Re: [PATCH net] datagram: When peeking datagrams with offset < 0 don't skip empty skbs

On Mon, Aug 14, 2017 at 12:06 PM, Thiago Macieira
[off-list ref] wrote:

On Monday, 14 August 2017 08:03:50 PDT Willem de Bruijn wrote:

quoted

quoted

I'm actually surprised that only unix sockets can have negative values.
Is
there a reason for that?  I had assumed that sk_set_peek_off would allow
negative values as the code already has to support negative values due to
what the initial value is.

A negative initial value indicates that PEEK_OFF is disabled. It only
makes sense to peek from a positive offset from the start of the data.

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.

Zero length packets are a special case. This did come up before and
we chose to signal their existence in the queue by returning 0 for each
once, even in the offset-enabled mode.

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.

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?