netdev - Re: [PATCH v4 bpf-next 2/4] libbpf: support libbpf-provided extern variables

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Message-ID: <20191217201613.iccqsqwuhitsyqyl@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 17 Dec 2019 12:16:14 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 2/4] libbpf: support libbpf-provided extern
 variables

On Tue, Dec 17, 2019 at 08:50:31PM +0100, Daniel Borkmann wrote:
> > 
> > Yes, name collision is a possibility, which means users should
> > restrain from using LINUX_KERNEL_VERSION and CONFIG_XXX names for
> > their variables. But if that is ever actually the problem, the way to
> > resolve this collision/ambiguity would be to put externs in a separate
> > sections. It's possible to annotate extern variable with custom
> > section.
> > 
> > But I guess putting Kconfig-provided externs into ".extern.kconfig"
> > might be a good idea, actually. That will make it possible to have
> > writable externs in the future.
> 
> Yep, and as mentioned it will make it more clear that these get special
> loader treatment as opposed to regular externs we need to deal with in
> future. A '.extern.kconfig' section sounds good to me and the BPF helper
> header could provide a __kconfig annotation for that as well.

I think annotating all extern vars into special section name will be quite
cumbersome from bpf program writer pov.
imo capital case extern variables LINUX_KERNEL_VERSION and CONFIG_XXX are
distinct enough and make it clear they should come from something other than
normal C. Traditional C coding style uses all capital letters for macroses. So
all capital extern variables are unlikely to conflict with any normal extern
vars. Like vars in vmlinux and vars in other bpf elf files.