Re: [PATCH bpf] tools/runqslower: Fix cross-build

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Fri, Nov 12, 2021 at 04:17:21PM +0000, Quentin Monnet wrote:
> 2021-11-12 15:51 UTC+0000 ~ Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Commit be79505caf3f ("tools/runqslower: Install libbpf headers when
> > building") uses the target libbpf to build the host bpftool, which
> > doesn't work when cross-building:
> > 
> >   make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/bpf/runqslower O=/tmp/runqslower
> >   ...
> >     LINK    /tmp/runqslower/bpftool/bpftool
> >   /usr/bin/ld: /tmp/runqslower/libbpf/libbpf.a(libbpf-in.o): Relocations in generic ELF (EM: 183)
> >   /usr/bin/ld: /tmp/runqslower/libbpf/libbpf.a: error adding symbols: file in wrong format
> >   collect2: error: ld returned 1 exit status
> > 
> > When cross-building, the target architecture differs from the host. The
> > bpftool used for building runqslower is executed on the host, and thus
> > must use a different libbpf than that used for runqslower itself.
> > Remove the LIBBPF_OUTPUT and LIBBPF_DESTDIR parameters, so the bpftool
> > build makes its own library if necessary.
> > 
> > In the selftests, pass the host bpftool, already a prerequisite for the
> > runqslower recipe, as BPFTOOL_OUTPUT. The runqslower Makefile will use
> > the bpftool that's already built for selftests instead of making a new
> > one.
> > 
> > Fixes: be79505caf3f ("tools/runqslower: Install libbpf headers when building")
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Reviewed-by: Quentin Monnet <quentin@isovalent.com>
> 
> I realised too late I should have cc-ed you on those patches, apologies
> for not doing so. Thank you for the fix!

No worries, I usually try to catch build issues in bpf-next but missed it
this time. I'm still slowly working towards getting automated testing on
Arm targets, which will catch regressions quicker.

Thanks,
Jean