RE: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF is enabled and pahole supports it

["Allan, Bruce W" <bruce.w.allan@intel.com>]

> -----Original Message-----
> From: Song Liu <songliubraving@fb.com>
> Sent: Tuesday, November 10, 2020 5:05 PM
> To: Andrii Nakryiko <andrii@kernel.org>
> Cc: bpf <bpf@vger.kernel.org>; Networking <netdev@vger.kernel.org>;
> Starovoitov, Alexei <ast@fb.com>; Daniel Borkmann <daniel@iogearbox.net>;
> Kernel Team <Kernel-team@fb.com>; open list <linux-
> kernel@vger.kernel.org>; rafael@kernel.org; jeyu@kernel.org; Arnaldo
> Carvalho de Melo <acme@redhat.com>; Greg Kroah-Hartman
> <gregkh@linuxfoundation.org>; Masahiro Yamada
> <yamada.masahiro@socionext.com>
> Subject: Re: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF is
> enabled and pahole supports it
> 
> 
> 
> > On Nov 9, 2020, at 5:19 PM, Andrii Nakryiko <andrii@kernel.org> wrote:
> 
> [...]
> 
> > SPLIT BTF
> > =========
> >
> > $ for f in $(find . -name '*.ko'); do size -A -d $f | grep BTF | awk '{print $2}';
> done | awk '{ s += $1 } END { print s }'
> > 5194047
> >
> > $ for f in $(find . -name '*.ko'); do printf "%s %d\n" $f $(size -A -d $f | grep
> BTF | awk '{print $2}'); done | sort -nr -k2 | head -n10
> > ./drivers/gpu/drm/i915/i915.ko 293206
> > ./drivers/gpu/drm/radeon/radeon.ko 282103
> > ./fs/xfs/xfs.ko 222150
> > ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko 198503
> > ./drivers/infiniband/hw/mlx5/mlx5_ib.ko 198356
> > ./drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko 113444
> > ./fs/cifs/cifs.ko 109379
> > ./arch/x86/kvm/kvm.ko 100225
> > ./drivers/gpu/drm/drm.ko 94827
> > ./drivers/infiniband/core/ib_core.ko 91188
> >
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> 
> Acked-by: Song Liu <songliubraving@fb.com>

This change, commit 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled and pahole
supports it") currently in net-next, linux-next, etc. breaks the use-case of compiling only a specific
kernel module (both in-tree and out-of-tree, e.g. 'make M=drivers/net/ethernet/intel/ice') after
first doing a 'make modules_prepare'.  Previously, that use-case would result in a warning noting
"Symbol info of vmlinux is missing. Unresolved symbol check will be entirely skipped" but now it
errors out after noting "No rule to make target 'vmlinux', needed by '<...>.ko'.  Stop."

Is that intentional?

Thanks,
Bruce.