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 98d69488e5c2..92cab904f1ba 100644
--- a/arch/parisc/boot/compressed/Makefile
+++ b/arch/parisc/boot/compressed/Makefile
@@ -21,7 +21,7 @@ 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 22dc393b5a83..287b853a7f09 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
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-asynchronous-unwind-tables
diff --git a/arch/s390/purgatory/Makefile b/arch/s390/purgatory/Makefile
index 2f4d8422bdfb..facd0b3db637 100644
--- a/arch/s390/purgatory/Makefile
+++ b/arch/s390/purgatory/Makefile
@@ -20,7 +20,7 @@ GCOV_PROFILE := n
 UBSAN_SANITIZE := n
 KASAN_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 69930ed5574b..424ee0e1f23d 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -31,7 +31,7 @@ endif
 CODE16GCC_CFLAGS := -m32 -Wa,$(srctree)/arch/x86/boot/code16gcc.h
 M16_CFLAGS	 := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))
 
-REALMODE_CFLAGS	:= -std=gnu11 $(M16_CFLAGS) -g -Os -DDISABLE_BRANCH_PROFILING \
+REALMODE_CFLAGS	:= -std=gnu89 $(M16_CFLAGS) -g -Os -DDISABLE_BRANCH_PROFILING \
 		   -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 e436819859d9..5771d5b43bb8 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -27,7 +27,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
 	vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
 
 KBUILD_CFLAGS := -m$(BITS) -O2
-KBUILD_CFLAGS += -std=gnu11
+KBUILD_CFLAGS += -std=gnu89
 KBUILD_CFLAGS += -fno-strict-aliasing $(call cc-option, -fPIE, -fPIC)
 KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
 cflags-$(CONFIG_X86_32) := -march=i386
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 0dc80564bbd2..00622027a49f 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__ -O2 -std=gnu11 \
+cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -O2 -std=gnu89 \
 				   -fPIC -fno-strict-aliasing -mno-red-zone \
 				   -mno-mmx -mno-sse -fshort-wchar \
 				   -Wno-pointer-sign \

-- 
2.51.0