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

[Andrii Nakryiko]

On Mon, Nov 16, 2020 at 11:55 AM Allan, Bruce W <bruce.w.allan@xxxxxxxxx> wrote:
>
> > -----Original Message-----
> > From: Song Liu <songliubraving@xxxxxx>
> > Sent: Tuesday, November 10, 2020 5:05 PM
> > To: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Cc: bpf <bpf@xxxxxxxxxxxxxxx>; Networking <netdev@xxxxxxxxxxxxxxx>;
> > Starovoitov, Alexei <ast@xxxxxx>; Daniel Borkmann <daniel@xxxxxxxxxxxxx>;
> > Kernel Team <Kernel-team@xxxxxx>; open list <linux-
> > kernel@xxxxxxxxxxxxxxx>; rafael@xxxxxxxxxx; jeyu@xxxxxxxxxx; Arnaldo
> > Carvalho de Melo <acme@xxxxxxxxxx>; Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx>; Masahiro Yamada
> > <yamada.masahiro@xxxxxxxxxxxxx>
> > 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> >
> > Acked-by: Song Liu <songliubraving@xxxxxx>
>
> 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?

I wasn't aware of such a use pattern, so definitely not intentional.
But vmlinux is absolutely necessary to generate the module BTF. So I'm
wondering what's the proper fix here? Leave it as is (that error
message is actually surprisingly descriptive, btw)? Force vmlinux
build? Or skip BTF generation for that module?

>
> Thanks,
> Bruce.