netdev - Re: [PATCH bpf-next v2 04/11] tcp_bpf: Don't let child socket inherit parent protocol ops on copy

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Date:   Mon, 13 Jan 2020 23:42:42 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Martin Lau <kafai@...com>
Cc:     "bpf\@vger.kernel.org" <bpf@...r.kernel.org>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        "kernel-team\@cloudflare.com" <kernel-team@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "John Fastabend" <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next v2 04/11] tcp_bpf: Don't let child socket inherit parent protocol ops on copy

On Mon, Jan 13, 2020 at 11:23 PM CET, Martin Lau wrote:
> On Fri, Jan 10, 2020 at 11:50:20AM +0100, Jakub Sitnicki wrote:
>> Prepare for cloning listening sockets that have their protocol callbacks
>> overridden by sk_msg. Child sockets must not inherit parent callbacks that
>> access state stored in sk_user_data owned by the parent.
>>
>> Restore the child socket protocol callbacks before the it gets hashed and
>> any of the callbacks can get invoked.
>>
>> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
>> ---
>>  include/net/tcp.h        |  1 +
>>  net/ipv4/tcp_bpf.c       | 13 +++++++++++++
>>  net/ipv4/tcp_minisocks.c |  2 ++
>>  3 files changed, 16 insertions(+)
>>
>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>> index 9dd975be7fdf..7cbf9465bb10 100644
>> --- a/include/net/tcp.h
>> +++ b/include/net/tcp.h
>> @@ -2181,6 +2181,7 @@ int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>  		    int nonblock, int flags, int *addr_len);
>>  int __tcp_bpf_recvmsg(struct sock *sk, struct sk_psock *psock,
>>  		      struct msghdr *msg, int len, int flags);
>> +void tcp_bpf_clone(const struct sock *sk, struct sock *child);
>>
>>  /* Call BPF_SOCK_OPS program that returns an int. If the return value
>>   * is < 0, then the BPF op failed (for example if the loaded BPF
>> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
>> index f6c83747c71e..6f96320fb7cf 100644
>> --- a/net/ipv4/tcp_bpf.c
>> +++ b/net/ipv4/tcp_bpf.c
>> @@ -586,6 +586,19 @@ static void tcp_bpf_close(struct sock *sk, long timeout)
>>  	saved_close(sk, timeout);
>>  }
>>
>> +/* If a child got cloned from a listening socket that had tcp_bpf
>> + * protocol callbacks installed, we need to restore the callbacks to
>> + * the default ones because the child does not inherit the psock state
>> + * that tcp_bpf callbacks expect.
>> + */
>> +void tcp_bpf_clone(const struct sock *sk, struct sock *newsk)
>> +{
>> +	struct proto *prot = newsk->sk_prot;
>> +
>> +	if (prot->recvmsg == tcp_bpf_recvmsg)
> A question not related to this patch (may be it is more for patch 6).
>
> How tcp_bpf_recvmsg may be used for a listening sock (sk here)?

It can't be used. It's a way of checking if sock has tcp_bpf callbacks
that I copied from sk_psock_get_checked:

static inline struct sk_psock *sk_psock_get_checked(struct sock *sk)
{
	struct sk_psock *psock;

	rcu_read_lock();
	psock = sk_psock(sk);
	if (psock) {
		if (sk->sk_prot->recvmsg != tcp_bpf_recvmsg) {
			psock = ERR_PTR(-EBUSY);
			goto out;
		}
        ...

This makes me think that perhaps it deserves a well-named helper.

>
>> +		newsk->sk_prot = sk->sk_prot_creator;
>> +}
>> +