Re: [PATCH v3 bpf-next 3/3] selftests/bpf: add custom SEC() handling selftest

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH v3 bpf-next 3/3] selftests/bpf: add custom SEC() handling selftest
Date: Mon, 14 Feb 2022 16:29:47 -0800	[thread overview]
Message-ID: <CAADnVQ+Rcim71k8z2ifsEbJRRFwPHCxhOmzv+VCL3Qk1kMtKsA@mail.gmail.com> (raw)
In-Reply-To: <alpine.LRH.2.23.451.2202142305590.25685@MyRouter>

On Mon, Feb 14, 2022 at 3:14 PM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On Mon, 14 Feb 2022, Alexei Starovoitov wrote:
>
> > Alan,
> > can you demo your "okprobe" feature based on this api?
> > Any rough patches would do.
>
> sure; see below.  Requires Andrii's v3 patches to be applied first,
> and demonstrates okprobe handling for a kprobe function that exists
> and one that doesn't - the important thing is skeleton attach
> can succeed even when a function is missing (as it would be if
> the associated module wasn't loaded).
>
> > The "o" handling will be done in which callback?
> >
>
> We set program type at init and do custom attach using the function
> name (specified in the program section after the "okprobe" prefix).
> However we make sure to catch -ENOENT attach failures and return 0
> with a NULL link so skeleton attach can proceed.
>
> From 9bbd615b71f8f59ff743608bc86d7a2a346da2a8 Mon Sep 17 00:00:00 2001
> From: Alan Maguire <alan.maguire@oracle.com>
> Date: Mon, 14 Feb 2022 22:57:56 +0000
> Subject: [PATCH bpf-next] selftests/bpf: demonstrate further use of custom
>  SEC() handling
>
> Register and use SEC() handling for "okprobe/" kprobe programs
> (Optional kprobe) which should be attached as kprobes but
> critically should not stop skeleton loading if attach fails
> due to non-existence of the to-be-probed function.  This mode
> of SEC() handling is useful for tracing module functions
> where the module might not be loaded.
>
> Note - this patch is based on the v3 of Andrii's section
> handling patches [1] and these need to be applied for it to
> apply cleanly.
>
> [1] https://lore.kernel.org/bpf/20220211211450.2224877-1-andrii@kernel.org/
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

Thanks. The patch certainly helps to understand the api usage.

>  static int custom_init_prog(struct bpf_program *prog, long cookie)
>  {
>         if (cookie == COOKIE_ABC1)
>                 bpf_program__set_autoload(prog, false);
> +       else if (cookie == COOKIE_OKPROBE)
> +               bpf_program__set_type(prog, BPF_PROG_TYPE_KPROBE);

I think bpf_program__set_type() would have worked
from the prepare_load_fn callback as well.

What would be a recommended way of setting it?