Re: [PATCH] [v2] Kbuild: move to -std=gnu11

Marco Elver
2022-02-28T11:48:24Z

On Mon, 28 Feb 2022 at 11:32, Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> 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 and gnu11, 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 -Werror, which happens with the
> 'make W=1' option, as well as for three drivers in the kernel that always
> enable -Werror, 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.

On the topic of Wdeclaration-after-statement, Clang only respects this
warning with C99 and later starting with Clang 14:
https://github.com/llvm/llvm-project/commit/c65186c89f35#diff-ec770381d76c859f5f572db789175fe44410a72608f58ad5dbb14335ba56eb97R61

Until Clang 14, -Wdeclaration-after-statement is ignored by Clang in
newer standards. If this is a big problem, we can probably convince
the Clang stable folks to backport the fixes. However, the build won't
fail, folks might just miss the warning if they don't also test with
GCC.

> 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@mail.gmail.com/
> Link: https://github.com/ClangBuiltLinux/linux/issues/1603
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: linux-kbuild@vger.kernel.org
> Cc: llvm@lists.linux.dev
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Marco Elver <elver@google.com>

Thanks,
-- Marco

> ---
> [v2]
> - added -std=gnu11 back, rather than just relying on the default
> - minor changes to changelog text
> ---
> Documentation/process/programming-language.rst | 4 ++--
> .../translations/it_IT/process/programming-language.rst | 4 ++--
> .../translations/zh_CN/process/programming-language.rst | 4 ++--
> .../translations/zh_TW/process/programming-language.rst | 4 ++--
> Makefile | 6 +++---
> 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, 17 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/process/programming-language.rst b/Documentation/process/programming-language.rst
> index ec474a70a02f..894f2a6eb9db 100644
> --- a/Documentation/process/programming-language.rst
> +++ b/Documentation/process/programming-language.rst
> @@ -5,8 +5,8 @@ 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
> +under ``-std=gnu11`` [gcc-c-dialect-options]_: the GNU dialect of ISO C11
> +(including some C17 features). ``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]_,
> diff --git a/Documentation/translations/it_IT/process/programming-language.rst b/Documentation/translations/it_IT/process/programming-language.rst
> index 41db2598ce11..aa21097737ae 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 (con l'aggiunta di alcune funzionalità da C17).
> 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..58d2b3bd2d85 100644
> --- a/Documentation/translations/zh_CN/process/programming-language.rst
> +++ b/Documentation/translations/zh_CN/process/programming-language.rst
> @@ -9,8 +9,8 @@
> ============
>
> 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（
> +包括一些C17特性）
>
> 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..235de05f7e2c 100644
> --- a/Documentation/translations/zh_TW/process/programming-language.rst
> +++ b/Documentation/translations/zh_TW/process/programming-language.rst
> @@ -12,8 +12,8 @@
> ============
>
> 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（
> +包括一些C17特性）
>
> 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..66496eaeb9ec 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -432,7 +432,7 @@ HOSTCXX = g++
> endif
>
> export KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
> - -O2 -fomit-frame-pointer -std=gnu89
> + -O2 -fomit-frame-pointer -std=gnu11
> export KBUILD_USERLDFLAGS :=
>
> KBUILD_HOSTCFLAGS := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
> @@ -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
>
> CC := $(CROSS_COMPILE)gcc
>
> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> index 4188ba3fd8c3..99f9995670ea 100644
> --- a/fs/btrfs/Makefile
> +++ b/fs/btrfs/Makefile
> @@ -17,6 +17,7 @@ subdir-ccflags-y += $(condflags)
> subdir-ccflags-y += -Wno-missing-field-initializers
> subdir-ccflags-y += -Wno-sign-compare
> subdir-ccflags-y += -Wno-type-limits
> +subdir-ccflags-y += -Wno-shift-negative-value
>
> obj-$(CONFIG_BTRFS_FS) := btrfs.o
>
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 8be892887d71..650d0b8ceec3 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -36,6 +36,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
> KBUILD_CFLAGS += -Wno-missing-field-initializers
> KBUILD_CFLAGS += -Wno-sign-compare
> KBUILD_CFLAGS += -Wno-type-limits
> +KBUILD_CFLAGS += -Wno-shift-negative-value
>
> KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
>
> --
> 2.29.2
>
>