Re: [PATCH bpf v3 01/15] selftests/bpf: Pass through build flags to bpftool and resolve_btfids

[bot+bpf-ci]

> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index 1733a6e93a07..ef083602b73a 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -65,6 +65,9 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
>
> +ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
> +ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)

The "|| echo -lzstd" fallback makes zstd an unconditional link
dependency. On systems where libzstd-dev is not installed, pkg-config
fails and the fallback provides -lzstd, but the linker cannot find the
library and the build fails.

bpftool handles the same transitive dependency conditionally via a
feature test:

tools/bpf/bpftool/Makefile:
    ifeq ($(feature-libelf-zstd),1)
    LIBS += -lzstd
    LIBS_BOOTSTRAP += -lzstd
    endif

Would it be safer to drop the fallback here so ZSTD_LIBS is empty
when zstd is not available?

    ZSTD_LIBS := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null)

This would still link zstd when present (needed for the static linking
case where libelf.a pulls in ZSTD symbols from elf_compress.o), but
avoid breaking dynamic builds on systems without libzstd-dev.

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22243753316

AI-authorship-score: low
AI-authorship-explanation: Human-characteristic typo in commit message, natural lore discussion with incremental debugging, and standard Makefile pattern reuse all indicate human authorship.
issues-found: 1
issue-severity-score: low
issue-severity-explanation: Unconditional zstd link fallback in resolve_btfids Makefile can break builds on systems without libzstd-dev, but the affected scenario is narrow and limited to the selftests build system.