, and apply fixes and workarounds needed to link the arm64 kernel with GNU gold.

Changes since v1:
- moved LD_DEAD_CODE_DATA_ELIMINATION fixes to the beginning of the patch set and removed mentions of gold
- renamed ld-if-name-version to __ld-ifversion for consistency
- cleaned up the erratum changes in arch/arm64/Makefile

Sami Tolvanen (7):
kbuild: fix LD_DEAD_CODE_DATA_ELIMINATION
arm64: keep .altinstructions and .altinstr_replacement
kbuild: add ld-name macro
kbuild: add __ld-ifversion and linker-specific macros
arm64: fix -m for GNU gold
arm64: explicitly pass --no-fix-cortex-a53-843419 to GNU gold
arm64: add a workaround for GNU gold with ARM64_MODULE_PLTS

 arch/arm64/Makefile               | 17 +++++++++++++++++
 arch/arm64/kernel/module.lds      |  4 ++--
 arch/arm64/kernel/vmlinux.lds.S   |  4 ++--
 include/asm-generic/vmlinux.lds.h |  8 ++++----
 scripts/Kbuild.include            | 16 ++++++++++++++++
 5 files changed, 41 insertions(+), 8 deletions(-)

2.15.0.531.g2ccb3012c9-goog

Don't remove .head.text or .exitcall.exit when linking with --gc-sections, and include .init.text.* in .init.text and .init.rodata.* in .init.rodata.

---
 include/asm-generic/vmlinux.lds.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index ee8b707d9fa9..eb4199c12bd2 100644
@@ -515,7 +515,7 @@
 		VMLINUX_SYMBOL(__softirqentry_text_end) = .;
 
 /* Section used for early init (in .S files) */
-#define HEAD_TEXT  *(.head.text)
+#define HEAD_TEXT  KEEP(*(.head.text))
 
 #define HEAD_TEXT_SECTION							\
 	.head.text : AT(ADDR(.head.text) - LOAD_OFFSET) {		\

@@ -560,7 +560,7 @@
 	MEM_DISCARD(init.data)						\
 	KERNEL_CTORS()							\
 	MCOUNT_REC()							\
-	*(.init.rodata)							\
+	*(.init.rodata .init.rodata.*)					\
 	FTRACE_EVENTS()							\
 	TRACE_SYSCALLS()						\
 	KPROBE_BLACKLIST()						\

@@ -579,7 +579,7 @@
 	EARLYCON_TABLE()
 
 #define INIT_TEXT							\
-	*(.init.text)							\
+	*(.init.text .init.text.*)					\
 	*(.text.startup)						\
 	MEM_DISCARD(init.text)
 

@@ -596,7 +596,7 @@
 	MEM_DISCARD(exit.text)
 
 #define EXIT_CALL							\
-	*(.exitcall.exit)
+	KEEP(*(.exitcall.exit))
 
 /*
  * bss (Block Started by Symbol) - uninitialized data

2.15.0.531.g2ccb3012c9-goog

Make sure the linker doesn't remove .altinstructions or .altinstr_replacement when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled.

---
 arch/arm64/kernel/vmlinux.lds.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 7da3e5c366a0..15479995869c 100644
@@ -138,11 +138,11 @@ SECTIONS
 	. = ALIGN(4);
 	.altinstructions : {
 		__alt_instructions = .;
-		*(.altinstructions)
+		KEEP(*(.altinstructions))
 		__alt_instructions_end = .;
 	}
 	.altinstr_replacement : {
-		*(.altinstr_replacement)
+		KEEP(*(.altinstr_replacement))
 	}
 
 	. = ALIGN(PAGE_SIZE);

2.15.0.531.g2ccb3012c9-goog

GNU gold may require different flags than GNU ld. Add a macro for detecting the linker.

---
 scripts/Kbuild.include | 4 ++++
 1 file changed, 4 insertions(+)

index eb2cf2780f6e..899863e4cd05 100644
@@ -273,6 +273,10 @@ ld-option = $(call try-run-cached,\
 # Important: no spaces around options
 ar-option = $(call try-run-cached, $(AR) rc$(1) "$$TMP",$(1),$(2))
 
+# ld-name
+# Expands to either bfd or gold
+ld-name = $(shell $(LD) -v 2>&1 | grep -q "GNU gold" && echo gold || echo bfd)
+
 # ld-version
 # Note this is mainly for HJ Lu's 3 number binutil versions
 ld-version = $(call shell-cached,$(LD) --version | $(srctree)/scripts/ld-version.sh)

2.15.0.531.g2ccb3012c9-goog

Add macros for testing both linker name and version.

