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Commit 18ded910 authored by Neal Cardwell's avatar Neal Cardwell Committed by Jakub Kicinski
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tcp: fix to update snd_wl1 in bulk receiver fast path



In the header prediction fast path for a bulk data receiver, if no
data is newly acknowledged then we do not call tcp_ack() and do not
call tcp_ack_update_window(). This means that a bulk receiver that
receives large amounts of data can have the incoming sequence numbers
wrap, so that the check in tcp_may_update_window fails:
   after(ack_seq, tp->snd_wl1)

If the incoming receive windows are zero in this state, and then the
connection that was a bulk data receiver later wants to send data,
that connection can find itself persistently rejecting the window
updates in incoming ACKs. This means the connection can persistently
fail to discover that the receive window has opened, which in turn
means that the connection is unable to send anything, and the
connection's sending process can get permanently "stuck".

The fix is to update snd_wl1 in the header prediction fast path for a
bulk data receiver, so that it keeps up and does not see wrapping
problems.

This fix is based on a very nice and thorough analysis and diagnosis
by Apollon Oikonomopoulos (see link below).

This is a stable candidate but there is no Fixes tag here since the
bug predates current git history. Just for fun: looks like the bug
dates back to when header prediction was added in Linux v2.1.8 in Nov
1996. In that version tcp_rcv_established() was added, and the code
only updates snd_wl1 in tcp_ack(), and in the new "Bulk data transfer:
receiver" code path it does not call tcp_ack(). This fix seems to
apply cleanly at least as far back as v3.2.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Reported-by: default avatarApollon Oikonomopoulos <apoikos@dmesg.gr>
Tested-by: default avatarApollon Oikonomopoulos <apoikos@dmesg.gr>
Link: https://www.spinics.net/lists/netdev/msg692430.html


Acked-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201022143331.1887495-1-ncardwell.kernel@gmail.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 700465fd
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