netdev - Re: [PATCH bpf-next v6 05/11] bpf: Adds field bpf_sock_ops_cb_flags to tcp

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Date:   Tue, 23 Jan 2018 09:29:35 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lawrence Brakmo <brakmo@...com>
Cc:     netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        Blake Matheny <bmatheny@...com>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH bpf-next v6 05/11] bpf: Adds field bpf_sock_ops_cb_flags
 to tcp_sock

On Fri, 2018-01-19 at 19:52 -0800, Alexei Starovoitov wrote:
> On Fri, Jan 19, 2018 at 05:45:42PM -0800, Lawrence Brakmo wrote:
> > Adds field bpf_sock_ops_cb_flags to tcp_sock and bpf_sock_ops. Its primary
> > use is to determine if there should be calls to sock_ops bpf program at
> > various points in the TCP code. The field is initialized to zero,
> > disabling the calls. A sock_ops BPF program can set it, per connection and
> > as necessary, when the connection is established.
> > 
> > It also adds support for reading and writting the field within a
> > sock_ops BPF program. Reading is done by accessing the field directly.
> > However, writing is done through the helper function
> > bpf_sock_ops_cb_flags_set, in order to return an error if a BPF program
> > is trying to set a callback that is not supported in the current kernel
> > (i.e. running an older kernel). The helper function returns 0 if it was
> > able to set all of the bits set in the argument, a positive number
> > containing the bits that could not be set, or -EINVAL if the socket is
> > not a full TCP socket.
> 
> ...
> > +/* Sock_ops bpf program related variables */
> > +#ifdef CONFIG_BPF
> > +	u8	bpf_sock_ops_cb_flags;  /* Control calling BPF programs
> > +					 * values defined in uapi/linux/tcp.h
> 
> I guess we can extend u8 into u16 or more if necessary in the future.
> 
> > + * int bpf_sock_ops_cb_flags_set(bpf_sock_ops, flags)
> > + *     Set callback flags for sock_ops
> > + *     @bpf_sock_ops: pointer to bpf_sock_ops_kern struct
> > + *     @flags: flags value
> > + *     Return: 0 for no error
> > + *             -EINVAL if there is no full tcp socket
> > + *             bits in flags that are not supported by current kernel
> 
> ...
> > +BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
> > +	   int, argval)
> > +{
> > +	struct sock *sk = bpf_sock->sk;
> > +	int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;
> > +
> > +	if (!sk_fullsock(sk))
> > +		return -EINVAL;
> > +
> > +#ifdef CONFIG_INET
> > +	if (val)
> > +		tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
> > +
> > +	return argval & (~BPF_SOCK_OPS_ALL_CB_FLAGS);
> 
> interesting idea! took me some time to realize the potential
> of such semantics, but now I like it a lot.
> It blends 'set good flag' with 'which flags are supported' logic
> into single helper. Nice.
> Thanks for adding a test for both ways.
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> 
> Eric, does this approach address your concerns?


Yes, this seems fine, thanks.