---
 scripts/Kbuild.include | 12 ++++++++++++
 1 file changed, 12 insertions(+)

index 899863e4cd05..2991b463b4ce 100644
@@ -285,6 +285,18 @@ ld-version = $(call shell-cached,$(LD) --version | $(srctree)/scripts/ld-version
 # Usage:  $(call ld-ifversion, -ge, 22252, y)
 ld-ifversion = $(shell [ $(ld-version) $(1) $(2) ] && echo $(3) || echo $(4))
 
+# __ld-ifversion
+# Usage:  $(call __ld-ifversion, gold, -ge, 112000000, y)
+__ld-ifversion = $(shell [ $(ld-name) == $(1) ] && [ $(ld-version) $(2) $(3) ] && echo $(4) || echo $(5))
+
+# bfd-ifversion
+# Usage:  $(call bfd-ifversion, -ge, 227000000, y)
+bfd-ifversion = $(call __ld-ifversion, bfd, $(1), $(2), $(3), $(4))
+
+# gold-ifversion
+# Usage:  $(call gold-ifversion, -ge, 112000000, y)
+gold-ifversion = $(call __ld-ifversion, gold, $(1), $(2), $(3), $(4))
+
 ######
 
 ###

2.15.0.531.g2ccb3012c9-goog

GNU gold supports different emulations than bfd. Use aarch64_elf64_*_vec instead of aarch64linux.

---
 arch/arm64/Makefile | 8 ++++++++
 1 file changed, 8 insertions(+)

index b35788c909f1..82a0df6e865e 100644
@@ -64,14 +64,22 @@ KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
 AS		+= -EB
 LD		+= -EB
+ifeq ($(ld-name),gold)
+LDFLAGS		+= -maarch64_elf64_be_vec
+else
 LDFLAGS		+= -maarch64linuxb
+endif
 UTS_MACHINE	:= aarch64_be
 else
 KBUILD_CPPFLAGS	+= -mlittle-endian
 CHECKFLAGS	+= -D__AARCH64EL__
 AS		+= -EL
 LD		+= -EL
+ifeq ($(ld-name),gold)
+LDFLAGS		+= -maarch64_elf64_le_vec
+else
 LDFLAGS		+= -maarch64linux
+endif
 UTS_MACHINE	:= aarch64
 endif
 

2.15.0.531.g2ccb3012c9-goog

Some versions of GNU gold are known to produce broken code with --fix-cortex-a53-843419 as explained in this bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=21491

If ARM64_ERRATUM_843419 is disabled and we're using GNU gold, pass --no-fix-cortex-a53-843419 to the linker to ensure the erratum fix is not used even if the linker is configured to enable it by default.

This change also adds a warning if the erratum fix is enabled and gold version <1.14 is used.

---
 arch/arm64/Makefile | 9 +++++++++
 1 file changed, 9 insertions(+)

index 82a0df6e865e..68eed687e468 100644
@@ -26,8 +26,17 @@ ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
   ifeq ($(call ld-option, --fix-cortex-a53-843419),)
 $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)
   else
+    ifeq ($(call gold-ifversion, -lt, 114000000, y), y)
+$(warning This version of GNU gold may generate incorrect code with --fix-cortex-a53-843419;\
+	see https://sourceware.org/bugzilla/show_bug.cgi?id=21491)
+    endif
 LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
   endif
+else
+  ifeq ($(ld-name),gold)
+# Pass --no-fix-cortex-a53-843419 to ensure the erratum fix is disabled
+LDFLAGS	+= --no-fix-cortex-a53-843419
+  endif
 endif
 
 KBUILD_DEFCONFIG := defconfig

2.15.0.531.g2ccb3012c9-goog

All current versions of GNU gold crash when linking kernel modules with ARM64_MODULE_PLTS due to a known bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=14592

To work around the problem, this change removes NOLOAD from .plt and .init.plt.

---
 arch/arm64/kernel/module.lds | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index f7c9781a9d48..eacb5c67f61e 100644
@@ -1,4 +1,4 @@
 SECTIONS {
-	.plt (NOLOAD) : { BYTE(0) }
-	.init.plt (NOLOAD) : { BYTE(0) }
+	.plt : { BYTE(0) }
+	.init.plt : { BYTE(0) }
 }

2.15.0.531.g2ccb3012c9-goog