netdev - 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 13:24:15 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     "Allan, Bruce W" <bruce.w.allan@...el.com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andrii@...nel.org>, 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

On Mon, 16 Nov 2020 12:34:17 -0800 Andrii Nakryiko wrote:
> > 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?

For an external out-of-tree module there is no guarantee that vmlinux
will even be on the system, no? So only the last option can work in
that case.