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

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Date:   Mon, 16 Nov 2020 19:55:15 +0000
From:   "Allan, Bruce W" <bruce.w.allan@...el.com>
To:     Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andrii@...nel.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Starovoitov, Alexei" <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        open list <linux-kernel@...r.kernel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: RE: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF
 is enabled and pahole supports it

> -----Original Message-----
> From: Song Liu <songliubraving@...com>
> Sent: Tuesday, November 10, 2020 5:05 PM
> To: Andrii Nakryiko <andrii@...nel.org>
> Cc: bpf <bpf@...r.kernel.org>; Networking <netdev@...r.kernel.org>;
> Starovoitov, Alexei <ast@...com>; Daniel Borkmann <daniel@...earbox.net>;
> Kernel Team <Kernel-team@...com>; open list <linux-
> kernel@...r.kernel.org>; rafael@...nel.org; jeyu@...nel.org; Arnaldo
> Carvalho de Melo <acme@...hat.com>; Greg Kroah-Hartman
> <gregkh@...uxfoundation.org>; Masahiro Yamada
> <yamada.masahiro@...ionext.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@...nel.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@...ionext.com>
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> 
> Acked-by: Song Liu <songliubraving@...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.