Re: [PATCH 1/3] [v3] Kbuild: move to -std=gnu11

From: Masahiro Yamada
Date: Fri Mar 04 2022 - 00:04:47 EST

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On Wed, Mar 2, 2022 at 1:53 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Tue, Mar 01, 2022 at 03:52:31PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> >
> > During a patch discussion, Linus brought up the option of changing
> > the C standard version from gnu89 to gnu99, which allows using variable
> > declaration inside of a for() loop. While the C99, C11 and later standards
> > introduce many other features, most of these are already available in
> > gnu89 as GNU extensions as well.
> >
> > An earlier attempt to do this when gcc-5 started defaulting to
> > -std=gnu11 failed because at the time that caused warnings about
> > designated initializers with older compilers. Now that gcc-5.1 is
> > the minimum compiler version used for building kernels, that is no
> > longer a concern. Similarly, the behavior of 'inline' functions changes
> > between gnu89 using gnu_inline behavior and gnu11 using standard c99+
> > behavior, but this was taken care of by defining 'inline' to include
> > __attribute__((gnu_inline)) in order to allow building with clang a
> > while ago.
> >
> > One minor issue that remains is an added gcc warning for shifts of
> > negative integers when building with -Wextra, which happens with the
> > 'make W=1' option, as well as for three drivers in the kernel that always
> > enable -Wextra, but it was only observed with the i915 driver so far.
> > To be on the safe side, add -Wno-shift-negative-value to any -Wextra
> > in a Makefile.
> >
> > Nathan Chancellor reported an additional -Wdeclaration-after-statement
> > warning that appears in a system header on arm, this still needs a
> > workaround.
> >
> > The differences between gnu99, gnu11, gnu1x and gnu17 are fairly
> > minimal and mainly impact warnings at the -Wpedantic level that the
> > kernel never enables. Between these, gnu11 is the newest version
> > that is supported by all supported compiler versions, though it is
> > only the default on gcc-5, while all other supported versions of
> > gcc or clang default to gnu1x/gnu17.
> >
> > Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVFW3TA@xxxxxxxxxxxxxx/
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1603
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Acked-by: Marco Elver <elver@xxxxxxxxxx>
> > Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx>
> > Acked-by: David Sterba <dsterba@xxxxxxxx>
> > Reviewed-by: Alex Shi <alexs@xxxxxxxxxx>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Reviewed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> > Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: linux-kbuild@xxxxxxxxxxxxxxx
> > Cc: llvm@xxxxxxxxxxxxxxx
> > Cc: linux-doc@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
>
> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
>
> One comment below.
>
> > ---
> > [v3]
> > - split out USERCFLAGS to a separate patch
> > - add -Wdeclaration-after-statement patch from Mark Rutland
> > - leave out C17 reference
> > - more rewording the descrption
> >
> > [v2]
> > - added -std=gnu11 back, rather than just relying on the default
> > - minor changes to changelog text
> > ---
> > Documentation/process/programming-language.rst | 6 +++---
> > .../translations/it_IT/process/programming-language.rst | 4 ++--
> > .../translations/zh_CN/process/programming-language.rst | 3 +--
> > .../translations/zh_TW/process/programming-language.rst | 3 +--
> > Makefile | 4 ++--
> > arch/arm64/kernel/vdso32/Makefile | 2 +-
> > drivers/gpu/drm/i915/Makefile | 1 +
> > drivers/staging/greybus/tools/Makefile | 3 ++-
> > fs/btrfs/Makefile | 1 +
> > scripts/Makefile.extrawarn | 1 +
> > 10 files changed, 15 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/process/programming-language.rst b/Documentation/process/programming-language.rst
> > index ec474a70a02f..5fc9160ca1fa 100644
> > --- a/Documentation/process/programming-language.rst
> > +++ b/Documentation/process/programming-language.rst
> > @@ -5,9 +5,9 @@ Programming Language
> >
> > The kernel is written in the C programming language [c-language]_.
> > More precisely, the kernel is typically compiled with ``gcc`` [gcc]_
> > -under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90
> > -(including some C99 features). ``clang`` [clang]_ is also supported, see
> > -docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> > +under ``-std=gnu11`` [gcc-c-dialect-options]_: the GNU dialect of ISO C11.
> > +``clang`` [clang]_ is also supported, see docs on
> > +:ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> >
> > This dialect contains many extensions to the language [gnu-extensions]_,
> > and many of them are used within the kernel as a matter of course.
> > diff --git a/Documentation/translations/it_IT/process/programming-language.rst b/Documentation/translations/it_IT/process/programming-language.rst
> > index 41db2598ce11..c1a9b481a6f9 100644
> > --- a/Documentation/translations/it_IT/process/programming-language.rst
> > +++ b/Documentation/translations/it_IT/process/programming-language.rst
> > @@ -10,8 +10,8 @@ Linguaggio di programmazione
> >
> > Il kernel è scritto nel linguaggio di programmazione C [it-c-language]_.
