From e3d8676f5722b7622685581e06e8f53e6138e3ab Mon Sep 17 00:00:00 2001
From: Felix Fietkau <nbd@nbd.name>
Date: Sat, 15 Jul 2017 23:42:36 +0200
Subject: use -ffunction-sections, -fdata-sections and --gc-sections

In combination with kernel symbol export stripping this significantly reduces
the kernel image size. Used on both ARM and MIPS architectures.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
---
 Makefile                          | 10 +++----
 arch/arm/Kconfig                  |  1 +
 arch/arm/boot/compressed/Makefile |  1 +
 arch/arm/kernel/vmlinux.lds.S     | 26 ++++++++--------
 arch/mips/Kconfig                 |  1 +
 arch/mips/kernel/vmlinux.lds.S    |  4 +--
 include/asm-generic/vmlinux.lds.h | 63 ++++++++++++++++++++-------------------
 7 files changed, 55 insertions(+), 51 deletions(-)

--- a/Makefile
+++ b/Makefile
@@ -406,6 +406,11 @@ KBUILD_CFLAGS_MODULE  := -DMODULE
 KBUILD_LDFLAGS_MODULE = -T $(srctree)/scripts/module-common.lds $(if $(CONFIG_PROFILING),,-s)
 GCC_PLUGINS_CFLAGS :=
 
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-ffunction-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-fdata-sections,)
+endif
+
 # Read KERNELRELEASE from include/config/kernel.release (if it exists)
 KERNELRELEASE = $(shell cat include/config/kernel.release 2> /dev/null)
 KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
@@ -638,11 +643,6 @@ KBUILD_CFLAGS	+= $(call cc-disable-warni
 KBUILD_CFLAGS	+= $(call cc-disable-warning, int-in-bool-context)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, attribute-alias)
 
-ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
-endif
-
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS	+= -Os $(call cc-disable-warning,maybe-uninitialized,) $(EXTRA_OPTIMIZATION)
 else
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -81,6 +81,7 @@ config ARM
 	select HAVE_UID16
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN
 	select IRQ_FORCED_THREADING
+	select LD_DEAD_CODE_DATA_ELIMINATION
 	select MODULES_USE_ELF_REL
 	select NO_BOOTMEM
 	select OF_EARLY_FLATTREE if OF
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -102,6 +102,7 @@ ifeq ($(CONFIG_FUNCTION_TRACER),y)
 ORIG_CFLAGS := $(KBUILD_CFLAGS)
 KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))
 endif
+KBUILD_CFLAGS_KERNEL := $(patsubst -f%-sections,,$(KBUILD_CFLAGS_KERNEL))
 
 # -fstack-protector-strong triggers protection checks in this code,
 # but it is being used too early to link to meaningful stack_chk logic.
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -17,7 +17,7 @@
 #define PROC_INFO							\
 	. = ALIGN(4);							\
 	VMLINUX_SYMBOL(__proc_info_begin) = .;				\
-	*(.proc.info.init)						\
+	KEEP(*(.proc.info.init))					\
 	VMLINUX_SYMBOL(__proc_info_end) = .;
 
 #define HYPERVISOR_TEXT							\
@@ -28,11 +28,11 @@
 #define IDMAP_TEXT							\
 	ALIGN_FUNCTION();						\
 	VMLINUX_SYMBOL(__idmap_text_start) = .;				\
-	*(.idmap.text)							\
+	KEEP(*(.idmap.text))						\
 	VMLINUX_SYMBOL(__idmap_text_end) = .;				\
 	. = ALIGN(PAGE_SIZE);						\
 	VMLINUX_SYMBOL(__hyp_idmap_text_start) = .;			\
-	*(.hyp.idmap.text)						\
+	KEEP(*(.hyp.idmap.text))					\
 	VMLINUX_SYMBOL(__hyp_idmap_text_end) = .;
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -105,7 +105,7 @@ SECTIONS
 		_stext = .;		/* Text and read-only data	*/
 			IDMAP_TEXT
 			__exception_text_start = .;
-			*(.exception.text)
+			KEEP(*(.exception.text))
 			__exception_text_end = .;
 			IRQENTRY_TEXT
 			SOFTIRQENTRY_TEXT
@@ -134,7 +134,7 @@ SECTIONS
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
 		__start___ex_table = .;
 #ifdef CONFIG_MMU
-		*(__ex_table)
+		KEEP(*(__ex_table))
 #endif
 		__stop___ex_table = .;
 	}
@@ -146,12 +146,12 @@ SECTIONS
 	. = ALIGN(8);
 	.ARM.unwind_idx : {
 		__start_unwind_idx = .;
-		*(.ARM.exidx*)
+		KEEP(*(.ARM.exidx*))
 		__stop_unwind_idx = .;
 	}
 	.ARM.unwind_tab : {
 		__start_unwind_tab = .;
-		*(.ARM.extab*)
+		KEEP(*(.ARM.extab*))
 		__stop_unwind_tab = .;
 	}
 #endif
@@ -171,14 +171,14 @@ SECTIONS
 	 */
 	__vectors_start = .;
 	.vectors 0xffff0000 : AT(__vectors_start) {
-		*(.vectors)
+		KEEP(*(.vectors))
 	}
 	. = __vectors_start + SIZEOF(.vectors);
 	__vectors_end = .;
 
