netdev - RE: Use of genradix in sctp

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Date:   Fri, 21 Aug 2020 21:39:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>
CC:     "'linux-sctp@...r.kernel.org'" <linux-sctp@...r.kernel.org>,
        "'netdev@...r.kernel.org'" <netdev@...r.kernel.org>
Subject: RE: Use of genradix in sctp

From: 'Marcelo Ricardo Leitner'
> Sent: 21 August 2020 21:47
...
> > 3) Defer the allocation until the stream is used.
> >    for outbound streams this could remove the extra buffer.
> 
> This can be tricky. What should happen if it gets a packet on a stream
> that it couldn't allocate, and then another on a stream that was
> already allocated? Just a drop, it will retransmit and recover, and
> then again.. While, OTOH, if the application requested such amount of
> streams, it is likely going to use it. If not, that's an application
> bug.

You'd probably need to (effectively) drop the ethernet frame
that contained the chunk.

But the problem I see is that GFP flags are passed in.
So there must me a path where the allocation can't sleep.
Now allocating a couple of pages is fine but if the
maximum is just over 300 for each of 'in' and 'out'.
I can well imagine that is likely to fail.
I suspect this happens because the remote system can
(if my quick scan of the code is right) negotiate a
much larger number on an active connection.

I don't know what applications might be doing such things.
But I can imagine someone will try to negotiate 64k-1
streams just because that is the maximum.
And/or deciding to use stream 65535 for 'special' traffic.

	David

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