netdev - Re: [PATCH net] datagram: When peeking datagrams with offset

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Message-ID: <13347366.nbnduK6AMs@tjmaciei-mobl1>
Date:   Mon, 14 Aug 2017 10:02:23 -0700
From:   Thiago Macieira <thiago.macieira@...el.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     Matthew Dawson <matthew@...systems.ca>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] datagram: When peeking datagrams with offset < 0 don't skip empty skbs

On Monday, 14 August 2017 09:33:48 PDT Willem de Bruijn wrote:
> > But here's a question: if the peek offset is equal to the length, should
> > the reading return an empty datagram? This would indicate to the caller
> > that there was a datagram there, which was skipped over.
> 
> In the general case, no, it should read at the offset, which is the next
> skb.

I beg to differ. In this particular case, we are talking about datagrams. If 
it were stream sockets, I would agree with you: just skip to the next. But in 
datagrams, the same way you do return zero-sized ones, I would return an empty 
one if you peeked at or past the end.

> Since we only need to change no-offset semantics to fix this bug,
> I would not change this behavior, which is also expected by some
> applications by now.

Do applications using SOCK_DGRAM rely on the behaviour of skipping over 
datagrams that are too short?

> > That's how we deal with empty datagrams anyway.
> 
> What is? With no-offset and a zero payload skb at the head, peek
> or recv returns 0, right?

Right.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center