Re: [PATCH v2 03/18] 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 1 potential issue(s) to consider:
- [High] Unconditionally redirecting the feature check pipeline directly to the target file (`> $@`) causes Kbuild to permanently cache failed detections as successful.
--

commit c952ccdf6ac3df2b44fe927cf8066c42efc727a9
Author: Ian Rogers <irogers@google.com>

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

This commit updates the test-clang-bpf-co-re.bin feature check to
redirect the grep output to the target file. This ensures the target file
is created on disk upon success, allowing Kbuild to cache the result.

> diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
> index dc828523b50ad..f43f85738b5db 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 > $@

Could this unconditional redirection cause Kbuild to permanently cache failed
detections as successful?

If the host compiler does not support BPF CO-RE, grep fails to find
BTF_KIND_VAR and exits with 1. However, the shell processes the redirection
by creating the target file before execution.

Since tools/build/feature/Makefile lacks the GNU Make .DELETE_ON_ERROR:
special target, a failed grep leaves a 0-byte $@ target file on disk.

On subsequent evaluations, make checks test-clang-bpf-co-re.bin, sees the
file exists, and considers the target up to date. This causes make to exit
with 0, which breaks negative feature caching as Kbuild incorrectly
interprets this as feature-clang-bpf-co-re=1.

Would it be better to write to a temporary file and move it on success
(e.g., > $@.tmp && mv $@.tmp $@) to ensure the target is only created when
the feature is actually supported?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260512174638.120445-1-irogers@google.com?part=3