Re: finding libelf

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Feb 03, 2021 at 12:06:10PM -0800, Andrii Nakryiko wrote:
> On Wed, Feb 3, 2021 at 11:39 AM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Feb 3, 2021 at 9:22 AM Randy Dunlap <rdunlap@infradead.org> wrote:
> > >
> > > On 2/3/21 2:57 AM, Toke Høiland-Jørgensen wrote:
> > > > Randy Dunlap <rdunlap@infradead.org> writes:
> > > >
> > > >> Hi,
> > > >>
> > > >> I see this sometimes when building a kernel: (on x86_64,
> > > >> with today's linux-next 20210202):
> > > >>
> > > >>
> > > >> CONFIG_CGROUP_BPF=y
> > > >> CONFIG_BPF=y
> > > >> CONFIG_BPF_SYSCALL=y
> > > >> CONFIG_ARCH_WANT_DEFAULT_BPF_JIT=y
> > > >> CONFIG_BPF_PRELOAD=y
> > > >> CONFIG_BPF_PRELOAD_UMD=m
> > > >> CONFIG_HAVE_EBPF_JIT=y
> > > >>
> > > >>
> > > >> Auto-detecting system features:
> > > >> ...                        libelf: [ [31mOFF[m ]
> > > >> ...                          zlib: [ [31mOFF[m ]
> > > >> ...                           bpf: [ [31mOFF[m ]
> > > >>
> > > >> No libelf found
> > > >> make[5]: [Makefile:287: elfdep] Error 1 (ignored)
> > > >> No zlib found
> > > >> make[5]: [Makefile:290: zdep] Error 1 (ignored)
> > > >> BPF API too old
> > > >> make[5]: [Makefile:293: bpfdep] Error 1 (ignored)
> > > >>
> > > >>
> > > >> but pkg-config tells me:
> > > >>
> > > >> $ pkg-config --modversion  libelf
> > > >> 0.168
> > > >> $ pkg-config --libs  libelf
> > > >> -lelf
> > > >>
> > > >>
> > > >> Any ideas?
> > > >
> > > > This usually happens because there's a stale cache of the feature
> > > > detection tests lying around somewhere. Look for a 'feature' directory
> > > > in whatever subdir you got that error. Just removing the feature
> > > > directory usually fixes this; I've fixed a couple of places where this
> > > > is not picked up by 'make clean' (see, e.g., 9d9aae53b96d ("bpf/preload:
> > > > Make sure Makefile cleans up after itself, and add .gitignore")) but I
> > > > wouldn't be surprised if there are still some that are broken.
> > >
> > > Hi,
> > >
> > > Thanks for replying.
> > >
> > > I removed the feature subdir and still got this build error, so I
> > > removed everything in BUILDDIR/kernel/bpf/preload and rebuilt --
> > > and still got the same libelf build error.
> >
> > I hate the complexity of feature detection framework to the point that
> > I'm willing to rip it out from libbpf's Makefile completely. I just
> > spent an hour trying to understand what's going on in a very similar
> > situation. Extremely frustrating.
> >
> > In your case, it might be feature detection triggered from
> > resolve_btfids, so try removing
> > $(OUTPUT)/tools/bpf/resolve_btfids/{feature/,FEATURE-DUMP.libbpf}.
> >
> > It seems like we don't do proper cleanup in resolve_btfids (it should
> > probably call libbpf's clean as well). And it's beyond me why `make -C
> > tools/build/feature clean` doesn't clean up FEATURE-DUMP.<use-case>
> > file as well.
> 
> So resolve_btfids does call libbpf's clean, but Linux Kbuild never
> calls resolve_btfids' clean. Jiri, do you think that could be
> improved? Basically, if something goes wrong with feature detection,
> no amount of `make clean` would help and users will be forced to
> struggle with frustrating experience trying to understand what's going
> on.

ok, that one is missing.. will add

> 
> I also still think that FEATURE-DUMP should be cleaned up by feature
> infra on clean and that's not happening today, but I'm unwilling to go
> and untangle all that complexity right now.

I haven't seen this error for some time so I thought we got rid of it,
I'll try to reproduce and fix

jirka