Re: linux-next: build failure after merge of the bpf-next tree

On Mon, Aug 24, 2020 at 11:50 PM Stephen Rothwell [off-list ref] wrote:

Hi Alexei,

On Mon, 24 Aug 2020 20:27:28 -0700 Alexei Starovoitov [off-list ref] wrote:

quoted

I didn't receive the first email you've replied to.
The build error is:
"
No libelf found
make[5]: *** [Makefile:284: elfdep] Error 1
"
and build process stops because libelf is not found, right?
That is expected and necessary.
bpf_preload needs libbpf that depends on libelf.
The only 'fix' is to turn off bpf_preload.
It's off by default.
allmodconfig cannot build bpf_preload umd if there is no libelf.
There is CC_CAN_LINK that does feature detection.
We can extend scripts/cc-can-link.sh or add another script that
will do CC_CAN_LINK_LIBELF, but such approach doesn't scale.
imo it's cleaner to rely on feature detection by libbpf Makefile with
an error above instead of adding such knobs to top Kconfig.
Does it make sense?

Sorry, but if this is not necessary to build the kernel, then an
allmodconfig build needs to succeed so you need to do the detection and
turn it off automatically.  Or you could make it so that it has to be
manually enabled in all circumstances.

what do you suggest to use to make it 'manually enabled' ?
All I could think of is to add:
depends on !COMPILE_TEST
so that allmodconfig doesn't pick it up.