linux-kernel - Re: [PATCH] bpf, sockmap: defer sk_psock_free

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Message-ID: <87a5kfwe8l.fsf@cloudflare.com>
Date: Fri, 24 May 2024 15:06:50 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: John Fastabend <john.fastabend@...il.com>,  Daniel Borkmann
 <daniel@...earbox.net>,  Hillf Danton <hdanton@...a.com>,  Tetsuo Handa
 <penguin-kernel@...ove.sakura.ne.jp>,  Eric Dumazet <edumazet@...gle.com>,
  Linus Torvalds <torvalds@...ux-foundation.org>,  bpf
 <bpf@...r.kernel.org>,  LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf, sockmap: defer sk_psock_free_link() using RCU

On Wed, May 22, 2024 at 07:57 AM -07, Alexei Starovoitov wrote:
> On Wed, May 22, 2024 at 5:12 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>>
>> On Wed, May 22, 2024 at 07:33 PM +08, Hillf Danton wrote:
>> > On Wed, 22 May 2024 11:50:49 +0200 Jakub Sitnicki <jakub@...udflare.com>
>> > On Wed, May 22, 2024 at 06:59 AM +08, Hillf Danton wrote:
>> >> > On Tue, 21 May 2024 08:38:52 -0700 Alexei Starovoitov <alexei.starovoitov@...il.com>
>> >> >> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> >> >> > --- a/net/core/sock_map.c
>> >> >> > +++ b/net/core/sock_map.c
>> >> >> > @@ -142,6 +142,7 @@ static void sock_map_del_link(struct sock *sk,
>> >> >> >         bool strp_stop =3D false, verdict_stop =3D false;
>> >> >> >         struct sk_psock_link *link, *tmp;
>> >> >> >
>> >> >> > +       rcu_read_lock();
>> >> >> >         spin_lock_bh(&psock->link_lock);
>> >> >>
>> >> >> I think this is incorrect.
>> >> >> spin_lock_bh may sleep in RT and it won't be safe to do in rcu cs.
>> >> >
>> >> > Could you specify why it won't be safe in rcu cs if you are right?
>> >> > What does rcu look like in RT if not nothing?
>> >>
>> >> RCU readers can't block, while spinlock RT doesn't disable preemption.
>> >>
>> >> https://docs.kernel.org/RCU/rcu.html
>> >> https://docs.kernel.org/locking/locktypes.html#spinlock-t-and-preempt-rt
>> >>
>> >> I've finally gotten around to testing proposed fix that just disallows
>> >> map_delete_elem on sockmap/sockhash from BPF tracing progs
>> >> completely. This should put an end to this saga of syzkaller reports.
>> >>
>> >> https://lore.kernel.org/all/87jzjnxaqf.fsf@cloudflare.com/
>
> Agree. Let's do that. According to John the delete path is not something
> that is used in production. It's only a source of trouble with syzbot.

Cool. The proposed API rule would be that if a BPF program type is
allowed to update a sockmap/sockhash, then it is also allowed to delete
from it.

So I need to tweak my patch to allow deletes from sock_ops progs.
We have a dedicated bpf_sock_map_update() helper there.

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