Re: finding libelf

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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.

-Toke