Re: [PATCH v7 06/23] tools build: Append -fzero-init-padding-bits=all to extra cflags

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] The `try-run` macro attempts to create a temporary directory in the source tree, breaking out-of-tree builds on read-only filesystems.
- [High] Unconditionally appending to `EXTRA_CFLAGS` in `Makefile.include` breaks `libbpf`'s default compilation flags, causing it to be built without debug info and optimizations.
--

commit e8ede2fdc1b643da62469cb30003522715558696
Author: Leo Yan <leo.yan@arm.com>
Subject: tools build: Append -fzero-init-padding-bits=all to extra cflags

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 by adding cc-option and host-cc-option to dynamically detect and
append it to EXTRA_CFLAGS and HOST_EXTRACFLAGS.

Link: https://gcc.gnu.org/gcc-15/changes.html

> diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> index 41971a68972dd..17d24cf84051a 100644
> --- a/tools/scripts/Makefile.include
> +++ b/tools/scripts/Makefile.include
[ ... ]
> @@ -139,6 +139,36 @@ 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);		\

[Severity: High]
Does this break out-of-tree builds on read-only filesystems?

Because TMPOUT is hardcoded as .tmp_$$$$ without an $(OUTPUT) prefix,
the mkdir -p command will attempt to write directly to the source directory.

When performing out-of-tree builds (like make O=/path/to/build) on read-only
source trees, this fails with a read-only file system error. Since the
compiler flag uses immediate assignment (:=), this failure might spam the
console and silently drop the option during parsing.

> +	if ($(1)) >/dev/null 2>&1;	\
> +	then echo "$(2)";		\
> +	else echo "$(3)";		\
> +	fi)
> +
[ ... ]
> +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)

[Severity: High]
Will this cause libbpf to lose its default debug and optimization flags?

In tools/lib/bpf/Makefile, there is a conditional block:

ifdef EXTRA_CFLAGS
  CFLAGS := $(EXTRA_CFLAGS)
else
  CFLAGS := -g -O2
endif

Because tools/scripts/Makefile.include is included prior to this check,
EXTRA_CFLAGS will unconditionally contain -fzero-init-padding-bits=all
if supported by the compiler.

This means EXTRA_CFLAGS is always defined, causing libbpf to completely drop
its default -g -O2 flags and compile with only the padding bits flag.

It appears libthermal was fixed to avoid this issue, but is a corresponding
fix needed for libbpf in this patch series?

> +HOST_FLAG_ZERO_INIT := $(call host-cc-option,-fzero-init-padding-bits=all)
> +override HOST_EXTRACFLAGS += $(HOST_FLAG_ZERO_INIT)
> +

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260602-tools_build_fix_zero_init-v7-0-631baf679fe7@arm.com?part=6