Subject: Re: Crash when receiving FIN-ACK in TCP_FIN_WAIT1 state

From: Subash Abhinov Kasiviswanathan <[email-masked]>
Date: Mon, 21 Oct 2019 18:04:43 -0600

> Interesting! As tcp_input.c summarizes, "packets_out is
> SND.NXT-SND.UNA counted in packets". In the normal operation of a
> socket, tp->packets_out should not be 0 if any of those other fields
> are non-zero.
> 
> The tcp_write_queue_purge() function sets packets_out to 0:
> 
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/tree/net/ipv4/tcp.c?h=v4.19#n2526
> 
> So the execution of tcp_write_queue_purge()  before this point is one
> way for the socket to end up in this weird state.
> 

In one of the instances, the values are tp->snd_nxt = 1016118098,
tp->snd_una = 1016047820

tp->mss_cache = 1378

I assume the number of outstanding segments should be
(tp->snd_nxt - tp->snd_una)/tp->mss_cache = 51

tp->packets_out = 0 and tp->sacked_out = 158 in this case.

>> > Yes, one guess would be that somehow the skbs in the retransmit queue
>> > have been freed, but tp->sacked_out is still non-zero and
>> > tp->highest_sack is still a dangling pointer into one of those freed
>> > skbs. The tcp_write_queue_purge() function is one function that fees
>> > the skbs in the retransmit queue and leaves tp->sacked_out as non-zero
>> > and  tp->highest_sack as a dangling pointer to a freed skb, AFAICT, so
>> > that's why I'm wondering about that function. I can't think of a
>> > specific sequence of events that would involve tcp_write_queue_purge()
>> > and then a socket that's still in FIN-WAIT1. Maybe I'm not being
>> > creative enough, or maybe that guess is on the wrong track. Would you
>> > be able to set a new bit in the tcp_sock in tcp_write_queue_purge()
>> > and log it in your instrumentation point, to see if
>> > tcp_write_queue_purge()  was called for these connections that cause
>> > this crash?

I've queued up a build which logs calls to tcp_write_queue_purge and
clears tp->highest_sack and tp->sacked_out. I will let you know how
it fares by end of week.

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