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Date:   Thu, 5 Apr 2018 23:50:25 +0900
From:   Masahiro Yamada <>
To:     Linux Kbuild mailing list <>
Cc:     Greg KH <>,
        Michal Marek <>,
        Linus Torvalds <>,
        Tom Rini <>, Joe Perches <>,
        Randy Dunlap <>,
        Sam Ravnborg <>,
        Geert Uytterhoeven <>,
        Masahiro Yamada <>,
        Linux Kernel Mailing List <>
Subject: Re: [PATCH v3] kbuild: use -fmacro-prefix-map to make __FILE__ a
 relative path

2018-03-30 13:15 GMT+09:00 Masahiro Yamada <>:
> 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 9da0763bdd82 ("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: Masahiro Yamada <>
> ---
> I tested this on GCC 8.
> (not released yet, but you can get the source code from the trunk.)

Applied to linux-kbuild.

Best Regards
Masahiro Yamada

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