 	__stubs_start = .;
 	.stubs ADDR(.vectors) + 0x1000 : AT(__stubs_start) {
-		*(.stubs)
+		KEEP(*(.stubs))
 	}
 	. = __stubs_start + SIZEOF(.stubs);
 	__stubs_end = .;
@@ -194,24 +194,24 @@ SECTIONS
 	}
 	.init.arch.info : {
 		__arch_info_begin = .;
-		*(.arch.info.init)
+		KEEP(*(.arch.info.init))
 		__arch_info_end = .;
 	}
 	.init.tagtable : {
 		__tagtable_begin = .;
-		*(.taglist.init)
+		KEEP(*(.taglist.init))
 		__tagtable_end = .;
 	}
 #ifdef CONFIG_SMP_ON_UP
 	.init.smpalt : {
 		__smpalt_begin = .;
-		*(.alt.smp.init)
+		KEEP(*(.alt.smp.init))
 		__smpalt_end = .;
 	}
 #endif
 	.init.pv_table : {
 		__pv_table_begin = .;
-		*(.pv_table)
+		KEEP(*(.pv_table))
 		__pv_table_end = .;
 	}
 	.init.data : {
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -55,6 +55,7 @@ config MIPS
 	select CLONE_BACKWARDS
 	select HAVE_DEBUG_STACKOVERFLOW
 	select HAVE_CC_STACKPROTECTOR
+	select LD_DEAD_CODE_DATA_ELIMINATION
 	select CPU_PM if CPU_IDLE
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_BINFMT_ELF_STATE
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -71,7 +71,7 @@ SECTIONS
 	/* Exception table for data bus errors */
 	__dbe_table : {
 		__start___dbe_table = .;
-		*(__dbe_table)
+		KEEP(*(__dbe_table))
 		__stop___dbe_table = .;
 	}
 
@@ -121,7 +121,7 @@ SECTIONS
 	. = ALIGN(4);
 	.mips.machines.init : AT(ADDR(.mips.machines.init) - LOAD_OFFSET) {
 		__mips_machines_start = .;
-		*(.mips.machines.init)
+		KEEP(*(.mips.machines.init))
 		__mips_machines_end = .;
 	}
 
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -105,7 +105,7 @@
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			VMLINUX_SYMBOL(__start_mcount_loc) = .; \
-			*(__mcount_loc)				\
+			KEEP(*(__mcount_loc))			\
 			VMLINUX_SYMBOL(__stop_mcount_loc) = .;
 #else
 #define MCOUNT_REC()
@@ -113,7 +113,7 @@
 
 #ifdef CONFIG_TRACE_BRANCH_PROFILING
 #define LIKELY_PROFILE()	VMLINUX_SYMBOL(__start_annotated_branch_profile) = .; \
-				*(_ftrace_annotated_branch)			      \
+				KEEP(*(_ftrace_annotated_branch))		      \
 				VMLINUX_SYMBOL(__stop_annotated_branch_profile) = .;
 #else
 #define LIKELY_PROFILE()
@@ -121,7 +121,7 @@
 
