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

[Vitaly Chikunov]

Ihor,

On Mon, Mar 02, 2026 at 02:42:11PM -0800, Ihor Solodrai wrote:
> On 3/2/26 12:27 PM, Vitaly Chikunov wrote:
> > 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@xxxxxxxxxx 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 $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null)
> >>>>>
> >>>>> This would still link zstd when present (needed for the static linking
> >>>>> case where libelf.a pulls in ZSTD symbols from elf_compress.o), but
> >>>>> avoid breaking dynamic builds on systems without libzstd-dev.
> >>>>
> >>>> [...]
> >>>>
> >>>>
> >>>> So, while AI may be technically correct that the build will fail if
> >>>> libelf is installed and libzstd is not, it seems a far-fetched
> >>>> situation, given that libelf depends on libzstd.
> >>>
> >>> This is not far-fetched, and we have build failure for v7.0-rc2 due to
> >>> this.
> >>>
> >>>   ld: cannot find -lzstd: No such file or directory
> >>>
> >>> Even though libelf is linked with libzstd, this does not imply
> >>> libzstd-devel (with headers and so library) is there when building.
> >>
> >> Does AI's suggestion make sense in your case then?
> >> That is, make ZSTD_LIBS empty in case pkg-config didn't find libzstd?
> >>
> >> I'm happy to fix this, the build shouldn't fail unless it must.
> >>
> >> But I am curious how and why an environment building Linux with BTF
> >> (requiring build and run of resolve_btfids), which needs libelf-dev
> >> and presumably its dependencies, would exclude/avoid installing
> >> libzstd-dev?
> > 
> > Are you providing -lzstd just to link with libelf? I don't think you need to
> 
> An explicit -lzstd flag was added to enable a static build [1].
> 
> > care about zstd in that case. libelf is already linked with libzstd. If you
> > don't use libzstd functions yourself you don't need to link with -lzstd.
> > 
> > Example build without -lzstd:
> > 
> >   builder@x86_64:~/RPM/BUILD/kernel-image-7.0-rc2$ 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.
> 
> The issue that AI has raised is whether to leave -lzstd link flag by
> default or not. I decided to leave it on the assumption that the
> environments building Linux with BTF (hence building and running
> resovle_btfids) would have libelf-dev installed (because -lelf has
> been a requirement forever [2]), and libzstd-dev is its dependency.
> 
> I checked a few recent distros, all of them have libzstd-dev as a
> direct dependency of libelf-dev, which supports my assumption:
> 
>   # Fedora
>   $ dnf repoquery --providers-of=depends elfutils-libelf-devel
>   Updating and loading repositories:
>   Repositories loaded.
>   elfutils-libelf-0:0.194-1.fc43.i686
>   elfutils-libelf-0:0.194-1.fc43.x86_64
>   libzstd-devel-0:1.5.7-2.fc43.i686
>   libzstd-devel-0:1.5.7-2.fc43.x86_64

Interesting, but looks like this is not needed package dependency.

  [fodora:rawhide@docker /]# grep zstd /usr/lib64/pkgconfig/libelf.pc
  Requires.private: zlib libzstd

It's only a private dependency not exposed to the application.
pkg-config will not add -lzstd to link with libelf.so. (Unless it's
-static linking.) See pkg-config(1):

  Requires.private:
    A list of packages required by this package. The difference from Requires is
    that the packages listed under Requires.private are not taken into account when
    a flag list is computed for dynamically linked executable (i.e., when --static
    was not specified).  In the situation where each .pc file corresponds to a
    library, Requires.private shall be used exclusively to specify the dependencies
    between the libraries.

Perhaps, for your static linking case, you just need to extract -static from
EXTRA_LDFLAGS and add it to `pkg-config libelf --libs` to get the list of
static libraries to link with libelf, without defining ZSTD_LIBS at all.
This may provide other important flags besides these, see for example:

  alt:sisyphus# pkg-config libelf --libs --static
  -lelf -lz -lzstd -pthread

I think it's better to rely on pkg-config instead of guessing all the required
linking flags.


>   pkgconf-pkg-config-0:2.3.0-3.fc43.i686
>   pkgconf-pkg-config-0:2.3.0-3.fc43.x86_64
>   zlib-ng-compat-devel-0:2.3.3-1.fc43.i686
>   zlib-ng-compat-devel-0:2.3.3-1.fc43.x86_64
> 
>   # Ubuntu
>   $ apt info libelf-dev
>   Package: libelf-dev
>   Version: 0.190-1.1ubuntu0.1
>   Priority: optional
>   Section: libdevel
>   Source: elfutils
>   Origin: Ubuntu
>   Maintainer: Ubuntu Developers <ubuntu-devel-discuss@xxxxxxxxxxxxxxxx>
>   Original-Maintainer: Debian Elfutils Maintainers <debian-gcc@xxxxxxxxxxxxxxxx>
>   Bugs: https://bugs.launchpad.net/ubuntu/+filebug
>   Installed-Size: 385 kB
>   Depends: libelf1t64 (= 0.190-1.1ubuntu0.1), zlib1g-dev, libzstd-dev
>   Conflicts: libelfg0-dev
>   [...]
> 
>   # Debian
>   $ apt info libelf-dev
>   Package: libelf-dev
>   Version: 0.192-4
>   Priority: optional
>   Section: libdevel
>   Source: elfutils
>   Maintainer: Debian Elfutils Maintainers <debian-gcc@xxxxxxxxxxxxxxxx>
>   Installed-Size: 420 kB
>   Depends: libelf1t64 (= 0.192-4), zlib1g-dev, libzstd-dev
>   Conflicts: libelfg0-dev
>   [...]

I don't know why these distros do this sort of dependency. In ALT Linux we
don't require libzstd-devel for linelf-devel, also, I just checked OpenMandriva
and it does not require this too. But, openSUSE requires libzstd-devel.

> Of course it's plausible to have a system where libelf-dev is present
> while libzstd-dev is not, as demonstrated by you running one.
> 
> Anyways this is easy to fix, I'll send a patch shortly.

Thanks,

> 
> [1] https://lore.kernel.org/bpf/4ff82800-2daa-4b9f-95a9-6f512859ee70@xxxxxxxxx/
> [2] https://lore.kernel.org/bpf/20200711215329.41165-2-jolsa@xxxxxxxxxx/
> 
> 
> > 
> > Thanks,
> > 
> >>
> >> Thanks.
> >>
> >>>
> >>> Thanks,
> >>>
> >>>>
> >>>> I think we can leave the default -lzstd to have an explicit
> >>>> dependency in the Makefile.
> >>>>
> >>>>
> >>>>>
> >>>>> [ ... ]
> >>