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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 3/4/26 9:23 AM, Paul Chaignon wrote:
> 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

Hi Paul, thanks for taking a look.

I'm aware, I sent a v2 yesterday (with changed subject):
https://lore.kernel.org/bpf/20260303193954.1879948-1-ihor.solodrai@linux.dev/

Your diagnosis is correct, the HOSTPKG_CONFIG wasn't set.
I fixed that by adding a default in resolve_btfids/Makefile,
but your diff makes sense too, I think we should apply both.

> 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
>>
>>