makefile: enable garbage collection of code and data when hiding symbols

This patch changes the main makefile to enable garbage collection of
code and data when linking kernel with most symbols hidden.

In essence it makes all object files compile with '-ffunction-sections
-fdata-sections' flags only when conf_hide_symbols is ON. This allows
linker to eliminate code and data (with --gc-sections) that is not
really used between various compilation units. As the result it allows
us to produce even smaller kernel ELF at around 4.3M, down from 5.1M
when we would hide symbols only. In order to see what exact code and
data gets eliminated, one can pass --print-gc-sections flags to the
linker like so:

./scripts/build image=native-example fs=rofs conf_hide_symbols=1 conf_linker_extra_options=--print-gc-sections

In addition this patch also changes the makefile to make linker use
version script generated by ./scripts/generate_version_script.sh which
is obnly enabled when conf_hide_symbols is ON. Using version script
(--version-script) plays some role in futher reducing number of symbols
down to what exactly we want to export in exported/<arch>/*symbols.

Signed-off-by: Waldemar Kozaczuk <jwkozaczuk@gmail.com>

Makefile

@@ -284,7 +284,7 @@ $(out)/libc/%.o: cc-hide-flags =
 $(out)/libc/%.o: cxx-hide-flags =
 $(out)/musl/%.o: cc-hide-flags =
 
-kernel-defines = -D_KERNEL $(source-dialects) $(cc-hide-flags)
+kernel-defines = -D_KERNEL $(source-dialects) $(cc-hide-flags) $(gc-flags)
 
 # This play the same role as "_KERNEL", but _KERNEL unfortunately is too
 # overloaded. A lot of files will expect it to be set no matter what, specially
@@ -323,6 +323,10 @@ cxx-hide-flags-0 =
 cxx-hide-flags-1 = -fvisibility-inlines-hidden
 cxx-hide-flags = $(cxx-hide-flags-$(conf_hide_symbols))
 
+gc-flags-0 =
+gc-flags-1 = -ffunction-sections -fdata-sections
+gc-flags = $(gc-flags-$(conf_hide_symbols))
+
 gcc-opt-Og := $(call compiler-flag, -Og, -Og, compiler/empty.cc)
 
 CXXFLAGS = -std=gnu++11 $(COMMON) $(cxx-hide-flags)
@@ -1944,7 +1948,7 @@ $(out)/dummy-shlib.so: $(out)/dummy-shlib.o
 	$(call quiet, $(CXX) -nodefaultlibs -shared $(gcc-sysroot) -o $@ $^, LINK $@)
 
 stage1_targets = $(out)/arch/$(arch)/boot.o $(out)/loader.o $(out)/runtime.o $(drivers:%=$(out)/%) $(objects:%=$(out)/%) $(out)/dummy-shlib.so
-stage1: $(stage1_targets) links
+stage1: $(stage1_targets) links $(out)/version_script
 .PHONY: stage1
 
 loader_options_dep = $(out)/arch/$(arch)/loader_options.ld
@@ -1955,17 +1959,21 @@ $(loader_options_dep): stage1
 	fi
 
 ifeq ($(conf_hide_symbols),1)
-linker_archives_options = --no-whole-archive $(libstdc++.a) $(libgcc.a) $(libgcc_eh.a) $(boost-libs) --exclude-libs libstdc++.a
+linker_archives_options = --no-whole-archive $(libstdc++.a) $(libgcc.a) $(libgcc_eh.a) $(boost-libs) \
+  --exclude-libs libstdc++.a --gc-sections --version-script=$(out)/version_script
 else
 linker_archives_options = --whole-archive $(libstdc++.a) $(libgcc_eh.a) $(boost-libs) --no-whole-archive $(libgcc.a)
 endif
 
+$(out)/version_script: exported_symbols/$(arch)/*.symbols
+	$(call quiet, scripts/generate_version_script.sh $(out)/version_script, GEN version_script)
+
 $(out)/loader.elf: $(stage1_targets) arch/$(arch)/loader.ld $(out)/bootfs.o $(loader_options_dep)
 	$(call quiet, $(LD) -o $@ --defsym=OSV_KERNEL_BASE=$(kernel_base) \
 	    --defsym=OSV_KERNEL_VM_BASE=$(kernel_vm_base) --defsym=OSV_KERNEL_VM_SHIFT=$(kernel_vm_shift) \
 		-Bdynamic --export-dynamic --eh-frame-hdr --enable-new-dtags -L$(out)/arch/$(arch) \
 	    $(^:%.ld=-T %.ld) \
-	    $(linker_archives_options), \
+	    $(linker_archives_options) $(conf_linker_extra_options), \
 		LINK loader.elf)
 	@# Build libosv.so matching this loader.elf. This is not a separate
 	@# rule because that caused bug #545.
@@ -1978,10 +1986,9 @@ $(out)/kernel.elf: $(stage1_targets) arch/$(arch)/loader.ld $(out)/empty_bootfs.
 	    --defsym=OSV_KERNEL_VM_BASE=$(kernel_vm_base) --defsym=OSV_KERNEL_VM_SHIFT=$(kernel_vm_shift) \
 		-Bdynamic --export-dynamic --eh-frame-hdr --enable-new-dtags -L$(out)/arch/$(arch) \
 	    $(^:%.ld=-T %.ld) \
-	    $(linker_archives_options), \
+	    $(linker_archives_options) $(conf_linker_extra_options), \
 		LINK kernel.elf)
 	$(call quiet, $(STRIP) $(out)/kernel.elf -o $(out)/kernel-stripped.elf, STRIP kernel.elf -> kernel-stripped.elf )
-	$(call very-quiet, cp $(out)/kernel-stripped.elf $(out)/kernel.elf)
 
 $(out)/bsd/%.o: COMMON += -DSMP -D'__FBSDID(__str__)=extern int __bogus__'
 

conf/base.mk

@@ -12,3 +12,4 @@ conf-DEBUG_BUILD=0
 
 conf-debug_elf=0
 conf_hide_symbols=0
+conf_linker_extra_options=