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

[Vitaly Chikunov <vt@altlinux.org>]

Ihor,

On Mon, Mar 02, 2026 at 10:01:59AM -0800, Ihor Solodrai wrote:
> On 3/2/26 7:22 AM, Vitaly Chikunov wrote:
> > On Fri, Feb 20, 2026 at 08:23:51PM -0800, Ihor Solodrai wrote:
> >> On 2/20/26 2:57 PM, bot+bpf-ci@kernel.org wrote:
> >>>> 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.
> >>
> >> [...]
> >>
> >>
> >> So, while AI may be technically correct that the build will fail if
> >> libelf is installed and libzstd is not, it seems a far-fetched
> >> situation, given that libelf depends on libzstd.
> > 
> > This is not far-fetched, and we have build failure for v7.0-rc2 due to
> > this.
> > 
> >   ld: cannot find -lzstd: No such file or directory
> > 
> > Even though libelf is linked with libzstd, this does not imply
> > libzstd-devel (with headers and so library) is there when building.
> 
> Does AI's suggestion make sense in your case then?
> That is, make ZSTD_LIBS empty in case pkg-config didn't find libzstd?
> 
> I'm happy to fix this, the build shouldn't fail unless it must.
> 
> But I am curious how and why an environment building Linux with BTF
> (requiring build and run of resolve_btfids), which needs libelf-dev
> and presumably its dependencies, would exclude/avoid installing
> libzstd-dev?

Are you providing -lzstd just to link with libelf? I don't think you need to
care about zstd in that case. libelf is already linked with libzstd. If you
don't use libzstd functions yourself you don't need to link with -lzstd.

Example build without -lzstd:

  builder@x86_64:~/RPM/BUILD/kernel-image-7.0-rc2$ grep zstd tools/bpf/resolve_btfids/Makefile
  ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null)

  builder@x86_64:~/RPM/BUILD/kernel-image-7.0-rc2$ ldd ./tools/bpf/resolve_btfids/resolve_btfids
	  linux-vdso.so.1 (0x00007ff8b2329000)
	  libelf.so.1 => /lib64/libelf.so.1 (0x00007ff8b2287000)
	  libz.so.1 => /lib64/libz.so.1 (0x00007ff8b2269000)
	  libc.so.6 => /lib64/libc.so.6 (0x00007ff8b206e000)
	  libzstd.so.1 => /lib64/libzstd.so.1 (0x00007ff8b1fc8000)
	  /lib64/ld-linux-x86-64.so.2 (0x00007ff8b232b000)
  builder@x86_64:~/RPM/BUILD/kernel-image-7.0-rc2$ rpm -q libzstd
  libzstd-1.5.5-alt2.x86_64
  builder@x86_64:~/RPM/BUILD/kernel-image-7.0-rc2$ rpm -q libzstd-devel
  package libzstd-devel is not installed

lib*-devel/-dev packages only required if your source is directly using the
target lib, in other causes this is already handled.

Thanks,

> 
> Thanks.
> 
> > 
> > Thanks,
> > 
> >>
> >> I think we can leave the default -lzstd to have an explicit
> >> dependency in the Makefile.
> >>
> >>
> >>>
> >>> [ ... ]
>