netdev - Re: [PATCH bpf-next v2] libbpf: fix probe code to return EPERM if encountered

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Date:   Mon, 4 May 2020 11:43:01 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Eelco Chaudron <echaudro@...hat.com>
Cc:     bpf <bpf@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v2] libbpf: fix probe code to return EPERM if encountered

On Mon, May 4, 2020 at 2:13 AM Eelco Chaudron <echaudro@...hat.com> wrote:
>
> When the probe code was failing for any reason ENOTSUP was returned, even
> if this was due to no having enough lock space. This patch fixes this by
> returning EPERM to the user application, so it can respond and increase
> the RLIMIT_MEMLOCK size.
>
> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
> ---
> v2: Split bpf_object__probe_name() in two functions as suggested by Andrii

Yeah, looks good, and this is good enough, so consider you have my
ack. But I think we can further improve the experience by:

1. Changing existing "Couldn't load basic 'r0 = 0' BPF program."
message to be something more meaningful and actionable for user. E.g.,

"Couldn't load trivial BPF program. Make sure your kernel supports BPF
(CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is set to big enough
value."

Then even complete kernel newbies can search for CONFIG_BPF_SYSCALL or
RLIMIT_MEMLOCK and hopefully find useful discussions. We can/should
add RLIMIT_MEMLOCK examples to some FAQ, probably as well (if it's not
there already).

2. I'd do bpf_object__probe_loading() before obj->loaded is set, so
that user can have a loop of bpf_object__load() that bump
RLIMIT_MEMLOCK in steps. After setting obj->loaded = true, user won't
be able to attemp loading again and will get "object should not be
loaded twice\n".

[...]