compiler.h: Allow arch-specific asm/compiler.h

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 28128c61e0 ("kconfig.h: Include compiler types to avoid missed
struct attributes") which caused linux/compiler_types.h to be included
in the compilation of every C file via the -include linux/kconfig.h flag
in c_flags.

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>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/20269/
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

arch/Kconfig

@@ -971,4 +971,12 @@ 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"

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