[PATCH 5.15.y 0/3] v5.15: fix build with GCC 15

[PATCH 5.15.y 0/3] v5.15: fix build with GCC 15

[Matthieu Baerts (NGI0)]

This kernel version doesn't build with GCC 15:

  In file included from include/uapi/linux/posix_types.h:5,
                   from include/uapi/linux/types.h:14,
                   from include/linux/types.h:6,
                   from arch/x86/realmode/rm/wakeup.h:11,
                   from arch/x86/realmode/rm/wakemain.c:2:
  include/linux/stddef.h:11:9: error: cannot use keyword 'false' as enumeration constant
     11 |         false   = 0,
        |         ^~~~~
  include/linux/stddef.h:11:9: note: 'false' is a keyword with '-std=c23' onwards
  include/linux/types.h:30:33: error: 'bool' cannot be defined via 'typedef'
     30 | typedef _Bool                   bool;
        |                                 ^~~~
  include/linux/types.h:30:33: note: 'bool' is a keyword with '-std=c23' onwards
  include/linux/types.h:30:1: warning: useless type name in empty declaration
     30 | typedef _Bool                   bool;
        | ^~~~~~~

I initially fixed this by adding -std=gnu11 in arch/x86/Makefile, then I
realised this fix was already done in an upstream commit, created before
the GCC 15 release and not mentioning the error I had. This is the first
patch.

When I was investigating my error, I noticed other commits were already
backported to v5.15. They were all adding -std=gnu11 in different
Makefiles. In their commit message, they were mentioning 'gnu11' was
picked to use the same as the one from the main Makefile. But this is
not the case in this kernel version. Patch 2 fixes that.

Finally, I noticed the documentation was not correct in this kernel
version: this is because a commit was backported to v5.15 while it was
not supposed to. Patch 3 fixes that.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Alexey Dobriyan (1):
      x86/boot: Compile boot code with -std=gnu11 too

Matthieu Baerts (NGI0) (2):
      arch: back to -std=gnu89 in < v5.18
      Revert "docs/process/howto: Replace C89 with C11"

 Documentation/process/howto.rst                    | 2 +-
 Documentation/translations/it_IT/process/howto.rst | 2 +-
 Documentation/translations/ja_JP/howto.rst         | 2 +-
 Documentation/translations/ko_KR/howto.rst         | 2 +-
 Documentation/translations/zh_CN/process/howto.rst | 2 +-
 Documentation/translations/zh_TW/process/howto.rst | 2 +-
 arch/parisc/boot/compressed/Makefile               | 2 +-
 arch/s390/Makefile                                 | 2 +-
 arch/s390/purgatory/Makefile                       | 2 +-
 arch/x86/Makefile                                  | 2 +-
 arch/x86/boot/compressed/Makefile                  | 2 +-
 drivers/firmware/efi/libstub/Makefile              | 2 +-
 12 files changed, 12 insertions(+), 12 deletions(-)
---
base-commit: 06cf22cc87e00b878c310d5441981b7750f04078
change-id: 20251017-v5-15-gcc-15-5ceda8ebe577

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH 5.15.y 1/3] x86/boot: Compile boot code with -std=gnu11 too

[Matthieu Baerts (NGI0)]

From: Alexey Dobriyan <adobriyan@gmail.com>

commit b3bee1e7c3f2b1b77182302c7b2131c804175870 upstream.

Use -std=gnu11 for consistency with main kernel code.

It doesn't seem to change anything in vmlinux.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Link: https://lore.kernel.org/r/2058761e-12a4-4b2f-9690-3c3c1c9902a5@p183
[ This kernel version doesn't build with GCC 15:

    In file included from include/uapi/linux/posix_types.h:5,
                     from include/uapi/linux/types.h:14,
                     from include/linux/types.h:6,
                     from arch/x86/realmode/rm/wakeup.h:11,
                     from arch/x86/realmode/rm/wakemain.c:2:
    include/linux/stddef.h:11:9: error: cannot use keyword 'false' as enumeration constant
       11 |         false   = 0,
          |         ^~~~~
    include/linux/stddef.h:11:9: note: 'false' is a keyword with '-std=c23' onwards
    include/linux/types.h:30:33: error: 'bool' cannot be defined via 'typedef'
       30 | typedef _Bool                   bool;
          |                                 ^~~~
    include/linux/types.h:30:33: note: 'bool' is a keyword with '-std=c23' onwards
    include/linux/types.h:30:1: warning: useless type name in empty declaration
       30 | typedef _Bool                   bool;
          | ^~~~~~~

  The fix is similar to commit ee2ab467bddf ("x86/boot: Use '-std=gnu11'
  to fix build with GCC 15") which has been backported to this kernel.