> > Più precisamente, il kernel viene compilato con ``gcc`` [it-gcc]_ usando
> > -l'opzione ``-std=gnu89`` [it-gcc-c-dialect-options]_: il dialetto GNU
> > -dello standard ISO C90 (con l'aggiunta di alcune funzionalità da C99).
> > +l'opzione ``-std=gnu11`` [it-gcc-c-dialect-options]_: il dialetto GNU
> > +dello standard ISO C11.
> > Linux supporta anche ``clang`` [it-clang]_, leggete la documentazione
> > :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> >
> > diff --git a/Documentation/translations/zh_CN/process/programming-language.rst b/Documentation/translations/zh_CN/process/programming-language.rst
> > index 2a47a1d2ec20..fabdc338dbfb 100644
> > --- a/Documentation/translations/zh_CN/process/programming-language.rst
> > +++ b/Documentation/translations/zh_CN/process/programming-language.rst
> > @@ -9,8 +9,7 @@
> > ============
> >
> > 内核是用C语言 :ref:`c-language <cn_c-language>` 编写的。更准确地说，内核通常是用 :ref:`gcc <cn_gcc>`
> > -在 ``-std=gnu89`` :ref:`gcc-c-dialect-options <cn_gcc-c-dialect-options>` 下编译的：ISO C90的 GNU 方言（
> > -包括一些C99特性）
> > +在 ``-std=gnu11`` :ref:`gcc-c-dialect-options <cn_gcc-c-dialect-options>` 下编译的：ISO C11的 GNU 方言
> >
> > 这种方言包含对语言 :ref:`gnu-extensions <cn_gnu-extensions>` 的许多扩展，当然，它们许多都在内核中使用。
> >
> > diff --git a/Documentation/translations/zh_TW/process/programming-language.rst b/Documentation/translations/zh_TW/process/programming-language.rst
> > index 54e3699eadf8..144bdaf81a41 100644
> > --- a/Documentation/translations/zh_TW/process/programming-language.rst
> > +++ b/Documentation/translations/zh_TW/process/programming-language.rst
> > @@ -12,8 +12,7 @@
> > ============
> >
> > 內核是用C語言 :ref:`c-language <tw_c-language>` 編寫的。更準確地說，內核通常是用 :ref:`gcc <tw_gcc>`
> > -在 ``-std=gnu89`` :ref:`gcc-c-dialect-options <tw_gcc-c-dialect-options>` 下編譯的：ISO C90的 GNU 方言（
> > -包括一些C99特性）
> > +在 ``-std=gnu11`` :ref:`gcc-c-dialect-options <tw_gcc-c-dialect-options>` 下編譯的：ISO C11的 GNU 方言
> >
> > 這種方言包含對語言 :ref:`gnu-extensions <tw_gnu-extensions>` 的許多擴展，當然，它們許多都在內核中使用。
> >
> > diff --git a/Makefile b/Makefile
> > index 289ce2be8032..94fa9a849a7a 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -515,7 +515,7 @@ KBUILD_CFLAGS := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
> > -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
> > -Werror=implicit-function-declaration -Werror=implicit-int \
> > -Werror=return-type -Wno-format-security \
> > - -std=gnu89
> > + -std=gnu11
> > KBUILD_CPPFLAGS := -D__KERNEL__
> > KBUILD_AFLAGS_KERNEL :=
> > KBUILD_CFLAGS_KERNEL :=
> > @@ -782,7 +782,7 @@ KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
> >
> > ifdef CONFIG_CC_IS_CLANG
> > KBUILD_CPPFLAGS += -Qunused-arguments
> > -# The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > +# The kernel builds with '-std=gnu11' so use of GNU extensions is acceptable.
> > KBUILD_CFLAGS += -Wno-gnu
> > # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > # source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> > index 6c01b63ff56d..9378ea055bf2 100644
> > --- a/arch/arm64/kernel/vdso32/Makefile
> > +++ b/arch/arm64/kernel/vdso32/Makefile
> > @@ -68,7 +68,7 @@ VDSO_CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
> > -fno-strict-aliasing -fno-common \
> > -Werror-implicit-function-declaration \
> > -Wno-format-security \
> > - -std=gnu89
> > + -std=gnu11
> > VDSO_CFLAGS += -O2
> > # Some useful compiler-dependent flags from top-level Makefile
> > VDSO_CFLAGS += $(call cc32-option,-Wdeclaration-after-statement,)
> > diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> > index 1b62b9f65196..1618a6e0af4e 100644
> > --- a/drivers/gpu/drm/i915/Makefile
> > +++ b/drivers/gpu/drm/i915/Makefile
> > @@ -17,6 +17,7 @@ subdir-ccflags-y += -Wno-unused-parameter
> > subdir-ccflags-y += -Wno-type-limits
> > subdir-ccflags-y += -Wno-missing-field-initializers
> > subdir-ccflags-y += -Wno-sign-compare
> > +subdir-ccflags-y += -Wno-shift-negative-value
> > subdir-ccflags-y += $(call cc-disable-warning, unused-but-set-variable)
> > subdir-ccflags-y += $(call cc-disable-warning, frame-address)
> > subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
> > diff --git a/drivers/staging/greybus/tools/Makefile b/drivers/staging/greybus/tools/Makefile
> > index ad0ae8053b79..a3bbd73171f2 100644
> > --- a/drivers/staging/greybus/tools/Makefile
> > +++ b/drivers/staging/greybus/tools/Makefile
> > @@ -12,7 +12,8 @@ CFLAGS += -std=gnu99 -Wall -Wextra -g \
> > -Wredundant-decls \
> > -Wcast-align \
> > -Wsign-compare \
> > - -Wno-missing-field-initializers
> > + -Wno-missing-field-initializers \
> > + -Wno-shift-negative-value
>
> This Makefile uses its own -std= option because it is a userland tool.
> Seems like this hunk could just be dropped or the -std= argument could
> be bumped here too?
>

Right, this is a userland tool.

I do not mind either way.

BTW, this makefile is somewhat corrupted
because ../greybus_protocols.h does not exist.
(but it is working, relying on the built-in rules)

If Arnd does not mind sending v4, maybe we
can split this into two paches:

1/4 : add -Wno-shift-negative-value where -Wextra is used
2/4 : Kbuild: move to -std=gnu11

1/4 adds -Wno-shift-negative-value
whether it is the kernel space or user-land.

--
Best Regards
Masahiro Yamada

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