Re: [PATCH 2/2] perf build: Add -Wno-missing-declarations for GEN_VMLINUX_H=1

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [Medium] Suppressing the missing-declarations warning instead of enabling Microsoft extensions causes Clang to silently drop anonymous fields from structures in `vmlinux.h`.
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commit 93aff8b1d8d6d53f0451d4c78852fef0b06314df
Author: Namhyung Kim <namhyung@kernel.org>

perf build: Add -Wno-missing-declarations for GEN_VMLINUX_H=1

This commit suppresses the missing-declarations warning by adding
-Wno-missing-declarations when compiling BPF skeletons in perf.

> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -1249,7 +1249,7 @@ $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h
>  	$(QUIET_CLANG)$(CLANG) -g -O2 -fno-stack-protector --target=bpf \
>  	  $(CLANG_OPTIONS) $(EXTRA_BPF_FLAGS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
>  	  -include $(OUTPUT)PERF-VERSION-FILE -include util/bpf_skel/perf_version.h \
> -	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
> +	  -Wno-missing-declarations -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

Does suppressing this warning instead of enabling Microsoft extensions cause
Clang to silently drop anonymous fields from structures in vmlinux.h?

bpftool generates anonymous structs in vmlinux.h using a Microsoft extension.
Without -fms-extensions, standard C treats these as empty forward declarations.
Clang ignores them, effectively removing them from the struct layout and
altering the structure's size and offsets.

By passing -Wno-missing-declarations instead of enabling the extension, could
this hide the layout corruption? Any perf BPF programs relying on the size of
such structs or performing non-CO-RE offset calculations might compute
incorrect values, leading to verifier rejections or incorrect tracing
statistics.

Would it be better to pass -fms-extensions and -Wno-microsoft-anon-tag to
properly support this in Clang, mirroring the fix applied to BPF selftests in
commit 835a50753579a?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260408055259.374744-1-namhyung@kernel.org?part=2