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Date:   Tue, 1 Mar 2022 09:53:24 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Marco Elver <elver@google.com>,
        Jani Nikula <jani.nikula@intel.com>,
        David Sterba <dsterba@suse.com>, Alex Shi <alexs@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-kbuild@vger.kernel.org, llvm@lists.linux.dev,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] [v3] Kbuild: move to -std=gnu11
Message-ID: <Yh5PhDDLcVe4kSqk@dev-arch.archlinux-ax161>
References: <20220301145233.3689119-1-arnd@kernel.org>
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On Tue, Mar 01, 2022 at 03:52:31PM +0100, Arnd Bergmann 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 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@mail.gmail.com/
> Link: https://github.com/ClangBuiltLinux/linux/issues/1603
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Acked-by: Marco Elver <elver@google.com>
> Acked-by: Jani Nikula <jani.nikula@intel.com>
> Acked-by: David Sterba <dsterba@suse.com>
> Reviewed-by: Alex Shi <alexs@kernel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-kbuild@vger.kernel.org
> Cc: llvm@lists.linux.dev
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

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?

>  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
> 
>