Re: [PATCH bpf v1] resolve_btfids: Drop -lzstd fallback if libzstd not found

[Paul Chaignon <paul.chaignon@gmail.com>]

On Mon, Mar 02, 2026 at 03:10:58PM -0800, Ihor Solodrai wrote:
> The "|| echo -lzstd" default makes zstd an unconditional link
> dependency of resolve_btfids. On systems where libzstd-dev is not
> installed and pkg-config fails, the linker fails:
> 
>   ld: cannot find -lzstd: No such file or directory
> 
> libzstd is a transitive dependency of libelf, so the -lzstd flag is
> strictly necessary only for static builds [1].
> 
> Drop the -lzstd fallback to enable builds on systems without
> libzstd-dev installed.
> 
> Reported-by: BPF CI Bot (Claude Opus 4.6) <bot+bpf-ci@kernel.org>
> Reported-by: Vitaly Chikunov <vt@altlinux.org>
> Closes: https://lore.kernel.org/bpf/aaWqMcK-2AQw5dx8@altlinux.org/
> Fixes: 4021848a903e ("selftests/bpf: Pass through build flags to bpftool and resolve_btfids")
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---
>  tools/bpf/resolve_btfids/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index ef083602b73a..5df897c881d3 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -66,7 +66,7 @@ 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)
> +ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null)

This change is currently failing in CI with "undefined reference to
`ZSTD_xxx'" when trying to build resolve_btfids. I think this is
happening because the selftests use static linking with
PKG_CONFIG='pkg-config --static' but then the same isn't done for
resolve_btfids, so "pkg-config libelf --libs" doesn't return -lzstd.

So maybe the solution is to define HOSTPKG_CONFIG when building
resolve_btfids, as in the following diff:

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 72a9ba41f95e..d5acbeba0383 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -409,6 +409,7 @@ $(RESOLVE_BTFIDS): $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/resolve_btfids        \
                CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" \
                LIBBPF_INCLUDE=$(HOST_INCLUDE_DIR) \
                EXTRA_LDFLAGS='$(SAN_LDFLAGS) $(EXTRA_LDFLAGS)' \
+               HOSTPKG_CONFIG=$(PKG_CONFIG) \
                OUTPUT=$(HOST_BUILD_DIR)/resolve_btfids/ BPFOBJ=$(HOST_BPFOBJ)

 # Get Clang's default includes on this system, as opposed to those seen by

>  
>  HOSTCFLAGS_resolve_btfids += -g \
>            -I$(srctree)/tools/include \
> -- 
> 2.53.0
> 
>