Re: [PATCH v3 03/17] tools build: Fix test-clang-bpf-co-re.bin to generate target file

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] Broken dependency tracking for `test-clang-bpf-co-re.bin` due to missing `.d` generation and prerequisites.
- [Low] Incomplete implementation: `test-bpftool-skeletons.bin` suffers from the identical missing target file flaw.
--

commit 57e84770100052aa7d6b943326d9f33e586ed45a
Author: Ian Rogers <irogers@google.com>

tools build: Fix test-clang-bpf-co-re.bin to generate target file

This commit fixes the test-clang-bpf-co-re.bin feature check to ensure
the target file is created on disk upon success. This allows Kbuild to
cache the detection result and avoids redundant sub-make forks.

> Because the target file was never created, Kbuild could never cache the
> result of the check. Consequently, Make treated the prerequisite as missing
> and continuously re-executed the Clang BPF backend compilation check on
> every single sub-make evaluation during build startup.

The commit message identifies that failing to create the target file
breaks Make's caching. In the same Makefile, test-bpftool-skeletons.bin
redirects its output but never creates the $@ target file:

$(OUTPUT)test-bpftool-skeletons.bin:
	$(SYSTEM_BPFTOOL) version | grep '^features:.*skeletons' \
		> $(@:.bin=.make.output) 2>&1

Should test-bpftool-skeletons.bin also be updated to ensure the target file is
touched upon success, to prevent the same redundant sub-make fork overhead?

> diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
> index dc828523b50ad..c34faccdf28bd 100644
> --- a/tools/build/feature/Makefile
> +++ b/tools/build/feature/Makefile
> @@ -385,7 +385,7 @@ $(OUTPUT)test-libzstd.bin:
>  
>  $(OUTPUT)test-clang-bpf-co-re.bin:
>  	$(CLANG) -S -g --target=bpf -o - $(patsubst %.bin,%.c,$(@F)) |	\
> -		grep BTF_KIND_VAR
> +		grep BTF_KIND_VAR > $@.tmp && mv $@.tmp $@

Since this manual $(CLANG) invocation doesn't use the -MD flag (unlike
standard targets using $(BUILD)), no .d dependency file is generated.
Additionally, there is no explicit prerequisite on the corresponding .c source
file.

Now that the target file is created on disk, will Make consider it
up-to-date indefinitely and silently ignore future modifications to
test-clang-bpf-co-re.c during incremental builds?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260514163409.927816-1-irogers@google.com?part=3