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Date:   Sat, 16 Nov 2019 23:12:04 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 0/6] Add libbpf-provided extern variables support

On Sat, Nov 16, 2019 at 11:08 PM Andrii Nakryiko <andriin@...com> wrote:
>
> It's often important for BPF program to know kernel version or some specific
> config values (e.g., CONFIG_HZ to convert jiffies to seconds) and change or
> adjust program logic based on their values. As of today, any such need has to
> be resolved by recompiling BPF program for specific kernel and kernel
> configuration. In practice this is usually achieved by using BCC and its
> embedded LLVM/Clang. With such set up #ifdef CONFIG_XXX and similar
> compile-time constructs allow to deal with kernel varieties.
>
> With CO-RE (Compile Once – Run Everywhere) approach, this is not an option,
> unfortunately. All such logic variations have to be done as a normal
> C language constructs (i.e., if/else, variables, etc), not a preprocessor
> directives. This patch series add support for such advanced scenarios through
> C extern variables. These extern variables will be recognized by libbpf and
> supplied through extra .extern internal map, similarly to global data. This
> .extern map is read-only, which allows BPF verifier to track its content
> precisely as constants. That gives an opportunity to have pre-compiled BPF
> program, which can potentially use BPF functionality (e.g., BPF helpers) or
> kernel features (types, fields, etc), that are available only on a subset of
> targeted kernels, while effectively eleminating (through verifier's dead code
> detection) such unsupported functionality for other kernels (typically, older
> versions). Patch #5 contains all the details. Patch #5 explicitly tests
> a scenario of using unsupported BPF helper, to validate the approach.
>
> As part of this patch set, libbpf also allows usage of initialized global
> (non-static) variables, which provides better Clang semantics, which is closer
> and better aligned witht kernel vs userspace BPF map contents sharing.
>
> Outline of the patch set:
> - patches #1-#3 do some preliminary refactorings of libbpf relocation logic
>   and some more clean ups;
> - patch #4 allows non-static variables and converts few tests to use them;
> - patch #5 adds support for externs to libbpf;
> - patch #6 adds tests for externs.

Forgot to mention, this patch set is based off of patch set adding
mmap()-support for BPF maps, because I use that functionality for
selftests. In the worst case scenario, I can switch tests to
old-fashioned bpf_map_update_elem()/bpf_map_lookup_elem() interface.

[...]

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