  Note: In < 5.18 version, -std=gnu89 is used instead of -std=gnu11, see
  commit e8c07082a810 ("Kbuild: move to -std=gnu11"). I suggest not to
  modify that in this commit here as all the other similar fixes to
  support GCC 15 set -std=gnu11. This can be done in a dedicated commit
  if needed. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 9c09bbd390ce..d5ee0b920dc0 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -24,7 +24,7 @@ endif
 
 # How to compile the 16-bit code.  Note we always compile for -march=i386;
 # that way we can complain to the user if the CPU is insufficient.
-REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
+REALMODE_CFLAGS	:= -std=gnu11 -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
 		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
 		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
 		   -mno-mmx -mno-sse $(call cc-option,-fcf-protection=none)

-- 
2.51.0

[PATCH 5.15.y 2/3] arch: back to -std=gnu89 in < v5.18

[Matthieu Baerts (NGI0)]

Recent fixes have been backported to < v5.18 to fix build issues with
GCC 5.15. They all force -std=gnu11 in the CFLAGS, "because [the kernel]
requests the gnu11 standard via '-std=' in the main Makefile".

This is true for >= 5.18 versions, but not before. This switch to
-std=gnu11 has been done in commit e8c07082a810 ("Kbuild: move to
-std=gnu11").

For a question of uniformity, force -std=gnu89, similar to what is done
in the main Makefile.

Note: the fixes tags below refers to upstream commits, but this fix is
only for kernels not having commit e8c07082a810 ("Kbuild: move to
-std=gnu11").

Fixes: 7cbb015e2d3d ("parisc: fix building with gcc-15")
Fixes: 3b8b80e99376 ("s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS")
Fixes: b3bee1e7c3f2 ("x86/boot: Compile boot code with -std=gnu11 too")
Fixes: ee2ab467bddf ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")
Fixes: 8ba14d9f490a ("efi: libstub: Use '-std=gnu11' to fix build with GCC 15")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Note:
  An alternative is to backport commit e8c07082a810 ("Kbuild: move to
  -std=gnu11"), but I guess we might not want to do that for stable, as
  it might introduce new warnings.

Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
---
 arch/parisc/boot/compressed/Makefile  | 2 +-
 arch/s390/Makefile                    | 2 +-
 arch/s390/purgatory/Makefile          | 2 +-
 arch/x86/Makefile                     | 2 +-
 arch/x86/boot/compressed/Makefile     | 2 +-
 drivers/firmware/efi/libstub/Makefile | 2 +-
 6 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/parisc/boot/compressed/Makefile b/arch/parisc/boot/compressed/Makefile
index 839a13a59f53..3eba999a2eb7 100644
--- a/arch/parisc/boot/compressed/Makefile
+++ b/arch/parisc/boot/compressed/Makefile
@@ -22,7 +22,7 @@ KBUILD_CFLAGS += -fno-PIE -mno-space-regs -mdisable-fpregs -Os
 ifndef CONFIG_64BIT
 KBUILD_CFLAGS += -mfast-indirect-calls
 endif
-KBUILD_CFLAGS += -std=gnu11
+KBUILD_CFLAGS += -std=gnu89
 
 OBJECTS += $(obj)/head.o $(obj)/real2.o $(obj)/firmware.o $(obj)/misc.o $(obj)/piggy.o
 
diff --git a/arch/s390/Makefile b/arch/s390/Makefile
index c8071eb82e2e..40265863ed36 100644
--- a/arch/s390/Makefile
+++ b/arch/s390/Makefile
@@ -23,7 +23,7 @@ endif
 aflags_dwarf	:= -Wa,-gdwarf-2
 KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
 KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
-KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -std=gnu11
+KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -std=gnu89
 KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-delete-null-pointer-checks -msoft-float -mbackchain
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-asynchronous-unwind-tables
diff --git a/arch/s390/purgatory/Makefile b/arch/s390/purgatory/Makefile
index 677cbb654024..414ba87e038b 100644
--- a/arch/s390/purgatory/Makefile
+++ b/arch/s390/purgatory/Makefile
@@ -21,7 +21,7 @@ UBSAN_SANITIZE := n
 KASAN_SANITIZE := n
 KCSAN_SANITIZE := n
 
