netdev - Re: [PATCH bpf-next v5 3/8] bpf: move prog->aux->linked_prog and trampoline into bpf

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Date:   Wed, 16 Sep 2020 11:22:08 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 3/8] bpf: move prog->aux->linked_prog and
 trampoline into bpf_link on attach

On Tue, Sep 15, 2020 at 5:50 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> In preparation for allowing multiple attachments of freplace programs, move
> the references to the target program and trampoline into the
> bpf_tracing_link structure when that is created. To do this atomically,
> introduce a new mutex in prog->aux to protect writing to the two pointers
> to target prog and trampoline, and rename the members to make it clear that
> they are related.
>
> With this change, it is no longer possible to attach the same tracing
> program multiple times (detaching in-between), since the reference from the
> tracing program to the target disappears on the first attach. However,
> since the next patch will let the caller supply an attach target, that will
> also make it possible to attach to the same place multiple times.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

Seems much more straightforward to me with mutex. And I don't have to
worry about various transient NULL states.

Acked-by: Andrii Nakryiko <andriin@...com>

>  include/linux/bpf.h     |   15 +++++++++------
>  kernel/bpf/btf.c        |    6 +++---
>  kernel/bpf/core.c       |    9 ++++++---
>  kernel/bpf/syscall.c    |   46 ++++++++++++++++++++++++++++++++++++++++------
>  kernel/bpf/trampoline.c |   12 ++++--------
>  kernel/bpf/verifier.c   |    9 +++++----
>  6 files changed, 67 insertions(+), 30 deletions(-)
>

[...]

> @@ -2583,19 +2598,38 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog)
>                       &bpf_tracing_link_lops, prog);
>         link->attach_type = prog->expected_attach_type;
>
> +       mutex_lock(&prog->aux->tgt_mutex);
> +
> +       if (!prog->aux->tgt_trampoline) {
> +               err = -ENOENT;
> +               goto out_unlock;
> +       }
> +       tr = prog->aux->tgt_trampoline;
> +       tgt_prog = prog->aux->tgt_prog;
> +
>         err = bpf_link_prime(&link->link, &link_primer);
>         if (err) {
> -               kfree(link);
> -               goto out_put_prog;
> +               goto out_unlock;
>         }

nit: unnecessary {} now

>
> -       err = bpf_trampoline_link_prog(prog);
> +       err = bpf_trampoline_link_prog(prog, tr);
>         if (err) {
>                 bpf_link_cleanup(&link_primer);
> -               goto out_put_prog;
> +               link = NULL;
> +               goto out_unlock;
>         }

[...]