[PATCH v1] build system: add support for building static pie busybox

[Charles Mirabile]

Static PIE binaries are basically what they say on the tin, a position
independant executible that is also statically linked. These binaries are
noteworthy because they can be loaded by the ELF fdpic binfmt loader from
the Linux Kernel on No-MMU systems, but are truly standalone because they
do not require a dynamic linker to function. (They also load just fine via
the regular ELF binfmt on MMU Linux too, so they are maximally portable).

It would be nice if simply passing both `-static` and `-pie` to gcc at the
same time worked to create such a binary, but as it stands whichever is
passed later is the one which actually takes affect and instead the
dedicated option `-static-pie` is needed. This unfortunate complexity is
reflected in the conditions needed in Makefile.flags, but to the user the
two options are now just presented orthogonally and can be selected
independantly removing the previous `depends on !STATIC` for `config PIE`.

Signed-off-by: Charles Mirabile <cmirabil at redhat.com>
---
 Config.in      |  1 -
 Makefile.flags | 10 +++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Config.in b/Config.in
index 6b925b517..8955ed37a 100644
--- a/Config.in
+++ b/Config.in
@@ -390,7 +390,6 @@ config STATIC
 config PIE
 	bool "Build position independent executable"
 	default n
-	depends on !STATIC
 	help
 	Hardened code option. PIE binaries are loaded at a different
 	address at each invocation. This has some overhead,
diff --git a/Makefile.flags b/Makefile.flags
index 97cb4dca2..64b2b2bcf 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -108,6 +108,7 @@ endif
 ARCH_FPIC ?= -fpic
 ARCH_FPIE ?= -fpie
 ARCH_PIE ?= -pie
+ARCH_SPIE ?= -static-pie
 
 # Usage: $(eval $(call pkg_check_modules,VARIABLE-PREFIX,MODULES))
 define pkg_check_modules
@@ -125,13 +126,20 @@ CFLAGS += $(call cc-option,-fvisibility=hidden)
 endif
 
 ifeq ($(CONFIG_STATIC),y)
-CFLAGS_busybox += -static
 PKG_CONFIG_FLAGS += --static
 endif
 
 ifeq ($(CONFIG_PIE),y)
+ifeq ($(CONFIG_STATIC),y)
+CFLAGS_busybox += $(ARCH_SPIE)
+else
 CFLAGS_busybox += $(ARCH_PIE)
+endif
 CFLAGS += $(ARCH_FPIE)
+else
+ifeq ($(CONFIG_STATIC),y)
+CFLAGS_busybox += -static
+endif
 endif
 
 ifneq ($(CONFIG_EXTRA_CFLAGS),)
-- 
2.53.0