-KBUILD_CFLAGS := -std=gnu11 -fno-strict-aliasing -Wall -Wstrict-prototypes
+KBUILD_CFLAGS := -std=gnu89 -fno-strict-aliasing -Wall -Wstrict-prototypes
 KBUILD_CFLAGS += -Wno-pointer-sign -Wno-sign-compare
 KBUILD_CFLAGS += -fno-zero-initialized-in-bss -fno-builtin -ffreestanding
 KBUILD_CFLAGS += -c -MD -Os -m64 -msoft-float -fno-common
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index d5ee0b920dc0..16267e85c5ad 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -24,7 +24,7 @@ endif
 
 # How to compile the 16-bit code.  Note we always compile for -march=i386;
 # that way we can complain to the user if the CPU is insufficient.
-REALMODE_CFLAGS	:= -std=gnu11 -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
+REALMODE_CFLAGS	:= -std=gnu89 -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
 		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
 		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
 		   -mno-mmx -mno-sse $(call cc-option,-fcf-protection=none)
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index f82b2cb24360..f54fa1579dcd 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -33,7 +33,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
 # avoid errors with '-march=i386', and future flags may depend on the target to
 # be valid.
 KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
-KBUILD_CFLAGS += -std=gnu11
+KBUILD_CFLAGS += -std=gnu89
 KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
 KBUILD_CFLAGS += -Wundef
 KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index f1a4f0154540..1195c9fb84de 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -7,7 +7,7 @@
 #
 cflags-$(CONFIG_X86_32)		:= -march=i386
 cflags-$(CONFIG_X86_64)		:= -mcmodel=small
-cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -std=gnu11 \
+cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -std=gnu89 \
 				   -fPIC -fno-strict-aliasing -mno-red-zone \
 				   -mno-mmx -mno-sse -fshort-wchar \
 				   -Wno-pointer-sign \

-- 
2.51.0

[PATCH 5.15.y 3/3] Revert "docs/process/howto: Replace C89 with C11"

[Matthieu Baerts (NGI0)]

This reverts commit dc52117cd797f71f9686fa0cec91509eb7a9623d.

In this kernel version, C89 is still the default ISO standard.

The reverted commit was fixing commit e8c07082a810 ("Kbuild: move to
-std=gnu11"), introduced in v5.18, and not backported to older versions.
It was then not supported to be backported to v5.15. It can then safely
be reverted.

Fixes: 2f3f53d62307 ("docs/process/howto: Replace C89 with C11")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <src.res@email.cn>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 Documentation/process/howto.rst                    | 2 +-
 Documentation/translations/it_IT/process/howto.rst | 2 +-
 Documentation/translations/ja_JP/howto.rst         | 2 +-
 Documentation/translations/ko_KR/howto.rst         | 2 +-
 Documentation/translations/zh_CN/process/howto.rst | 2 +-
 Documentation/translations/zh_TW/process/howto.rst | 2 +-
 6 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/Documentation/process/howto.rst b/Documentation/process/howto.rst
index 12a4e7ebcbab..e4beeca57e5f 100644
--- a/Documentation/process/howto.rst
+++ b/Documentation/process/howto.rst
@@ -36,7 +36,7 @@ experience, the following books are good for, if anything, reference:
  - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
 
 The kernel is written using GNU C and the GNU toolchain.  While it
-adheres to the ISO C11 standard, it uses a number of extensions that are
+adheres to the ISO C89 standard, it uses a number of extensions that are
 not featured in the standard.  The kernel is a freestanding C
 environment, with no reliance on the standard C library, so some
 portions of the C standard are not supported.  Arbitrary long long
diff --git a/Documentation/translations/it_IT/process/howto.rst b/Documentation/translations/it_IT/process/howto.rst
index d02df35d0f6b..9554368a2ae2 100644
--- a/Documentation/translations/it_IT/process/howto.rst
+++ b/Documentation/translations/it_IT/process/howto.rst
@@ -44,7 +44,7 @@ altro, utili riferimenti:
 - "C:  A Reference Manual" di Harbison and Steele [Prentice Hall]
 
