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.)
Makefile | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/Makefile b/Makefile
index 7ba478a..d2d44f9 100644
--- a/Makefile
+++ b/Makefile
@@ -813,6 +813,11 @@ KBUILD_CFLAGS += $(call cc-option,-ffunction-sections,)
KBUILD_CFLAGS += $(call cc-option,-fdata-sections,)
endif
+#ifneq ($(KBUILD_SRC),)
+# Change __FILE__ to a relative path from the srctree
+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
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