 #ifdef CONFIG_PROFILE_ALL_BRANCHES
 #define BRANCH_PROFILE()	VMLINUX_SYMBOL(__start_branch_profile) = .;   \
-				*(_ftrace_branch)			      \
+				KEEP(*(_ftrace_branch))			      \
 				VMLINUX_SYMBOL(__stop_branch_profile) = .;
 #else
 #define BRANCH_PROFILE()
@@ -130,7 +130,7 @@
 #ifdef CONFIG_KPROBES
 #define KPROBE_BLACKLIST()	. = ALIGN(8);				      \
 				VMLINUX_SYMBOL(__start_kprobe_blacklist) = .; \
-				*(_kprobe_blacklist)			      \
+				KEEP(*(_kprobe_blacklist))		      \
 				VMLINUX_SYMBOL(__stop_kprobe_blacklist) = .;
 #else
 #define KPROBE_BLACKLIST()
@@ -139,10 +139,10 @@
 #ifdef CONFIG_EVENT_TRACING
 #define FTRACE_EVENTS()	. = ALIGN(8);					\
 			VMLINUX_SYMBOL(__start_ftrace_events) = .;	\
-			*(_ftrace_events)				\
+			KEEP(*(_ftrace_events))				\
 			VMLINUX_SYMBOL(__stop_ftrace_events) = .;	\
 			VMLINUX_SYMBOL(__start_ftrace_enum_maps) = .;	\
-			*(_ftrace_enum_map)				\
+			KEEP(*(_ftrace_enum_map))			\
 			VMLINUX_SYMBOL(__stop_ftrace_enum_maps) = .;
 #else
 #define FTRACE_EVENTS()
@@ -163,7 +163,7 @@
 #ifdef CONFIG_FTRACE_SYSCALLS
 #define TRACE_SYSCALLS() . = ALIGN(8);					\
 			 VMLINUX_SYMBOL(__start_syscalls_metadata) = .;	\
-			 *(__syscalls_metadata)				\
+			 KEEP(*(__syscalls_metadata))			\
 			 VMLINUX_SYMBOL(__stop_syscalls_metadata) = .;
 #else
 #define TRACE_SYSCALLS()
@@ -172,7 +172,7 @@
 #ifdef CONFIG_SERIAL_EARLYCON
 #define EARLYCON_TABLE() . = ALIGN(8);				\
 			 VMLINUX_SYMBOL(__earlycon_table) = .;	\
-			 *(__earlycon_table)			\
+			 KEEP(*(__earlycon_table))		\
 			 VMLINUX_SYMBOL(__earlycon_table_end) = .;
 #else
 #define EARLYCON_TABLE()
@@ -185,8 +185,8 @@
 #define _OF_TABLE_1(name)						\
 	. = ALIGN(8);							\
 	VMLINUX_SYMBOL(__##name##_of_table) = .;			\
-	*(__##name##_of_table)						\
-	*(__##name##_of_table_end)
+	KEEP(*(__##name##_of_table))					\
+	KEEP(*(__##name##_of_table_end))
 
 #define CLKSRC_OF_TABLES()	OF_TABLE(CONFIG_CLKSRC_OF, clksrc)
 #define IRQCHIP_OF_MATCH_TABLE() OF_TABLE(CONFIG_IRQCHIP, irqchip)
@@ -209,7 +209,7 @@
 #define KERNEL_DTB()							\
 	STRUCT_ALIGN();							\
 	VMLINUX_SYMBOL(__dtb_start) = .;				\
-	*(.dtb.init.rodata)						\
+	KEEP(*(.dtb.init.rodata))					\
 	VMLINUX_SYMBOL(__dtb_end) = .;
 
 /*
@@ -227,16 +227,17 @@
 	/* implement dynamic printk debug */				\
 	. = ALIGN(8);                                                   \
 	VMLINUX_SYMBOL(__start___jump_table) = .;                       \
-	*(__jump_table)                                                 \
+	KEEP(*(__jump_table))                                           \
 	VMLINUX_SYMBOL(__stop___jump_table) = .;                        \
 	. = ALIGN(8);							\
 	VMLINUX_SYMBOL(__start___verbose) = .;                          \
-	*(__verbose)                                                    \
+	KEEP(*(__verbose))                                              \
 	VMLINUX_SYMBOL(__stop___verbose) = .;				\
 	LIKELY_PROFILE()		       				\
 	BRANCH_PROFILE()						\
 	TRACE_PRINTKS()							\
-	TRACEPOINT_STR()
+	TRACEPOINT_STR()                                                \
+	*(.data.[a-zA-Z_]*)
 
