On Wed, Mar 2, 2022 at 5:29 PM Arnd Bergmann <arnd@xxxxxxxxxx> 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> When I saw the first patches in this area, I was thinking what about the tools/ build-system? This ECO system has its own rules. I use CONFIG_DEBUG_INFO_BTF=y in my kernels which means tools/bpf/resolve_btfids will be built. This uses hardcoded -std=gnu89. $ git grep 'std=gnu89' tools/ tools/lib/bpf/Makefile:override CFLAGS += -std=gnu89 tools/testing/selftests/sync/Makefile:CFLAGS += -O2 -g -std=gnu89 -pthread -Wall -Wextra Furthermore, we have several places using C99 which can be replaced by C11 now. $ git grep 'std=gnu99' tools/ | wc -l 20 We already have C11 in tools/: $ git grep 'std=gnu11' tools/ tools/testing/selftests/wireguard/qemu/Makefile: $(MUSL_CC) -o $@ $(CFLAGS) $(LDFLAGS) -std=gnu11 $< What about having a central place where we define the STD_GNU_C_VERSION (as a suggestion)? This might be helpful for people dealing with diverse C version standards. Also with STD_GNU_CXX_VERSION which is used AFAICS only in tools/: $ git grep 'std=gnu++' tools/ tools/build/feature/Makefile: $(BUILDXX) -std=gnu++11 tools/build/feature/Makefile: $(BUILDXX) -std=gnu++14 \ tools/build/feature/Makefile: $(BUILDXX) -std=gnu++14 \ tools/build/feature/Makefile: $(BUILDXX) -std=gnu++14 \ tools/perf/Makefile.config:CXXFLAGS += -std=gnu++14 -fno-exceptions -fno-rtti Looks like one place should be -std=gnu++14. ( I had no deeper look. No testing. ) Just some thoughts. - Sedat - > --- > [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 > > 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 >