 Il kernel è stato scritto usando GNU C e la toolchain GNU.
-Sebbene si attenga allo standard ISO C11, esso utilizza una serie di
+Sebbene si attenga allo standard ISO C89, esso utilizza una serie di
 estensioni che non sono previste in questo standard. Il kernel è un
 ambiente C indipendente, che non ha alcuna dipendenza dalle librerie
 C standard, così alcune parti del C standard non sono supportate.
diff --git a/Documentation/translations/ja_JP/howto.rst b/Documentation/translations/ja_JP/howto.rst
index 6a00e43868a2..d667f9d8a02a 100644
--- a/Documentation/translations/ja_JP/howto.rst
+++ b/Documentation/translations/ja_JP/howto.rst
@@ -65,7 +65,7 @@ Linux カーネル開発のやり方
  - 『新・詳説 C 言語 H&S リファレンス』 (サミュエル P ハービソン/ガイ L スティール共著 斉藤 信男監訳)[ソフトバンク]
 
 カーネルは GNU C と GNU ツールチェインを使って書かれています。カーネル
-は ISO C11 仕様に準拠して書く一方で、標準には無い言語拡張を多く使って
+は ISO C89 仕様に準拠して書く一方で、標準には無い言語拡張を多く使って
 います。カーネルは標準 C ライブラリに依存しない、C 言語非依存環境です。
 そのため、C の標準の中で使えないものもあります。特に任意の long long
 の除算や浮動小数点は使えません。カーネルがツールチェインや C 言語拡張
diff --git a/Documentation/translations/ko_KR/howto.rst b/Documentation/translations/ko_KR/howto.rst
index a787d31dcdbf..e3cdf0c84892 100644
--- a/Documentation/translations/ko_KR/howto.rst
+++ b/Documentation/translations/ko_KR/howto.rst
@@ -62,7 +62,7 @@ Documentation/process/howto.rst
  - "Practical C Programming" by Steve Oualline [O'Reilly]
  - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
 
-커널은 GNU C와 GNU 툴체인을 사용하여 작성되었다. 이 툴들은 ISO C11 표준을
+커널은 GNU C와 GNU 툴체인을 사용하여 작성되었다. 이 툴들은 ISO C89 표준을
 따르는 반면 표준에 있지 않은 많은 확장기능도 가지고 있다. 커널은 표준 C
 라이브러리와는 관계없이 freestanding C 환경이어서 C 표준의 일부는
 지원되지 않는다. 임의의 long long 나누기나 floating point는 지원되지 않는다.
diff --git a/Documentation/translations/zh_CN/process/howto.rst b/Documentation/translations/zh_CN/process/howto.rst
index 2a910e3e904e..ee3dee476d57 100644
--- a/Documentation/translations/zh_CN/process/howto.rst
+++ b/Documentation/translations/zh_CN/process/howto.rst
@@ -45,7 +45,7 @@ Linux内核大部分是由C语言写成的，一些体系结构相关的代码
  - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
    《C语言参考手册（原书第5版）》（邱仲潘 等译）[机械工业出版社]
 
-Linux内核使用GNU C和GNU工具链开发。虽然它遵循ISO C11标准，但也用到了一些
+Linux内核使用GNU C和GNU工具链开发。虽然它遵循ISO C89标准，但也用到了一些
 标准中没有定义的扩展。内核是自给自足的C环境，不依赖于标准C库的支持，所以
 并不支持C标准中的部分定义。比如long long类型的大数除法和浮点运算就不允许
 使用。有时候确实很难弄清楚内核对工具链的要求和它所使用的扩展，不幸的是目
diff --git a/Documentation/translations/zh_TW/process/howto.rst b/Documentation/translations/zh_TW/process/howto.rst
index ce14d4ed5c5b..2043691b92e3 100644
--- a/Documentation/translations/zh_TW/process/howto.rst
+++ b/Documentation/translations/zh_TW/process/howto.rst
@@ -48,7 +48,7 @@ Linux內核大部分是由C語言寫成的，一些體系結構相關的代碼
  - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
    《C語言參考手冊（原書第5版）》（邱仲潘 等譯）[機械工業出版社]
 
