Em 12-08-2015 12:33, David Laight escreveu:
From: Marcelo Ricardo Leitner
Sent: 12 August 2015 14:16
Em 12-08-2015 07:23, David Laight escreveu:
From: Marcelo Ricardo Leitner
Sent: 11 August 2015 23:22
DLM is using 1-to-many API but in a 1-to-1 fashion. That is, it's not
needed but this causes it to use sctp_do_peeloff() to mimic an
kernel_accept() and this causes a symbol dependency on sctp module.
By switching it to 1-to-1 API we can avoid this dependency and also
reduce quite a lot of SCTP-specific code in lowcomms.c.
...
You still need to enable sctp notifications (I think the patch deleted
that code).
Otherwise you don't get any kind of indication if the remote system
'resets' (ie sends an new INIT chunk) on an existing connection.
Right, it would miss the restart event and could generate a corrupted
tx/rx buffers by glueing parts of old messages with new ones.
Except that it is SCTP so you'd expect DATA chunks to contain entire
messages and so get unexpected message sequences rather than corrupt
messages.
I was thinking on cases where the buf for recvmsg is not enough to hold
the chunk, so that the remaining is left for another attempt
(sctp_recvmsg, around line 2130), but sounds like we won't purge rx
buffer when the reset happens so that doesn't matter. The association is
replaced, but the buffers are kept.
Out of order messages aren't a problem for dlm. It can recover from that
just fine, as it doesn't have a specific handshake at beginning or
something like that and upper layers are agnostic to that state
transition (disconnect/reconnect/...), this should be fine.
The problem is that the recovery is likely to be another reset.
(Particularly with M3UA where the source and destination port numbers
are likely to be the same and fixed.)
It is probably enough to treat the MSG_NOTIFICATION as a fatal error
and close the socket.
Just so we are on the same page, you mean that after accepting the new
association and enabling notifications on it, any further notification
on it can be treated as fatal errors, right? Seems reasonable to me.
That's what I had to do.
The far end will probably see an additional disconnect, but it shouldn't
matter.
Okay
This is probably a bug in the sctp stack - if a connection is reset
but the user hasn't requested notifications then it should be
converted to a disconnect indication and a new incoming connection.
Maybe in such case resets shouldn't be allowed at all? Because unless it
happens on a moment of silence it will always lead to application buffer
corruption. Checked the RFCs now but couldn't find anything restricting
them to some condition.
As said above, such corruption doesn't exist, and while checking this,
the reset is actually reported by a double report of established state
via sk_state_change(). The reset will trigger a call to
sctp_sf_do_dupcook_a() which will later schedule a state transition to
established for !udp sockets. For users in kernel at least, one could
use that as the reconnect signal.
I certainly expected the 'reset' to cause an inwards abortive disconnect
on the old socket and a new indication on the listening socket.
I was thinking that too but now seeing that it seems to work out of the
box with dlm, I liked the feature. ;)
I think (hope) that is what you get for a TCP SYN that matches an existing
connection.
In our case I think they were happening when the remote system was power
cycled.
And it has to be a fast one, so that heartbeats won't catch it in time.
Thanks,
Marcelo
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