linux-kernel - Re: [PATCH v2 bpf 02/11] bpftool: define a local bpf_perf

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Date:   Fri, 14 Apr 2023 11:54:57 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Shung-Hsi Yu <shung-hsi.yu@...e.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Song Liu <songliubraving@...com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 bpf 02/11] bpftool: define a local bpf_perf_link to
 fix accessing its fields

Hello,

On Thu, Apr 21, 2022 at 12:38:58AM +0000, Alexander Lobakin wrote:
> When building bpftool with !CONFIG_PERF_EVENTS:
> 
> skeleton/pid_iter.bpf.c:47:14: error: incomplete definition of type 'struct bpf_perf_link'
>         perf_link = container_of(link, struct bpf_perf_link, link);
>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:74:22: note: expanded from macro 'container_of'
>                 ((type *)(__mptr - offsetof(type, member)));    \
>                                    ^~~~~~~~~~~~~~~~~~~~~~
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:68:60: note: expanded from macro 'offsetof'
>  #define offsetof(TYPE, MEMBER)  ((unsigned long)&((TYPE *)0)->MEMBER)
>                                                   ~~~~~~~~~~~^
> skeleton/pid_iter.bpf.c:44:9: note: forward declaration of 'struct bpf_perf_link'
>         struct bpf_perf_link *perf_link;
>                ^
> 
> &bpf_perf_link is being defined and used only under the ifdef.
> Define struct bpf_perf_link___local with the `preserve_access_index`
> attribute inside the pid_iter BPF prog to allow compiling on any
> configs. CO-RE will substitute it with the real struct bpf_perf_link
> accesses later on.
> container_of() is not CO-REd, but it is a noop for
> bpf_perf_link <-> bpf_link and the local copy is a full mirror of
> the original structure.
> 
> Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")

This does not solve the problem completely. Kernels that don't have
CONFIG_PERF_EVENTS in the first place are also missing the enum value
BPF_LINK_TYPE_PERF_EVENT which is used as the condition for handling the
cookie.

Thanks

Michal