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Date:   Mon, 11 May 2020 13:54:35 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <dccp@...r.kernel.org>, <kernel-team@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Gerrit Renker <gerrit@....abdn.ac.uk>,
        Jakub Kicinski <kuba@...nel.org>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next 02/17] bpf: Introduce SK_LOOKUP program type
 with a dedicated attach point

On Mon, May 11, 2020 at 09:26:02PM +0200, Jakub Sitnicki wrote:
> On Mon, May 11, 2020 at 08:59 PM CEST, Martin KaFai Lau wrote:
> > On Mon, May 11, 2020 at 11:08:15AM +0200, Jakub Sitnicki wrote:
> >> On Fri, May 08, 2020 at 08:39 PM CEST, Martin KaFai Lau wrote:
> >> > On Fri, May 08, 2020 at 12:45:14PM +0200, Jakub Sitnicki wrote:
> >> >> On Fri, May 08, 2020 at 09:06 AM CEST, Martin KaFai Lau wrote:
> >> >> > On Wed, May 06, 2020 at 02:54:58PM +0200, Jakub Sitnicki wrote:
> >>
> >> [...]
> >>
> >> >> >> +		return -ESOCKTNOSUPPORT;
> >> >> >> +
> >> >> >> +	/* Check if socket is suitable for packet L3/L4 protocol */
> >> >> >> +	if (sk->sk_protocol != ctx->protocol)
> >> >> >> +		return -EPROTOTYPE;
> >> >> >> +	if (sk->sk_family != ctx->family &&
> >> >> >> +	    (sk->sk_family == AF_INET || ipv6_only_sock(sk)))
> >> >> >> +		return -EAFNOSUPPORT;
> >> >> >> +
> >> >> >> +	/* Select socket as lookup result */
> >> >> >> +	ctx->selected_sk = sk;
> >> >> > Could sk be a TCP_ESTABLISHED sk?
> >> >>
> >> >> Yes, and what's worse, it could be ref-counted. This is a bug. I should
> >> >> be rejecting ref counted sockets here.
> >> > Agree. ref-counted (i.e. checking rcu protected or not) is the right check
> >> > here.
> >> >
> >> > An unrelated quick thought, it may still be fine for the
> >> > TCP_ESTABLISHED tcp_sk returned from sock_map because of the
> >> > "call_rcu(&psock->rcu, sk_psock_destroy);" in sk_psock_drop().
> >> > I was more thinking about in the future, what if this helper can take
> >> > other sk not coming from sock_map.
> >>
> >> I see, psock holds a sock reference and will not release it until a full
> >> grace period has elapsed.
> >>
> >> Even if holding a ref wasn't a problem, I'm not sure if returning a
> >> TCP_ESTABLISHED socket wouldn't trip up callers of inet_lookup_listener
> >> (tcp_v4_rcv and nf_tproxy_handle_time_wait4), that look for a listener
> >> when processing a SYN to TIME_WAIT socket.
> > Not suggesting the sk_assign helper has to support TCP_ESTABLISHED in TCP
> > if there is no use case for it.
> 
> Ack, I didn't think you were. Just explored the consequences.
> 
> > Do you have a use case on supporting TCP_ESTABLISHED sk in UDP?
> > From the cover letter use cases, it is not clear to me it is
> > required.
> >
> > or both should only support unconnected sk?
> 
> No, we don't have a use case for selecting a connected UDP socket.
> 
> I left it as a possiblity because __udp[46]_lib_lookup, where BPF
> sk_lookup is invoked from, can return one.
> 
> Perhaps the user would like to connect the selected receiving socket
> (for instance to itself) to ensure its not used for TX?
> 
> I've pulled this scenario out of the hat. Happy to limit bpf_sk_assign
> to select only unconnected UDP sockets, if returning a connected one
> doesn't make sense.
OTOH, my concern is:
TCP's SK_LOOKUP can override the kernel choice on TCP_LISTEN sk.
UDP's SK_LOOKUP can override the kernel choice on unconnected sk but
not the connected sk.

It could be quite confusing to bpf user if a bpf_prog was written to return
both connected and unconnected UDP sk and logically expect both
will be done before the kernel's choice.

> 
> > Regardless, this details will be useful in the helper's doc.
> 
> I've reworded the helper doc in v2 to say:
> 
>         Description
>                 ...
> 
>                 Only TCP listeners and UDP sockets, that is sockets
>                 which have *SOCK_RCU_FREE* flag set, can be selected.
> 
>                 ...
>         Return
>                 ...
> 
>                 **-ESOCKTNOSUPPORT** if socket does not use RCU freeing.

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