-Linux內核使用GNU C和GNU工具鏈開發。雖然它遵循ISO C11標準，但也用到了一些
+Linux內核使用GNU C和GNU工具鏈開發。雖然它遵循ISO C89標準，但也用到了一些
 標準中沒有定義的擴展。內核是自給自足的C環境，不依賴於標準C庫的支持，所以
 並不支持C標準中的部分定義。比如long long類型的大數除法和浮點運算就不允許
 使用。有時候確實很難弄清楚內核對工具鏈的要求和它所使用的擴展，不幸的是目

-- 
2.51.0

Re: [PATCH 5.15.y 2/3] arch: back to -std=gnu89 in < v5.18

[Greg Kroah-Hartman]

On Fri, Oct 17, 2025 at 06:24:01PM +0200, Matthieu Baerts (NGI0) wrote:
> Recent fixes have been backported to < v5.18 to fix build issues with
> GCC 5.15. They all force -std=gnu11 in the CFLAGS, "because [the kernel]
> requests the gnu11 standard via '-std=' in the main Makefile".
> 
> This is true for >= 5.18 versions, but not before. This switch to
> -std=gnu11 has been done in commit e8c07082a810 ("Kbuild: move to
> -std=gnu11").
> 
> For a question of uniformity, force -std=gnu89, similar to what is done
> in the main Makefile.
> 
> Note: the fixes tags below refers to upstream commits, but this fix is
> only for kernels not having commit e8c07082a810 ("Kbuild: move to
> -std=gnu11").
> 
> Fixes: 7cbb015e2d3d ("parisc: fix building with gcc-15")
> Fixes: 3b8b80e99376 ("s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS")
> Fixes: b3bee1e7c3f2 ("x86/boot: Compile boot code with -std=gnu11 too")
> Fixes: ee2ab467bddf ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")
> Fixes: 8ba14d9f490a ("efi: libstub: Use '-std=gnu11' to fix build with GCC 15")
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---
> Note:
>   An alternative is to backport commit e8c07082a810 ("Kbuild: move to
>   -std=gnu11"), but I guess we might not want to do that for stable, as
>   it might introduce new warnings.

I would rather do that, as that would allow us to make things align up
and be easier to support over the next two years that this kernel needs
to be alive for.  How much work would that entail?

thanks,

greg k-h

Re: [PATCH 5.15.y 2/3] arch: back to -std=gnu89 in < v5.18

[Matthieu Baerts]

Hi Greg,

On 20/10/2025 15:30, Greg Kroah-Hartman wrote:
> On Fri, Oct 17, 2025 at 06:24:01PM +0200, Matthieu Baerts (NGI0) wrote:
>> Recent fixes have been backported to < v5.18 to fix build issues with
>> GCC 5.15. They all force -std=gnu11 in the CFLAGS, "because [the kernel]
>> requests the gnu11 standard via '-std=' in the main Makefile".
>>
>> This is true for >= 5.18 versions, but not before. This switch to
>> -std=gnu11 has been done in commit e8c07082a810 ("Kbuild: move to
>> -std=gnu11").
>>
>> For a question of uniformity, force -std=gnu89, similar to what is done
>> in the main Makefile.
>>
>> Note: the fixes tags below refers to upstream commits, but this fix is
>> only for kernels not having commit e8c07082a810 ("Kbuild: move to
>> -std=gnu11").
>>
>> Fixes: 7cbb015e2d3d ("parisc: fix building with gcc-15")
>> Fixes: 3b8b80e99376 ("s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS")
>> Fixes: b3bee1e7c3f2 ("x86/boot: Compile boot code with -std=gnu11 too")
>> Fixes: ee2ab467bddf ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")
>> Fixes: 8ba14d9f490a ("efi: libstub: Use '-std=gnu11' to fix build with GCC 15")
>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
>> ---
>> Note:
>>   An alternative is to backport commit e8c07082a810 ("Kbuild: move to
>>   -std=gnu11"), but I guess we might not want to do that for stable, as
>>   it might introduce new warnings.
> 
> I would rather do that, as that would allow us to make things align up
> and be easier to support over the next two years that this kernel needs
> to be alive for.  How much work would that entail?

