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    Subject[PATCH v9 1/2] kbuild: Allow arch-specific asm/compiler.h
    Date
    We have a need to override the definition of
    barrier_before_unreachable() for MIPS, which means we either need to add
    architecture-specific code into linux/compiler-gcc.h or we need to allow
    the architecture to provide a header that can define the macro before
    the generic definition. The latter seems like the better approach.

    A straightforward approach to the per-arch header is to make use of
    asm-generic to provide a default empty header & adjust architectures
    which don't need anything specific to make use of that by adding the
    header to generic-y. Unfortunately this doesn't work so well due to
    commit a95b37e20db9 ("kbuild: get <linux/compiler_types.h> out of
    <linux/kconfig.h>") which moved the inclusion of linux/compiler.h to
    cflags using the -include compiler flag.

    Because the -include flag is present for all C files we compile, we need
    the architecture-provided header to be present before any C files are
    compiled. If any C files can be compiled prior to the asm-generic header
    wrappers being generated then we hit a build failure due to missing
    header. Such cases do exist - one pointed out by the kbuild test robot
    is the compilation of arch/ia64/kernel/nr-irqs.c, which occurs as part
    of the archprepare target [1].

    This leaves us with a few options:

    1) Use generic-y & fix any build failures we find by enforcing
    ordering such that the asm-generic target occurs before any C
    compilation, such that linux/compiler_types.h can always include
    the generated asm-generic wrapper which in turn includes the empty
    asm-generic header. This would rely on us finding all the
    problematic cases - I don't know for sure that the ia64 issue is
    the only one.

    2) Add an actual empty header to each architecture, so that we don't
    need the generated asm-generic wrapper. This seems messy.

    3) Give up & add #ifdef CONFIG_MIPS or similar to
    linux/compiler_types.h. This seems messy too.

    4) Include the arch header only when it's actually needed, removing
    the need for the asm-generic wrapper for all other architectures.

    This patch allows us to use approach 4, by including an asm/compiler.h
    header from linux/compiler_types.h after the inclusion of the
    compiler-specific linux/compiler-*.h header(s). We do this
    conditionally, only when CONFIG_HAVE_ARCH_COMPILER_H is selected, in
    order to avoid the need for asm-generic wrappers & the associated build
    ordering issue described above. The asm/compiler.h header is included
    after the generic linux/compiler-*.h header(s) for consistency with the
    way linux/compiler-intel.h & linux/compiler-clang.h are included after
    the linux/compiler-gcc.h header that they override.

    [1] https://lists.01.org/pipermail/kbuild-all/2018-August/051175.html

    Signed-off-by: Paul Burton <paul.burton@mips.com>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: James Hogan <jhogan@kernel.org>
    Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: linux-arch@vger.kernel.org
    Cc: linux-kbuild@vger.kernel.org
    Cc: linux-mips@linux-mips.org

    ---

    Changes in v9:
    - Use Kconfig & a #include directive as Masahiro suggested.
    - Go with asm/compiler.h rather than asm/compiler_types.h as it's really
    definitions from linux/compiler-*.h that we want to override & the
    conditional include means we don't need to worry about existing
    asm/compiler.h headers.
    - Tweak subject & commit message to reflect the changes above.

    Changes in v8:
    - New patch.

    arch/Kconfig | 8 ++++++++
    include/linux/compiler_types.h | 12 ++++++++++++
    2 files changed, 20 insertions(+)

    diff --git a/arch/Kconfig b/arch/Kconfig
    index c6148166a7b4..c0b56b0d86b0 100644
    --- a/arch/Kconfig
    +++ b/arch/Kconfig
    @@ -841,6 +841,14 @@ config REFCOUNT_FULL
    against various use-after-free conditions that can be used in
    security flaw exploits.

    +config HAVE_ARCH_COMPILER_H
    + bool
    + help
    + An architecture can select this if it provides an
    + asm/compiler.h header that should be included after
    + linux/compiler-*.h in order to override macro definitions that those
    + headers generally provide.
    +
    source "kernel/gcov/Kconfig"

    source "scripts/gcc-plugins/Kconfig"
    diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
    index fbf337933fd8..66239549d240 100644
    --- a/include/linux/compiler_types.h
    +++ b/include/linux/compiler_types.h
    @@ -78,6 +78,18 @@ extern void __chk_io_ptr(const volatile void __iomem *);
    #include <linux/compiler-clang.h>
    #endif

    +/*
    + * Some architectures need to provide custom definitions of macros provided
    + * by linux/compiler-*.h, and can do so using asm/compiler.h. We include that
    + * conditionally rather than using an asm-generic wrapper in order to avoid
    + * build failures if any C compilation, which will include this file via an
    + * -include argument in c_flags, occurs prior to the asm-generic wrappers being
    + * generated.
    + */
    +#ifdef CONFIG_HAVE_ARCH_COMPILER_H
    +#include <asm/compiler.h>
    +#endif
    +
    /*
    * Generic compiler-dependent macros required for kernel
    * build go below this comment. Actual compiler/compiler version
    --
    2.18.0
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