netdev - Re: [PATCH 10/14] bpf: Re-initialize lnode in bpf_ksym

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Message-ID: <CAEf4Bzb-J67oKcKtB-7TsO7wD7bnp57NAgqNJW9giZrhrqu_+g@mail.gmail.com>
Date:   Tue, 11 Feb 2020 10:28:50 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH 10/14] bpf: Re-initialize lnode in bpf_ksym_del

On Sat, Feb 8, 2020 at 7:43 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> When bpf_prog is removed from kallsyms it's on the way
> out to be removed, so we don't care about lnode state.
>
> However the bpf_ksym_del will be used also by bpf_trampoline
> and bpf_dispatcher objects, which stay allocated even when
> they are not in kallsyms list, hence the lnode re-init.
>
> The list_del_rcu commentary states that we need to call
> synchronize_rcu, before we can change/re-init the list_head
> pointers.
>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---

Wouldn't it make more sense to have patches 7 though 10 as a one
patch? It's a generalization of ksym from being bpf_prog-specific to
be more general (which this initialization fix is part of, arguably).

>  kernel/bpf/core.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 73242fd07893..66b17bea286e 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -676,6 +676,13 @@ void bpf_ksym_del(struct bpf_ksym *ksym)
>         spin_lock_bh(&bpf_lock);
>         __bpf_ksym_del(ksym);
>         spin_unlock_bh(&bpf_lock);
> +
> +       /*
> +        * As explained in list_del_rcu, We must call synchronize_rcu
> +        * before changing list_head pointers.
> +        */
> +       synchronize_rcu();
> +       INIT_LIST_HEAD_RCU(&ksym->lnode);
>  }
>
>  static bool bpf_prog_kallsyms_candidate(const struct bpf_prog *fp)
> --
> 2.24.1
>