[PATCH v2] kbuild: use -fmacro-prefix-map to make __FILE__ a relative path

[Masahiro Yamada]

The __FILE__ macro is used everywhere in the kernel to locate the file
path printing the log message.  The biggest users of this macro are
WARN_ON() and friends.  If the kernel is built out of tree, this could
be a long absolute path, like this:

  WARNING: CPU: 1 PID: 1 at /path/to/build/directory/arch/arm64/kernel/foo.c:...

This is because Kbuild runs in the objtree instead of the srctree.

Commit 9da0763bdd82 ("kbuild: Use relative path when building in a
subdir of the source tree") mitigated the pain to some extent;
$(srctree) becomes ".." if the objtree is a child of the srctree.

For other cases of out-of-tree build, __FILE__ is still the absolute
path.  It also means the kernel image depends on where it was built.

A brand-new option from GCC solves this problem.

-fmacro-prefix-map=<old>=<new>
  When preprocessing files residing in directory <old>, expand the
  __FILE__ and __BASE_FILE__ macros as if the files resided in
  directory <new> instead.  This can be used to change an absolute
  path to a relative path by using . for <new> which can result in
  more reproducible builds that are location independent.  This
  option also affects __builtin_FILE() during compilation.

If your compiler supports this option, __FILE__ is the relative path
from the root of srctree regardless of O= option.

Please note __FILE__ is always an absolute path for external modules.

Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

I tested this on GCC 8.
(not released yet, but you can get the source code from the trunk.)


Changes in v2:
  - Comment-in the ifeq

 Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Makefile b/Makefile
index 7ba478a..5acee54 100644
--- a/Makefile
+++ b/Makefile
@@ -813,6 +813,11 @@ KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
 KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
 endif
 
+# Change __FILE__ to a relative path from the srctree
+ifneq ($(KBUILD_SRC),)
+KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
+endif
+
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(call shell-cached,$(CC) -print-file-name=include)
 CHECKFLAGS     += $(NOSTDINC_FLAGS)
-- 
2.7.4