From: Jakub Sitnicki <jakub@cloudflare.com>
To: Martin Lau <kafai@fb.com>
Cc: "bpf\@vger.kernel.org" <bpf@vger.kernel.org>,
"netdev\@vger.kernel.org" <netdev@vger.kernel.org>,
"kernel-team\@cloudflare.com" <kernel-team@cloudflare.com>,
John Fastabend <john.fastabend@gmail.com>
Subject: Re: [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy
Date: Tue, 26 Nov 2019 19:36:09 +0100 [thread overview]
Message-ID: <87d0deo57q.fsf@cloudflare.com> (raw)
In-Reply-To: <20191126171607.pzrg5qhbavh7enwh@kafai-mbp.dhcp.thefacebook.com>
On Tue, Nov 26, 2019 at 06:16 PM CET, Martin Lau wrote:
> On Tue, Nov 26, 2019 at 04:54:33PM +0100, Jakub Sitnicki wrote:
>> On Mon, Nov 25, 2019 at 11:38 PM CET, Martin Lau wrote:
>> > On Sat, Nov 23, 2019 at 12:07:47PM +0100, Jakub Sitnicki wrote:
>> > [ ... ]
>> >
>> >> @@ -370,6 +378,11 @@ static inline void sk_psock_restore_proto(struct sock *sk,
>> >> sk->sk_prot = psock->sk_proto;
>> >> psock->sk_proto = NULL;
>> >> }
>> >> +
>> >> + if (psock->icsk_af_ops) {
>> >> + icsk->icsk_af_ops = psock->icsk_af_ops;
>> >> + psock->icsk_af_ops = NULL;
>> >> + }
>> >> }
>> >
>> > [ ... ]
>> >
>> >> +static struct sock *tcp_bpf_syn_recv_sock(const struct sock *sk,
>> >> + struct sk_buff *skb,
>> >> + struct request_sock *req,
>> >> + struct dst_entry *dst,
>> >> + struct request_sock *req_unhash,
>> >> + bool *own_req)
>> >> +{
>> >> + const struct inet_connection_sock_af_ops *ops;
>> >> + void (*write_space)(struct sock *sk);
>> >> + struct sk_psock *psock;
>> >> + struct proto *proto;
>> >> + struct sock *child;
>> >> +
>> >> + rcu_read_lock();
>> >> + psock = sk_psock(sk);
>> >> + if (likely(psock)) {
>> >> + proto = psock->sk_proto;
>> >> + write_space = psock->saved_write_space;
>> >> + ops = psock->icsk_af_ops;
>> > It is not immediately clear to me what ensure
>> > ops is not NULL here.
>> >
>> > It is likely I missed something. A short comment would
>> > be very useful here.
>>
>> I can see the readability problem. Looking at it now, perhaps it should
>> be rewritten, to the same effect, as:
>>
>> static struct sock *tcp_bpf_syn_recv_sock(...)
>> {
>> const struct inet_connection_sock_af_ops *ops = NULL;
>> ...
>>
>> rcu_read_lock();
>> psock = sk_psock(sk);
>> if (likely(psock)) {
>> proto = psock->sk_proto;
>> write_space = psock->saved_write_space;
>> ops = psock->icsk_af_ops;
>> }
>> rcu_read_unlock();
>>
>> if (!ops)
>> ops = inet_csk(sk)->icsk_af_ops;
>> child = ops->syn_recv_sock(sk, skb, req, dst, req_unhash, own_req);
>>
>> If psock->icsk_af_ops were NULL, it would mean we haven't initialized it
>> properly. To double check what happens here:
> I did not mean the init path. The init path is fine since it init
> eveything on psock before publishing the sk to the sock_map.
>
> I was thinking the delete path (e.g. sock_map_delete_elem). It is not clear
> to me what prevent the earlier pasted sk_psock_restore_proto() which sets
> psock->icsk_af_ops to NULL from running in parallel with
> tcp_bpf_syn_recv_sock()? An explanation would be useful.
Ah, I misunderstood. Nothing prevents the race, AFAIK.
