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From:   Stanislav Fomichev <sdf@google.com>
Date:   Fri, 6 Jan 2023 16:47:35 -0800
Message-ID: <CAKH8qBuakT6qtY5TZomWEAB=1ZJfdgXYt2A7WVOjtHAYsdbrVA@mail.gmail.com>
Subject: Re: [PATCH bpf-next 0/3] Annotate kfuncs with new __bpf_kfunc macro
To:     David Vernet <void@manifault.com>
Cc:     bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
        andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
        yhs@meta.com, john.fastabend@gmail.com, kpsingh@kernel.org,
        haoluo@google.com, jolsa@kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com
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On Fri, Jan 6, 2023 at 11:51 AM David Vernet <void@manifault.com> wrote:
>
> BPF kfuncs are kernel functions that can be invoked by BPF programs.
> kfuncs can be kernel functions which are also called elsewhere in the
> main kernel (such as crash_kexec()), or may be functions that are only
> meant to be used by BPF programs, such as bpf_task_acquire(), and which
> are not called from anywhere else in the kernel.
>
> While thus far we haven't observed any issues such as kfuncs being
> elided by the compiler, at some point we could easily run into problems
> such as the following:
>
> - static kernel functions that are also used as kfuncs could be inlined
>   and/or elided by the compiler.
> - BPF-specific kfuncs with external linkage may at some point be elided
>   by the compiler in LTO builds, when it's determined that they aren't
>   called anywhere.
>
> To address this, this patch set introduces a new __bpf_kfunc macro which
> should be added to all kfuncs, and which will protect kfuncs from such
> problems. Note that some kfuncs kind of try to do this already by
> specifying noinline or __used. We are inconsistent in how this is
> applied. __bpf_kfunc should provide a uniform and more-future-proof way
> to do this.

The series looks reasonable to me. Would be nice if we can somehow
prevent (with a checkpatch?) adding new kfuncs without this new tag,
but I don't see an easy way.
I was waiting in case other would like to comment, but if nothing to discuss:

Acked-by: Stanislav Fomichev <sdf@google.com>


> David Vernet (3):
>   bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs
>   bpf: Document usage of the new __bpf_kfunc macro
>   bpf: Add __bpf_kfunc tag to all kfuncs
>
>  Documentation/bpf/kfuncs.rst                  | 18 +++++
>  Documentation/conf.py                         |  3 +
>  include/linux/btf.h                           |  9 +++
>  kernel/bpf/helpers.c                          | 19 +++++
>  kernel/cgroup/rstat.c                         |  2 +
>  kernel/kexec_core.c                           |  2 +
>  kernel/trace/bpf_trace.c                      |  4 +
>  net/bpf/test_run.c                            | 76 ++++++++++++-------
>  net/ipv4/tcp_bbr.c                            |  8 ++
>  net/ipv4/tcp_cong.c                           |  5 ++
>  net/ipv4/tcp_cubic.c                          |  6 ++
>  net/ipv4/tcp_dctcp.c                          |  6 ++
>  net/netfilter/nf_conntrack_bpf.c              | 14 +++-
>  net/netfilter/nf_nat_bpf.c                    |  1 +
>  net/xfrm/xfrm_interface_bpf.c                 |  4 +-
>  .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  2 +-
>  16 files changed, 146 insertions(+), 33 deletions(-)
>
> --
> 2.39.0
>