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
@@ -272,6 +272,11 @@ else
 scripts/Kbuild.include: ;
 include scripts/Kbuild.include
 
+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)
@@ -786,11 +791,6 @@ ifdef CONFIG_DEBUG_SECTION_MISMATCH
 KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
-ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
-endif
-
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 CHECKFLAGS     += $(NOSTDINC_FLAGS)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -92,6 +92,7 @@ config ARM
 	select HAVE_UID16
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN
 	select IRQ_FORCED_THREADING
+	select LD_DEAD_CODE_DATA_ELIMINATION
 	select MODULES_USE_ELF_REL
 	select NO_BOOTMEM
 	select OF_EARLY_FLATTREE if OF
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -105,6 +105,7 @@ ifeq ($(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
@@ -31,7 +31,7 @@
 #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							\
@@ -42,11 +42,11 @@
 #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
@@ -119,7 +119,7 @@ SECTIONS
 		_stext = .;		/* Text and read-only data	*/
 			IDMAP_TEXT
 			__exception_text_start = .;
-			*(.exception.text)
+			KEEP(*(.exception.text))
 			__exception_text_end = .;
 			IRQENTRY_TEXT
 			SOFTIRQENTRY_TEXT
@@ -148,7 +148,7 @@ SECTIONS
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
 		__start___ex_table = .;
 #ifdef CONFIG_MMU
-		*(__ex_table)
+		KEEP(*(__ex_table))
 #endif
 		__stop___ex_table = .;
 	}
@@ -160,12 +160,12 @@ 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
@@ -186,13 +186,13 @@ SECTIONS
 	__vectors_lma = .;
 	OVERLAY 0xffff0000 : NOCROSSREFS AT(__vectors_lma) {
 		.vectors {
-			*(.vectors)
+			KEEP(*(.vectors))
 		}
 		.vectors.bhb.loop8 {
-			*(.vectors.bhb.loop8)
+			KEEP(*(.vectors.bhb.loop8))
 		}
 		.vectors.bhb.bpiall {
-			*(.vectors.bhb.bpiall)
+			KEEP(*(.vectors.bhb.bpiall))
 		}
 	}
 	ARM_LMA(__vectors, .vectors);
@@ -204,7 +204,7 @@ SECTIONS
 
 	__stubs_lma = .;
 	.stubs ADDR(.vectors) + 0x1000 : AT(__stubs_lma) {
-		*(.stubs)
+		KEEP(*(.stubs))
 	}
 	ARM_LMA(__stubs, .stubs);
 	. = __stubs_lma + SIZEOF(.stubs);
@@ -220,24 +220,24 @@ 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
@@ -40,6 +40,7 @@ config MIPS
 	select HAVE_CBPF_JIT if (!64BIT && !CPU_MICROMIPS)
 	select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
 	select HAVE_CC_STACKPROTECTOR
+	select LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_CONTEXT_TRACKING
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_C_RECORDMCOUNT
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -72,7 +72,7 @@ SECTIONS
 	/* Exception table for data bus errors */
 	__dbe_table : {
 		__start___dbe_table = .;
-		*(__dbe_table)
+		KEEP(*(__dbe_table))
 		__stop___dbe_table = .;
 	}
 
@@ -124,7 +124,7 @@ 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()
@@ -237,7 +237,8 @@
 	LIKELY_PROFILE()		       				\
 	BRANCH_PROFILE()						\
 	TRACE_PRINTKS()							\
-	TRACEPOINT_STR()
+	TRACEPOINT_STR()                                                \
+	*(.data.[a-zA-Z_]*)
 
 /*
  * Data section helpers
@@ -501,7 +502,7 @@
 #define ENTRY_TEXT							\
 		ALIGN_FUNCTION();					\
 		VMLINUX_SYMBOL(__entry_text_start) = .;			\
-		*(.entry.text)						\
+		KEEP(*(.entry.text))					\
 		VMLINUX_SYMBOL(__entry_text_end) = .;
 
 #define IRQENTRY_TEXT							\
@@ -608,7 +609,7 @@
 	. = ALIGN(sbss_align);						\
 	.sbss : AT(ADDR(.sbss) - LOAD_OFFSET) {				\
 		*(.dynsbss)						\
-		*(.sbss)						\
+		*(.sbss .sbss.*)					\
 		*(.scommon)						\
 	}
 
