Re: [PATCH bpf v2] resolve_btfids: Fix linker flags detection

[Vitaly Chikunov <vt@altlinux.org>]

Ihor,

On Tue, Mar 03, 2026 at 11:39:54AM -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].
> 
> Remove ZSTD_LIBS variable, and instead set LIBELF_LIBS depending on
> whether the build is static or not. Use $(HOSTPKG_CONFIG) as primary
> source of the flags list.
> 
> Also add a default value for HOSTPKG_CONFIG in case it's not built via
> the toplevel Makefile.
> 
> [1] https://lore.kernel.org/bpf/4ff82800-2daa-4b9f-95a9-6f512859ee70@linux.dev/
> 
> 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>
> 
> ---
> 
> v1: https://lore.kernel.org/bpf/20260302231058.916946-1-ihor.solodrai@linux.dev/
> 
> ---
>  tools/bpf/resolve_btfids/Makefile | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index ef083602b73a..7672208f65e4 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -23,6 +23,7 @@ RM      ?= rm
>  HOSTCC  ?= gcc
>  HOSTLD  ?= ld
>  HOSTAR  ?= ar
> +HOSTPKG_CONFIG ?= pkg-config
>  CROSS_COMPILE =
>  
>  OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
> @@ -63,10 +64,14 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
>  		    $(abspath $@) install_headers
>  
>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
> +
> +ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
> +LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
> +else
>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
> +endif

Btw, pkg-config accepts -static, so you can do just

-LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs $(filter -static,$(EXTRA_LDFLAGS)) 2>/dev/null || echo -lelf)

Thanks,

>  
>  ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
> -ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)
>  
>  HOSTCFLAGS_resolve_btfids += -g \
>            -I$(srctree)/tools/include \
> @@ -76,7 +81,7 @@ HOSTCFLAGS_resolve_btfids += -g \
>            $(LIBELF_FLAGS) \
>            -Wall -Werror
>  
> -LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS) $(ZSTD_LIBS)
> +LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
>  
>  export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
>  include $(srctree)/tools/build/Makefile.include
> -- 
> 2.53.0