[PATCH v5 06/26] tools build: Append -fzero-init-padding-bits=all to extra cflags

Leo Yan
2026-03-18T09:18:06Z

GCC-15 release claims [1]:

{0} initializer in C or C++ for unions no longer guarantees clearing
of the whole union (except for static storage duration initialization),
it just initializes the first union member to zero. If initialization
of the whole union including padding bits is desirable, use {} (valid
in C23 or C++) or use -fzero-init-padding-bits=unions option to
restore old GCC behavior.

As a result, this new behaviour might cause unexpected data when we
initialize a union with using the '{ 0 }' initializer.

Since commit dce4aab8441d ("kbuild: Use -fzero-init-padding-bits=all"),
the kernel has enabled -fzero-init-padding-bits=all to zero padding bits
in unions and structures. This commit applies the same option for tools
building.

The option is not supported by any version older than GCC 15, nor is it
supported by LLVM. This patch adds the cc-option and host-cc-option
functions to dynamically detect compiler option and append it to the
EXTRA_CFLAGS and HOST_EXTRACFLAGS respectively.

[1] https://gcc.gnu.org/gcc-15/changes.html

Acked-by: Quentin Monnet <qmo@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Leo Yan <leo.yan@arm.com>
---
tools/scripts/Makefile.include | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index b5ecf137febcae59f506e107a7f2e2ad72f4bef4..a2397ceae512c1bc54adb15cb1a111ff34e28e43 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -137,6 +137,36 @@ else
EXTRA_WARNINGS += -Wshadow
endif

+# output directory for tests below
+TMPOUT = .tmp_$$$$
+
+# try-run
+# Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise)
+# Exit code chooses option. "$$TMP" serves as a temporary file and is
+# automatically cleaned up.
+try-run = $(shell set -e; \
+ TMP=$(TMPOUT)/tmp; \
+ trap "rm -rf $(TMPOUT)" EXIT; \
+ mkdir -p $(TMPOUT); \
+ if ($(1)) >/dev/null 2>&1; \
+ then echo "$(2)"; \
+ else echo "$(3)"; \
+ fi)
+
+# cc-option
+# Usage: CFLAGS += $(call cc-option,-march=winchip-c6,-march=i586)
+cc-option = $(call try-run, \
+ $(CC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
+
+host-cc-option = $(call try-run, \
+ $(HOSTCC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
+
+# Explicitly clear padding bits with the initializer '{ 0 }'
+FLAG_ZERO_INIT := $(call cc-option,-fzero-init-padding-bits=all)
+override EXTRA_CFLAGS += $(FLAG_ZERO_INIT)
+HOST_FLAG_ZERO_INIT := $(call host-cc-option,-fzero-init-padding-bits=all)
+override HOST_EXTRACFLAGS += $(HOST_FLAG_ZERO_INIT)
+
ifneq ($(findstring $(MAKEFLAGS), w),w)
PRINT_DIR = --no-print-directory
else

--
2.34.1