Re: [PATCH v1 05/14] perf build: Move BPF skeleton generation out of Makefile.perf

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Low] The `prepare` target evaluates the `$(BPFTOOL)` variable before it is defined, causing it to silently expand to an empty string.
- [Low] The `bpf-skel-clean` target fails to clean the intermediate `.tmp` directory for bench skeletons, causing a build artifact leak after `make clean`.
--

commit ba0748386f00351749ad3ff0cb9d88e512a55543
Author: Ian Rogers <irogers@google.com>

perf build: Move BPF skeleton generation out of Makefile.perf

This commit refactors BPF skeleton generation by extracting the infrastructure
out of Makefile.perf into a shared bpf_skel.mak file. It isolates the directory
structure and prevents unrelated sub-makes from serializing behind bpftool and
clang BPF target evaluations.

> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index 24581941e912e..d4fc10f36781f 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
[ ... ]
> @@ -914,44 +913,13 @@ python-clean:
>  
>  SKEL_OUT := $(abspath $(OUTPUT)util/bpf_skel)
>  SKEL_TMP_OUT := $(abspath $(SKEL_OUT)/.tmp)
[ ... ]
>  ifeq ($(CONFIG_PERF_BPF_SKEL),y)
> +prepare: $(BPFTOOL) $(SKEL_OUT)/vmlinux.h
>  BPFTOOL := $(SKEL_TMP_OUT)/bootstrap/bpftool

Does the prepare target evaluate the $(BPFTOOL) variable before it is defined?

In GNU Make, variables in target prerequisites are expanded immediately.
Because the prepare target precedes the BPFTOOL assignment, $(BPFTOOL)
silently expands to an empty string. The build seems to succeed only
circumstantially because vmlinux.h has a transitive dependency on bpftool.

I note this regression is fixed later in the series by commit e42b6c2f3098a
("perf build: Encapsulate vmlinux.h and bpftool in bpf_skel.mak").

[ ... ]
>  bpf-skel-clean:
> -	$(call QUIET_CLEAN, bpf-skel) $(RM) -r $(SKEL_TMP_OUT) $(SKELETONS) $(SKEL_OUT)/vmlinux.h
> +	$(call QUIET_CLEAN, bpf-skel) $(RM) -r $(SKEL_TMP_OUT) $(SKEL_OUT)/*.skel.h $(SKEL_OUT)/vmlinux.h $(OUTPUT)bench/bpf_skel/*.skel.h

Is the intermediate .tmp directory for bench skeletons left behind during
cleanup?

While the target explicitly removes the generated headers from bench/bpf_skel/,
it misses the temporary object directory $(OUTPUT)bench/bpf_skel/.tmp where
intermediate .bpf.o files are generated, causing a build artifact leak after
running make clean.

This regression is also fixed later in the series by commit e42b6c2f3098a
("perf build: Encapsulate vmlinux.h and bpftool in bpf_skel.mak").

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Sashiko AI review · https://sashiko.dev/#/patchset/20260512053539.3410189-1-irogers@google.com?part=5