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tipc: compensate for double accounting in socket rcv buffer
The function net/core/sock.c::__release_sock() runs a tight loop to move buffers from the socket backlog queue to the receive queue. As a security measure, sk_backlog.len of the receiving socket is not set to zero until after the loop is finished, i.e., until the whole backlog queue has been transferred to the receive queue. During this transfer, the data that has already been moved is counted both in the backlog queue and the receive queue, hence giving an incorrect picture of the available queue space for new arriving buffers. This leads to unnecessary rejection of buffers by sk_add_backlog(), which in TIPC leads to unnecessarily broken connections. In this commit, we compensate for this double accounting by adding a counter that keeps track of it. The function socket.c::backlog_rcv() receives buffers one by one from __release_sock(), and adds them to the socket receive queue. If the transfer is successful, it increases a new atomic counter 'tipc_sock::dupl_rcvcnt' with 'truesize' of the transferred buffer. If a new buffer arrives during this transfer and finds the socket busy (owned), we attempt to add it to the backlog. However, when sk_add_backlog() is called, we adjust the 'limit' parameter with the value of the new counter, so that the risk of inadvertent rejection is eliminated. It should be noted that this change does not invalidate the original purpose of zeroing 'sk_backlog.len' after the full transfer. We set an upper limit for dupl_rcvcnt, so that if a 'wild' sender (i.e., one that doesn't respect the send window) keeps pumping in buffers to sk_add_backlog(), he will eventually reach an upper limit, (2 x TIPC_CONN_OVERLOAD_LIMIT). After that, no messages can be added to the backlog, and the connection will be broken. Ordinary, well- behaved senders will never reach this buffer limit at all. Signed-off-by:Jon Maloy <jon.maloy@ericsson.com> Reviewed-by: