linux-kernel - Re: [PATCH] libbpf: fix BPF_MAP_TYPE_PERF_EVENT

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Date:   Thu, 24 Feb 2022 17:38:17 +0200
From:   Stijn Tintel <stijn@...ux-ipv6.be>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Song Liu <song@...nel.org>
Cc:     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 24/02/2022 12:08, Stijn Tintel wrote:
> On 24/02/2022 01:15, Andrii Nakryiko wrote:
>> 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?
>>
> Will do!

Unfortunately that doesn't work. In bpf_object__create_maps, we call
bpf_object__reuse_map and map_is_reuse_compat before
bpf_object__create_map, so we check map_info.max_entries ==
map->def.max_entries before the latter is being overwritten.

So I propose to send a v2 based on my initial submission, but use __u32
for def_max_entries instead of int, unless someone has another suggestion?

Thanks,
Stijn