netdev - Re: [PATCH bpf-next v2 3/6] libbpf: Propagate EPERM to caller on program load

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Date:   Fri, 8 Nov 2019 14:50:43 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 3/6] libbpf: Propagate EPERM to caller on
 program load

On Fri, Nov 8, 2019 at 1:33 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> When loading an eBPF program, libbpf overrides the return code for EPERM
> errors instead of returning it to the caller. This makes it hard to figure
> out what went wrong on load.
>
> In particular, EPERM is returned when the system rlimit is too low to lock
> the memory required for the BPF program. Previously, this was somewhat
> obscured because the rlimit error would be hit on map creation (which does
> return it correctly). However, since maps can now be reused, object load
> can proceed all the way to loading programs without hitting the error;
> propagating it even in this case makes it possible for the caller to react
> appropriately (and, e.g., attempt to raise the rlimit before retrying).
>
> Acked-by: Song Liu <songliubraving@...com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  tools/lib/bpf/libbpf.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index cea61b2ec9d3..582c0fd16697 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3721,7 +3721,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
>                 free(log_buf);
>                 goto retry_load;
>         }
> -       ret = -LIBBPF_ERRNO__LOAD;
> +       ret = (errno == EPERM) ? -errno : -LIBBPF_ERRNO__LOAD;
>         cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>         pr_warn("load bpf program failed: %s\n", cp);
>
> @@ -3749,7 +3749,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
>                         }
>                 }
>
> -               if (log_buf)
> +               if (log_buf && ret != -EPERM)
>                         ret = -LIBBPF_ERRNO__KVER;

This whole special casing of EPERM looks weird. Should we just pass
through all the errors instead?

But also, I don't think you can assume that if you get EPERM, then it
must be setrlimit problem...


>         }
>
>