Setting psock->icsk_af_ops to null on restore and not checking for it
here was a bad move on my side. Also I need to revisit what to do about
psock->sk_proto so the child socket doesn't end up with null sk_proto.
This race should be easy enough to trigger. Will give it a shot.
Thank you for bringing this up,
Jakub
>
>>
>> In sock_map_link we do a setup dance where we first create the psock and
>> later initialize the socket callbacks (tcp_bpf_init).
>>
>> static int sock_map_link(struct bpf_map *map, struct sk_psock_progs *progs,
>> struct sock *sk)
>> {
>> ...
>> if (psock) {
>> ...
>> } else {
>> psock = sk_psock_init(sk, map->numa_node);
>> if (!psock) {
>> ret = -ENOMEM;
>> goto out_progs;
>> }
>> sk_psock_is_new = true;
>> }
>> ...
>> if (sk_psock_is_new) {
>> ret = tcp_bpf_init(sk);
>> if (ret < 0)
>> goto out_drop;
>> } else {
>> tcp_bpf_reinit(sk);
>> }
>>
>> The "if (sk_psock_new)" branch triggers the call chain that leads to
>> saving & overriding socket callbacks.
>>
>> tcp_bpf_init -> tcp_bpf_update_sk_prot -> sk_psock_update_proto
>>
>> Among them, icsk_af_ops.
>>
>> static inline void sk_psock_update_proto(...)
>> {
>> ...
>> psock->icsk_af_ops = icsk->icsk_af_ops;
>> icsk->icsk_af_ops = af_ops;
>> }
>>
>> Goes without saying that a comment is needed.
>>
>> Thanks for the feedback,
>> Jakub
next prev parent reply other threads:[~2019-11-26 18:36 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-23 11:07 [PATCH bpf-next 0/8] Extend SOCKMAP to store listening sockets Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 1/8] bpf, sockmap: Return socket cookie on lookup from syscall Jakub Sitnicki
2019-11-24 5:32 ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 2/8] bpf, sockmap: Let all kernel-land lookup values in SOCKMAP Jakub Sitnicki
2019-11-24 5:35 ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 3/8] bpf, sockmap: Allow inserting listening TCP sockets into SOCKMAP Jakub Sitnicki
2019-11-24 5:38 ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy Jakub Sitnicki
2019-11-24 5:56 ` John Fastabend
2019-11-25 22:38 ` Martin Lau
2019-11-26 15:54 ` Jakub Sitnicki
2019-11-26 17:16 ` Martin Lau
2019-11-26 18:36 ` Jakub Sitnicki [this message]
[not found] ` <87sglsfdda.fsf@cloudflare.com>
2019-12-11 17:20 ` Martin Lau
2019-12-12 11:27 ` Jakub Sitnicki
2019-12-12 19:23 ` Martin Lau
2019-12-17 15:06 ` Jakub Sitnicki
2019-11-26 18:43 ` John Fastabend
2019-11-27 22:18 ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a SOCKMAP Jakub Sitnicki
2019-11-24 5:57 ` John Fastabend
2019-11-25 1:24 ` Alexei Starovoitov
2019-11-25 4:17 ` John Fastabend
2019-11-25 10:40 ` Jakub Sitnicki
2019-11-25 22:07 ` Martin Lau
2019-11-26 14:30 ` Jakub Sitnicki
2019-11-26 19:03 ` Martin Lau
2019-11-27 21:34 ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 6/8] libbpf: Recognize SK_REUSEPORT programs from section name Jakub Sitnicki
2019-11-24 5:57 ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 7/8] selftests/bpf: Extend SK_REUSEPORT tests to cover SOCKMAP Jakub Sitnicki
2019-11-24 6:00 ` John Fastabend
2019-11-25 22:30 ` Martin Lau
2019-11-26 14:32 ` Jakub Sitnicki
2019-12-12 10:30 ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 8/8] selftests/bpf: Tests for SOCKMAP holding listening sockets Jakub Sitnicki
2019-11-24 6:04 ` John Fastabend
2019-11-24 6:10 ` [PATCH bpf-next 0/8] Extend SOCKMAP to store " John Fastabend
2019-11-25 9:22 ` Jakub Sitnicki
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