netdev - Re: [PATCH] libbpf: fix BPF_MAP_TYPE_PERF_EVENT

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Message-ID: <CAEf4BzYuk2Rur-pae7gbuXSb=ayJ0fUREStdWyorWgd_q1D9zQ@mail.gmail.com>
Date:   Wed, 23 Feb 2022 15:15:29 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <song@...nel.org>
Cc:     Stijn Tintel <stijn@...ux-ipv6.be>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        KP Singh <kpsingh@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Yonghong Song <yhs@...com>, Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] libbpf: fix BPF_MAP_TYPE_PERF_EVENT_ARRAY auto-pinning

On Tue, Feb 22, 2022 at 6:37 PM Song Liu <song@...nel.org> wrote:
>
> On Tue, Feb 22, 2022 at 12:51 PM Stijn Tintel <stijn@...ux-ipv6.be> wrote:
> >
> > When a BPF map of type BPF_MAP_TYPE_PERF_EVENT_ARRAY doesn't have the
> > max_entries parameter set, this parameter will be set to the number of
> > possible CPUs. Due to this, the map_is_reuse_compat function will return
> > false, causing the following error when trying to reuse the map:
> >
> > libbpf: couldn't reuse pinned map at '/sys/fs/bpf/m_logging': parameter mismatch
> >
> > Fix this by checking against the number of possible CPUs if the
> > max_entries parameter is not set in the map definition.
> >
> > Fixes: 57a00f41644f ("libbpf: Add auto-pinning of maps when loading BPF objects")
> > Signed-off-by: Stijn Tintel <stijn@...ux-ipv6.be>
>
> Acked-by: Song Liu <songliubraving@...com>
>
> I think the following fix would be more future proof, but the patch
> as-is is better for
> stable backport? How about we add a follow up patch on top of current
> patch to fix
> def->max_entries once for all?

Keeping special logic for PERF_EVENT_ARRAY in one place is
preferrable. With this, the changes in map_is_reuse_compat() shouldn't
be necessary at all. Stijn, can you please send v2 with Song's
proposed changes?

>
> Thanks,
> Song
>
> diff --git i/tools/lib/bpf/libbpf.c w/tools/lib/bpf/libbpf.c
> index ad43b6ce825e..a1bc1c80bc69 100644
> --- i/tools/lib/bpf/libbpf.c
> +++ w/tools/lib/bpf/libbpf.c
> @@ -4881,10 +4881,9 @@ static int bpf_object__create_map(struct
> bpf_object *obj, struct bpf_map *map, b
>                         return nr_cpus;
>                 }
>                 pr_debug("map '%s': setting size to %d\n", map->name, nr_cpus);
> -               max_entries = nr_cpus;
> -       } else {
> -               max_entries = def->max_entries;
> +               def->max_entries = nr_cpus;
>         }
> +       max_entries = def->max_entries;
>
>         if (bpf_map__is_struct_ops(map))
>                 create_attr.btf_vmlinux_value_type_id =
> map->btf_vmlinux_value_type_id;