Re: [PATCH] sctp: Honour SCTP_PARTIAL_DELIVERY_POINT even under memory pressure

Hi!

On Thu, Sep 03, 2020 at 01:21:48PM +0200, Petr Malat wrote:

Hi!
On Wed, Sep 02, 2020 at 11:58:35AM -0300, Marcelo Ricardo Leitner wrote:

quoted

On Tue, Sep 01, 2020 at 11:00:07AM +0200, Petr Malat wrote:

quoted

Command SCTP_CMD_PART_DELIVER issued under memory pressure calls
sctp_ulpq_partial_delivery(), which tries to fetch and partially deliver
the first message it finds without checking if the message is longer than
SCTP_PARTIAL_DELIVERY_POINT. According to the RFC 6458 paragraph 8.1.21.
such a behavior is invalid. Fix it by returning the first message only if
its part currently available is longer than SCTP_PARTIAL_DELIVERY_POINT.

Okay but AFAICT this patch then violates the basic idea behind partial
delivery. It will cause such small message to just not be delivered
anymore, and keep using the receive buffer which it is trying to free
some bits at this moment.

By default the pd_point is set to 0, so there will not be a change in the
behavior, but if the user changes it to some other value, it should be
respected by the stack - for example when the largest message the user
exchanges is 1kB and the user sets it to 1kB, his application is not
prepared to handle fragmented messages at all and it's not a good idea to
pass such a message to the app.

Then, if it doesn't support fragmented messages at all, the app just
shouldn't be setting pd_point at all. :-) Anyhow, I see how the patch
works now.

The fix is also needed on sctp_intl_retrieve_first() and
sctp_intl_retrieve_first_uo(). Same issue is in there, but for stream
interleaving.

Thanks.