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

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

On Wed, Mar 04, 2026 at 11:24:54AM -0800, Ihor Solodrai wrote:
> 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/

Ah, v1 was still marked as New in patchwork (probably due to changed
subject) so I missed it wasn't the latest :')

> 
> 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
> >>
> >>
>