Commit a73619a8 authored by Masahiro Yamada's avatar Masahiro Yamada

kbuild: use -fmacro-prefix-map to make __FILE__ a relative path

The __FILE__ macro is used everywhere in the kernel to locate the file
printing the log message, such as WARN_ON(), etc.  If the kernel is
built out of tree, this can 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,
then __FILE__ is expanded to a file path prefixed with $(srctree)/.

Commit 9da0763b ("kbuild: Use relative path when building in a
subdir of the source tree") improved this 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, -fmacro-prefix-map, solves this problem.
If your compiler supports it, __FILE__ is the relative path from the
srctree regardless of O= option.  This provides more readable log and
more reproducible builds.

Please note __FILE__ is always an absolute path for external modules.
Signed-off-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
parent 54a702f7
...@@ -846,6 +846,9 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=incompatible-pointer-types) ...@@ -846,6 +846,9 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=incompatible-pointer-types)
# Require designated initializers for all marked structures # Require designated initializers for all marked structures
KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init) KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)
# change __FILE__ to the relative path from the srctree
KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
# use the deterministic mode of AR if available # use the deterministic mode of AR if available
KBUILD_ARFLAGS := $(call ar-option,D) KBUILD_ARFLAGS := $(call ar-option,D)
......
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