Good question. I'm not an expert in this area, but I just did a quick
test: I backported commit e8c07082a810 ("Kbuild: move to -std=gnu11")
and its parent commit 4d94f910e79a ("Kbuild: use
-Wdeclaration-after-statement"). A build with 'make allyesconfig' and
GCC 5.15 looks OK to me, no warnings.

But when looking a bit around, I noticed these patches have already been
suggested to be backported to v5.15 in 2023, but they got removed --
except the doc update, see patch 3/3 -- because they were causing build
issues with GCC 8 and 12, see:

 https://lore.kernel.org/a2fbbaa2-51d2-4a8c-b032-5331e72cd116@linaro.org

I didn't try to reproduce the issues, but maybe we can re-add them to
the v5.15 queue, and ask the CIs to test that?

Note that even if we do that, the first patch will still be needed to
have GCC 15 support in v5.15.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.

Re: [PATCH 5.15.y 2/3] arch: back to -std=gnu89 in < v5.18

[Greg Kroah-Hartman]

On Mon, Oct 20, 2025 at 05:58:12PM +0200, Matthieu Baerts wrote:
> Hi Greg,
> 
> On 20/10/2025 15:30, Greg Kroah-Hartman wrote:
> > On Fri, Oct 17, 2025 at 06:24:01PM +0200, Matthieu Baerts (NGI0) wrote:
> >> Recent fixes have been backported to < v5.18 to fix build issues with
> >> GCC 5.15. They all force -std=gnu11 in the CFLAGS, "because [the kernel]
> >> requests the gnu11 standard via '-std=' in the main Makefile".
> >>
> >> This is true for >= 5.18 versions, but not before. This switch to
> >> -std=gnu11 has been done in commit e8c07082a810 ("Kbuild: move to
> >> -std=gnu11").
> >>
> >> For a question of uniformity, force -std=gnu89, similar to what is done
> >> in the main Makefile.
> >>
> >> Note: the fixes tags below refers to upstream commits, but this fix is
> >> only for kernels not having commit e8c07082a810 ("Kbuild: move to
> >> -std=gnu11").
> >>
> >> Fixes: 7cbb015e2d3d ("parisc: fix building with gcc-15")
> >> Fixes: 3b8b80e99376 ("s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS")
> >> Fixes: b3bee1e7c3f2 ("x86/boot: Compile boot code with -std=gnu11 too")
> >> Fixes: ee2ab467bddf ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")
> >> Fixes: 8ba14d9f490a ("efi: libstub: Use '-std=gnu11' to fix build with GCC 15")
> >> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> >> ---
> >> Note:
> >>   An alternative is to backport commit e8c07082a810 ("Kbuild: move to
> >>   -std=gnu11"), but I guess we might not want to do that for stable, as
> >>   it might introduce new warnings.
> > 
> > I would rather do that, as that would allow us to make things align up
> > and be easier to support over the next two years that this kernel needs
> > to be alive for.  How much work would that entail?
> 
> Good question. I'm not an expert in this area, but I just did a quick
> test: I backported commit e8c07082a810 ("Kbuild: move to -std=gnu11")
> and its parent commit 4d94f910e79a ("Kbuild: use
> -Wdeclaration-after-statement"). A build with 'make allyesconfig' and
> GCC 5.15 looks OK to me, no warnings.
> 
> But when looking a bit around, I noticed these patches have already been
> suggested to be backported to v5.15 in 2023, but they got removed --
> except the doc update, see patch 3/3 -- because they were causing build
> issues with GCC 8 and 12, see:
> 
>  https://lore.kernel.org/a2fbbaa2-51d2-4a8c-b032-5331e72cd116@linaro.org
> 
> I didn't try to reproduce the issues, but maybe we can re-add them to
> the v5.15 queue, and ask the CIs to test that?
> 
> Note that even if we do that, the first patch will still be needed to
> have GCC 15 support in v5.15.

Ok, 5.15.y and 5.10.y will be alive for a while, so I'll go queue these
up now as odds are, I'll end up hitting this issue myself :)

thanks for the patches!

greg k-h