config: add and use KERNEL_CC check for -Wno-format-zero-length

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>
Closes #16997

Author: intelfx

config/always-compiler-options.m4

@@ -155,6 +155,34 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_CC_NO_FORMAT_ZERO_LENGTH], [
 	AC_SUBST([NO_FORMAT_ZERO_LENGTH])
 ])
 
+dnl #
+dnl # Check if kernel cc supports -Wno-format-zero-length option.
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_KERNEL_CC_NO_FORMAT_ZERO_LENGTH], [
+	saved_cc="$CC"
+	AS_IF(
+		[ test -n "$KERNEL_CC" ], [ CC="$KERNEL_CC" ],
+		[ test -n "$KERNEL_LLVM" ], [ CC="clang" ],
+		[ CC="gcc" ]
+	)
+	AC_MSG_CHECKING([whether $CC supports -Wno-format-zero-length])
+
+	saved_flags="$CFLAGS"
+	CFLAGS="$CFLAGS -Werror -Wno-format-zero-length"
+
+	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], [
+		KERNEL_NO_FORMAT_ZERO_LENGTH=-Wno-format-zero-length
+		AC_MSG_RESULT([yes])
+	], [
+		KERNEL_NO_FORMAT_ZERO_LENGTH=
+		AC_MSG_RESULT([no])
+	])
+
+	CC="$saved_cc"
+	CFLAGS="$saved_flags"
+	AC_SUBST([KERNEL_NO_FORMAT_ZERO_LENGTH])
+])
+
 dnl #
 dnl # Check if cc supports -Wno-clobbered option.
 dnl #

config/zfs-build.m4

@@ -256,6 +256,7 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
 	ZFS_AC_CONFIG_ALWAYS_CC_FRAME_LARGER_THAN
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_FORMAT_TRUNCATION
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_FORMAT_ZERO_LENGTH
+	ZFS_AC_CONFIG_ALWAYS_KERNEL_CC_NO_FORMAT_ZERO_LENGTH
 	ZFS_AC_CONFIG_ALWAYS_CC_FORMAT_OVERFLOW
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_OMIT_FRAME_POINTER
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_IPA_SRA

module/Kbuild.in

@@ -4,7 +4,7 @@
 
 ZFS_MODULE_CFLAGS += -std=gnu99 -Wno-declaration-after-statement
 ZFS_MODULE_CFLAGS += -Wmissing-prototypes
-ZFS_MODULE_CFLAGS += @KERNEL_DEBUG_CFLAGS@  @NO_FORMAT_ZERO_LENGTH@
+ZFS_MODULE_CFLAGS += @KERNEL_DEBUG_CFLAGS@ @KERNEL_NO_FORMAT_ZERO_LENGTH@
 
 ifneq ($(KBUILD_EXTMOD),)
 zfs_include = @abs_top_srcdir@/include