Re: finding libelf

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Feb 3, 2021 at 12:57 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 2/3/21 12:41 PM, Andrii Nakryiko wrote:
> > On Wed, Feb 3, 2021 at 12:36 PM Randy Dunlap <rdunlap@infradead.org> wrote:
> >>
> >> On 2/3/21 12:33 PM, Andrii Nakryiko wrote:
> >>> On Wed, Feb 3, 2021 at 12:15 PM Randy Dunlap <rdunlap@infradead.org> wrote:
> >>>>
> >>>> On 2/3/21 12:12 PM, Andrii Nakryiko wrote:
> >>>>> On Wed, Feb 3, 2021 at 12:09 PM Randy Dunlap <rdunlap@infradead.org> wrote:
> >>>>>>
> >>>>>> On 2/3/21 11:39 AM, Andrii Nakryiko 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.
> >>>>>>
> >>>>>>
> >>>>>> I don't think it's related to improper cleanup or old files/dirs
> >>>>>> laying around. I say that because I did a full build in a new output dir.
> >>>>>> and it still failed in the same way.
> >>>>>
> >>>>> If you cd tools/lib/bpf and run make there, does it detect those libraries?
> >>>>
> >>>> Yes:
> >>>>
> >>>> Auto-detecting system features:
> >>>> ...                        libelf: [ on  ]
> >>>> ...                          zlib: [ on  ]
> >>>> ...                           bpf: [ on  ]
> >>>>
> >>>>
> >>>
> >>> Sounds exactly like my case. I removed
> >>> $(O)/tools/bpf/resolve_btfids/{feature/,FEATURE-DUMP.libbpf} and it
> >>> started working.
> >>
> >> I already tried that with no success.
> >>
> >> I suppose that it could be related to how I do builds:
> >>
> >> make ARCH=x86_64 O=subdir -j4 all
> >>
> >> so subdir is a relative path, not an absolute path.
> >
> > so can you confirm this by specifying the absolute path to subdir?
>
> Yes, absolute output path works for libelf, zlib, and bpf:
>
> Auto-detecting system features:
> ...                        libelf: [  [32mon [m  ]
> ...                          zlib: [  [32mon [m  ]
> ...                           bpf: [  [32mon [m  ]
>
>
> Are {feature,FEATURE-DUMP} created in multiple places?
> I don't see them in $(OUTDIR)/tools -- there is no /bpf/ subdir there
> at all.
> I do see them in $(OUTDIR)/kernel/bpf/preload/ -- are those different
> feature files?
>

Yes, they are different. Preload builds its own copy of libbpf, while
resolve_btfids builds its own.

So it seems like [0] should fix your issue by not using feature
detection at all.

  [0] https://patchwork.kernel.org/project/netdevbpf/patch/20210203203445.3356114-1-andrii@kernel.org/

>
> thanks.
> --
> ~Randy
>