Subject: Re: [PATCH net] sctp: fix a success return may hide an error

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Tue, 16 Aug 2016 14:24:47 -0300

On Tue, Aug 16, 2016 at 04:01:50PM +0000, David Laight wrote:
> From: Xin Long
> > Sent: 16 August 2016 12:34
> >
> > >> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
> > >> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
> > >> sctp_packet_transmit.
> > >
> > > What is the effect of the error?
> > > If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> > > is freed) then the protocol will recover.
> > > If it is anything else then the error path is probably wrong.
> >
> > This err returns back to sctp_sendmsg, there sctp will abort asoc.

That's not right I think. sctp_sendmsg will only free the asoc if it was
created to send that specific chunk. And in this case, this change
should have no effect as it can't have sctp_outq_flush() touching
several transports in a row.

I'm basing on:
out_free:
        if (new_asoc)
                sctp_association_free(asoc);

and sctp_recvmsg will just fetch, return and clear the error via
sctp_skb_recv_datagram, but not free it.

Do you see any other place freeing it?

> 
> That doesn't seem a good idea.
> You don't want to abort the association if there is a transient
> memory allocation failure.
> You also can't drop data chunks.

>>From a system-wise POV, this behavior - to free the new asoc in case of
transient memory allocation failure - doesn't seem bad to me.
That's what will have to happen if any allocation before it failed and
also it helps the system to reduce the stress a little bit. I don't see
any inconsistency/problems here because we are not dropping a single
random chunk but instead we are actually refusing to initialize a new
asoc in such conditions.

Nevertheless, I agree that letting the application see ENOMEM errors when
the data actually got queued and is being fully handled, as in, it will
be retransmitted later, is not be wise, as the application probably
won't be able to distinguish from ENOMEMs that it should retry or not.
Here I see a problem, yet it's not due to this specific change, perhaps
it just got attention because of it. In this situation, we should handle
ENOMEMs internally if possible so the application can know that if it
hits an ENOMEM, it's real and it has to retry.

Fixing this inconsistency may very well cause us to let that new asoc to
live longer, works for me too.

> 
> > in this function, sctp tries to do 3 things:
> > 1. flush rtx queue
> > 2. transmit the packet of current transport
> > 3. flush all the transports.
> > Now sctp would do them one by one, even if one of them returns err.
> 
> You probably need to explain what 'flush' means here.
> I think it means 'process and send', but it might mean 'discard the
> contents of'.

Yes, the first. He probably use the work 'flush' because the function is
called .._flush_..

> Last time I looked at the sctp code my head exploded.
> ISTR it is a mess of timing errors waiting to happen
> (and I write comms protocol stack code for a living).

Well, it may be, but we are trying to improve it.  Please continue
discussing the fixes so we can keep improving it. :)

  Marcelo