[PATCH v2] arch/x86/boot: use prefix map to avoid embedded paths

[Ross Burton]

From: Bruce Ashfield <bruce.ashfield@xxxxxxxxx>

It was observed that the kernel embeds the path in the x86 boot
artifacts.

>From https://bugzilla.yoctoproject.org/show_bug.cgi?id=13458:

[
   If you turn on the buildpaths QA test, or try a reproducible build, you
   discover that the kernel image contains build paths.

   $ strings bzImage-5.0.19-yocto-standard |grep tmp/
   out of pgt_buf in
   /data/poky-tmp/reproducible/tmp/work-shared/qemux86-64/kernel-source/arch/x86/boot/compressed/kaslr_64.c!?

   But what's this in the top-level Makefile:

   $ git grep prefix-map
   Makefile:KBUILD_CFLAGS  += $(call
   cc-option,-fmacro-prefix-map=$(srctree)/=)

   So the __FILE__ shouldn't be using the full path.  However
   arch/x86/boot/compressed/Makefile has this:

   KBUILD_CFLAGS := -m$(BITS) -O2

   So that clears KBUILD_FLAGS, removing the -fmacro-prefix-map option.
]

Other architectures do not clear the flags, but instead prune before
adding boot or specific options. There's no obvious reason why x86 isn't
doing the same thing (pruning vs clearing) and no build or boot issues
have been observed.

So we make x86 can do the same thing, and we no longer have embedded paths.

Signed-off-by: Bruce Ashfield <bruce.ashfield@xxxxxxxxx>
Signed-off-by: Ross Burton <ross.burton@xxxxxxxxx>
---
 arch/x86/boot/Makefile            | 1 +
 arch/x86/boot/compressed/Makefile | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index e2839b5c246c..cd9c56496106 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -66,6 +66,7 @@ clean-files += cpustr.h
 # ---------------------------------------------------------------------------
 
 KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
+KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 6b84afdd7538..b246f18c5857 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -38,6 +38,7 @@ KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
 KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
 KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
 KBUILD_CFLAGS += -Wno-pointer-sign
+KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
 KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
 GCOV_PROFILE := n
-- 
2.20.1