build system: add PIE build option

Config.in

@@ -262,6 +262,14 @@ config STATIC
 
 	  Most people will leave this set to 'N'.
 
+config PIE
+	bool "Build BusyBox as a position independent executable"
+	default n
+	depends on !STATIC
+	help
+	  (TODO: what is it and why/when is it useful?)
+	  Most people will leave this set to 'N'.
+
 config NOMMU
 	bool "Force NOMMU build"
 	default n

Makefile

@@ -315,6 +315,8 @@ AFLAGS_KERNEL	=
 # Use LINUXINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
 CFLAGS		:= $(CFLAGS)
+# Added only to final link stage of busybox binary
+CFLAGS_busybox	:= $(CFLAGS_busybox)
 CPPFLAGS	:= $(CPPFLAGS)
 AFLAGS		:= $(AFLAGS)
 LDFLAGS		:= $(LDFLAGS)
@@ -580,7 +582,7 @@ quiet_cmd_busybox__ ?= LINK    $@
       cmd_busybox__ ?= $(srctree)/scripts/trylink \
       "$@" \
       "$(CC)" \
-      "$(CFLAGS)" \
+      "$(CFLAGS) $(CFLAGS_busybox)" \
       "$(LDFLAGS) $(EXTRA_LDFLAGS)" \
       "$(core-y)" \
       "$(libs-y)" \

Makefile.flags

@@ -50,17 +50,31 @@ ifeq ($(CONFIG_DEBUG),y)
 CFLAGS += $(call cc-option,-g)
 endif
 
+# If arch/$(ARCH)/Makefile did not override it (with, say, -fPIC)...
+ARCH_FPIC ?= -fpic
+ARCH_FPIE ?= -fpie
+ARCH_PIE ?= -pie
+
 ifeq ($(CONFIG_BUILD_LIBBUSYBOX),y)
 # on i386: 14% smaller libbusybox.so
 # (code itself is 9% bigger, we save on relocs/PLT/GOT)
-CFLAGS += -fpic
+CFLAGS += $(ARCH_FPIC)
 # and another 4% reduction of libbusybox.so:
 # (external entry points must be marked EXTERNALLY_VISIBLE)
 CFLAGS += $(call cc-option,-fvisibility=hidden)
 endif
 
 ifeq ($(CONFIG_STATIC),y)
-LDFLAGS += -static
+CFLAGS_busybox += -static
+endif
+
+ifeq ($(CONFIG_PIE),y)
+CFLAGS_busybox += $(ARCH_PIE)
+CFLAGS += $(ARCH_FPIE)
+# No switch() jump tables. Code growth +1k, binary size down -12k
+# due to reduced number of code pointers.
+# (TODO: make overridable: some arches may want to not do this)
+CFLAGS += $(call cc-option,-fno-jump-tables)
 endif
 
 LDLIBS += m crypt
@@ -81,8 +95,6 @@ ifeq ($(CONFIG_DMALLOC),y)
 LDLIBS += dmalloc
 endif
 
-#LDFLAGS += -nostdlib
-
 LDFLAGS_ELF2FLT = -Wl,-elf2flt
 ifneq (,$(findstring $(LDFLAGS_ELF2FLT),$(LDFLAGS)))
 SKIP_STRIP = y

scripts/Makefile.IMA

@@ -180,7 +180,7 @@ busybox_unstripped.o: $(usage_stuff) include/applet_tables.h include/autoconf.h
 busybox: busybox_unstripped.o
 	$(srctree)/scripts/trylink \
 		busybox_unstripped \
-		"$(CC)" \
+		"$(CC) $(CFLAGS_busybox)" \
 		"$(CFLAGS)" \
 		"$(LDFLAGS)" \
 		"busybox_unstripped.o" \