netdev - Re: [RFC bpf-next 00/11] Add socket lookup support

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Message-ID: <20180516200410.utgdgcgxfbd5zu5y@ast-mbp>
Date:   Wed, 16 May 2018 13:04:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Joe Stringer <joe@...d.net.nz>
Cc:     daniel@...earbox.net, netdev <netdev@...r.kernel.org>,
        ast@...nel.org, john fastabend <john.fastabend@...il.com>,
        tgraf@...g.ch, Martin KaFai Lau <kafai@...com>
Subject: Re: [RFC bpf-next 00/11] Add socket lookup support

On Wed, May 16, 2018 at 12:05:06PM -0700, Joe Stringer wrote:
> >
> > A few open points:
> > * Currently, the lookup interface only returns either a valid socket or a NULL
> >   pointer. This means that if there is any kind of issue with the tuple, such
> >   as it provides an unsupported protocol number, or the socket can't be found,
> >   then we are unable to differentiate these cases from one another. One natural
> >   approach to improve this could be to return an ERR_PTR from the
> >   bpf_sk_lookup() helper. This would be more complicated but maybe it's
> >   worthwhile.
> 
> This suggestion would add a lot of complexity, and there's not many
> legitimately different error cases. There's:
> * Unsupported socket type
> * Cannot find netns
> * Tuple argument is the wrong size
> * Can't find socket
> 
> If we split the helpers into protocol-specific types, the first one
> would be addressed. The last one is addressed by returning NULL. It
> seems like a reasonable compromise to me to return NULL also in the
> middle two cases as well, and rely on the BPF writer to provide valid
> arguments.
> 
> > * No ordering is defined between sockets. If the tuple could find multiple
> >   sockets, then it will arbitrarily return one. It is up to the caller to
> >   handle this. If we wish to handle this more reliably in future, we could
> >   encode an ordering preference in the flags field.
> 
> Doesn't need to be addressed with this series, there is scope for
> addressing these cases when the use case arises.

Thanks for summarizing the conf call discussion.
Looking forward to non-rfc patches :)