 /*
  * Data section helpers
@@ -304,35 +305,35 @@
 	/* PCI quirks */						\
 	.pci_fixup        : AT(ADDR(.pci_fixup) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start_pci_fixups_early) = .;		\
-		*(.pci_fixup_early)					\
+		KEEP(*(.pci_fixup_early))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_early) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_header) = .;		\
-		*(.pci_fixup_header)					\
+		KEEP(*(.pci_fixup_header))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_header) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_final) = .;		\
-		*(.pci_fixup_final)					\
+		KEEP(*(.pci_fixup_final))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_final) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_enable) = .;		\
-		*(.pci_fixup_enable)					\
+		KEEP(*(.pci_fixup_enable))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_enable) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_resume) = .;		\
-		*(.pci_fixup_resume)					\
+		KEEP(*(.pci_fixup_resume))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_resume) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_resume_early) = .;	\
-		*(.pci_fixup_resume_early)				\
+		KEEP(*(.pci_fixup_resume_early))			\
 		VMLINUX_SYMBOL(__end_pci_fixups_resume_early) = .;	\
 		VMLINUX_SYMBOL(__start_pci_fixups_suspend) = .;		\
-		*(.pci_fixup_suspend)					\
+		KEEP(*(.pci_fixup_suspend))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_suspend) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_suspend_late) = .;	\
-		*(.pci_fixup_suspend_late)				\
+		KEEP(*(.pci_fixup_suspend_late))			\
 		VMLINUX_SYMBOL(__end_pci_fixups_suspend_late) = .;	\
 	}								\
 									\
 	/* Built-in firmware blobs */					\
 	.builtin_fw        : AT(ADDR(.builtin_fw) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start_builtin_fw) = .;			\
-		*(.builtin_fw)						\
+		KEEP(*(.builtin_fw))					\
 		VMLINUX_SYMBOL(__end_builtin_fw) = .;			\
 	}								\
 									\
@@ -410,7 +411,7 @@
 									\
 	/* Kernel symbol table: strings */				\
         __ksymtab_strings : AT(ADDR(__ksymtab_strings) - LOAD_OFFSET) {	\
-		KEEP(*(__ksymtab_strings))				\
+		*(__ksymtab_strings)					\
 	}								\
 									\
 	/* __*init sections */						\
@@ -423,14 +424,14 @@
 	/* Built-in module parameters. */				\
 	__param : AT(ADDR(__param) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start___param) = .;			\
-		*(__param)						\
+		KEEP(*(__param))					\
 		VMLINUX_SYMBOL(__stop___param) = .;			\
 	}								\
 									\
 	/* Built-in module versions. */					\
 	__modver : AT(ADDR(__modver) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start___modver) = .;			\
-		*(__modver)						\
+		KEEP(*(__modver))					\
 		VMLINUX_SYMBOL(__stop___modver) = .;			\
 		. = ALIGN((align));					\
 		VMLINUX_SYMBOL(__end_rodata) = .;			\
@@ -496,7 +497,7 @@
 #define ENTRY_TEXT							\
 		ALIGN_FUNCTION();					\
 		VMLINUX_SYMBOL(__entry_text_start) = .;			\
-		*(.entry.text)						\
+		KEEP(*(.entry.text))					\
 		VMLINUX_SYMBOL(__entry_text_end) = .;
 
 #if defined(CONFIG_FUNCTION_GRAPH_TRACER) || defined(CONFIG_KASAN)
@@ -534,7 +535,7 @@
 	. = ALIGN(align);						\
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__start___ex_table) = .;			\
-		*(__ex_table)						\
+		KEEP(*(__ex_table))						\
 		VMLINUX_SYMBOL(__stop___ex_table) = .;			\
 	}
 
@@ -550,9 +551,9 @@
 #ifdef CONFIG_CONSTRUCTORS
 #define KERNEL_CTORS()	. = ALIGN(8);			   \
 			VMLINUX_SYMBOL(__ctors_start) = .; \
-			*(.ctors)			   \
+			KEEP(*(.ctors))			   \
 			*(SORT(.init_array.*))		   \
-			*(.init_array)			   \
+			KEEP(*(.init_array))		   \
 			VMLINUX_SYMBOL(__ctors_end) = .;
 #else
 #define KERNEL_CTORS()
@@ -609,7 +610,7 @@
 #define SBSS(sbss_align)						\
 	. = ALIGN(sbss_align);						\
 	.sbss : AT(ADDR(.sbss) - LOAD_OFFSET) {				\
-		*(.sbss)						\
+		*(.sbss .sbss.*)					\
 		*(.scommon)						\
 	}
 
@@ -676,7 +677,7 @@
 	. = ALIGN(8);							\
 	__bug_table : AT(ADDR(__bug_table) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__start___bug_table) = .;		\
-		*(__bug_table)						\
+		KEEP(*(__bug_table))					\
 		VMLINUX_SYMBOL(__stop___bug_table) = .;			\
 	}
 #else
@@ -688,7 +689,7 @@
 	. = ALIGN(4);							\
 	.tracedata : AT(ADDR(.tracedata) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__tracedata_start) = .;			\
-		*(.tracedata)						\
+		KEEP(*(.tracedata))					\
 		VMLINUX_SYMBOL(__tracedata_end) = .;			\
 	}
 #else
@@ -705,7 +706,7 @@
 #define INIT_SETUP(initsetup_align)					\
 		. = ALIGN(initsetup_align);				\
 		VMLINUX_SYMBOL(__setup_start) = .;			\
-		*(.init.setup)						\
+		KEEP(*(.init.setup))					\
 		VMLINUX_SYMBOL(__setup_end) = .;
 
 #define INIT_CALLS_LEVEL(level)						\
