netdev - Re: [PATCH bpf-next] libbpf: Skip the pinning of global data map for old kernels.

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Message-ID: <CAEf4BzbtQGnGZTLbTdy1GHK54f5S7YNFQak7BuEfaqGEwqNNJA@mail.gmail.com>
Date:   Thu, 9 Dec 2021 09:26:10 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Shuyi Cheng <chengshuyi@...ux.alibaba.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Skip the pinning of global data map for
 old kernels.

On Thu, Dec 9, 2021 at 12:44 AM Shuyi Cheng
<chengshuyi@...ux.alibaba.com> wrote:
>
>
> Fix error: "failed to pin map: Bad file descriptor, path:
> /sys/fs/bpf/_rodata_str1_1."
>
> In the old kernel, the global data map will not be created, see [0]. So
> we should skip the pinning of the global data map to avoid
> bpf_object__pin_maps returning error.
>
> [0]: https://lore.kernel.org/bpf/20211123200105.387855-1-andrii@kernel.org
>
> Signed-off-by: Shuyi Cheng <chengshuyi@...ux.alibaba.com>
> ---
>   tools/lib/bpf/libbpf.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6db0b5e8540e..d96cf49cebab 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -7884,6 +7884,10 @@ int bpf_object__pin_maps(struct bpf_object *obj,
> const char *path)
>                 char *pin_path = NULL;
>                 char buf[PATH_MAX];
>
> +               if (bpf_map__is_internal(map) &&
> +                   !kernel_supports(obj, FEAT_GLOBAL_DATA))


doing the same check in 3 different places sucks. Let's add "bool
skipped" to struct bpf_map, which will be set in one place (at the map
creation time) and then check during relocation and during pinning?

> +                       continue;
> +
>                 if (path) {
>                         int len;
>
> --
> 2.19.1.6.gb485710b