Re: [PATCH v6 3/3] tcp: allow retransmit when peer receive window is zero

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 17, 2024 at 11:24:14AM -0400, Jon Maloy wrote:
> A bug in kernel TCP may lead to a deadlock where a zero window is sent
> from the peer, while it is unable to send out window updates even after
> reads have freed up enough buffer space to permit a larger window.
> In this situation, new window advertisemnts from the peer can only be
> triggered by packets arriving from this side.
> 
> However, such packets are never sent, because the zero-window condition
> currently prevents this side from sending out any packets whatsoever
> to the peer.
> 
> We notice that the above bug is triggered *only* after the peer has
> dropped an arriving packet because of severe memory squeeze, and that we
> hence always enter a retransmission situation when this occurs. This
> also means that it goes against the RFC 9293 recommendation that a
> previously advertised window never should shrink.
> 
> RFC 9293 gives the solution to this situation. In chapter 3.6.1 we find
> the following statement:
> "A TCP receiver SHOULD NOT shrink the window, i.e., move the right
> window edge to the left (SHLD-14). However, a sending TCP peer MUST
> be robust against window shrinking, which may cause the
> "usable window" (see Section 3.8.6.2.1) to become negative (MUST-34).
> 
> If this happens, the sender SHOULD NOT send new data (SHLD-15), but
> SHOULD retransmit normally the old unacknowledged data between SND.UNA
> and SND.UNA+SND.WND (SHLD-16). The sender MAY also retransmit old data
> beyond SND.UNA+SND.WND (MAY-7)"

So... I'm beginning to think this section of the rfc isn't really
helpful or useful here.  For starters, it doesn't seem to cover all of
what we're trying to do here - particularly the fact that we try to
send keepalive probes when in this situation...

> We never see the window become negative, but we interpret this as a
> recommendation to use the previously available window during
> retransmission even when the currently advertised window is zero.

... but also, looking at the RFC, I'm really not convinced of this
interpretation.  SND.WND generally refers to the last window we've
seen advertised by the guest, and I don't see any indication that in
this specific case we should instead consider the previous version it
had.

Indeed the "usable window" value it's discussing is elsewhere
described in terms of SND.WND, and if we used the previous SND.WND
value it would *not* become negative.

I believe that last MAY-7 bit means we're not violating the RFC by
using the previous window edge, but I don't think there's anything
there to suggest we must or should be doing so.

[In fact, I wonder if the reason behind MAY-7 is that it allows an
 implementation to satisfy this by just ignoring ignore window updates
 which would move the right edge backwards]

So.. moving on from the RFC to what we actually need to do to
workaround this bug.  Do we actually need anything more than
continuing to send keep-alive probes even when the window is zero?

> We use the above mechanism only for timer-induced retransmits, while
> the fast-retransmit mechanism won't trigger on this condition.
> 
> It should be noted that although this solves the problem we have at
> hand, it is not a genuine solution to the kernel bug. There may well
> be TCP stacks around in other OS-es which don't do this, nor have
> keep-alive probing as an alternatve way to solve the situation.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> 
> ---
> v2: - Using previously advertised window during retransmission, instead
>       highest send sequencece number in the cycle.
> v3: - Rebased to newest code
>     - Changes based on feedback from PASST team
>     - Sending out empty probe message at timer expiration when
>       we are not in retransmit situation.
> v4: - Some small changes based on feedback from PASST team.
>     - Replaced fast retransmit with a one-time 'fast probe' when
>       window is zero.
> v5: - Gave up on 'fast probing' for now. When I got the sequence
>       numbers right in the flag message (after having emptied the tap
>       queue), it turns out an empty message does *not* force a new peer
>       window update as was my previous understanding of the code.
>     - Added cppcheck suppression line (which I was unable to verify)
>       as suggested by S. Brivio.
>     - Removed sending of empty probe when window from tap side is zero.
>       It looks pointless at the moment, at least for solving the above
>       described situation.
> v6: - Ensure that arrival of new data doesn´t cause us to ignore a
>       zero-window situation.
>     - Removed the pointless probing referred to in v5 comment.
> ---
>  tcp.c      | 26 ++++++++++++++++++++------
>  tcp_conn.h |  2 ++
>  2 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index fa13292..38c3480 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1764,9 +1764,17 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,
>   */
>  static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd)
>  {
> +	uint32_t wnd_edge;
> +
>  	wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);
> +	/* cppcheck-suppress [knownConditionTrueFalse, unmatchedSuppression] */

If I recall from earlier, we thought this suppression was needed
because of the cppcheck bug referenced in tcp_update_seqack_wnd().  If
that's the case we need something like that comment here as well:
knownConditionTrueFalse is not a check we should be suppressing
lightly.

But also... is it actually that bug?  In that case the check tripped
when we did an if based on the result of the MIN - it thought it was
always zero.  But here the suppression is on the MIN itself, which
suggests something different.  Is it instead that cppcheck is managing
to deduce that wnd >> conn->ws_from_tap cannot be greater than
USHRT_MAX.  Which should indeed be the case, although I can't quickly
see how you'd statically deduce it.

I'm also not sure why this is showing up now, because these lines
aren't changed.

> +

I also don't think inserting a blank line between the suppression and
the line where the error is occuring is a good idea.

>  	conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);
>  
> +	wnd_edge = conn->seq_ack_from_tap + wnd;
> +	if (wnd && SEQ_GT(wnd_edge, conn->seq_wnd_edge_from_tap))
> +		conn->seq_wnd_edge_from_tap = wnd_edge;
> +
>  	/* FIXME: reflect the tap-side receiver's window back to the sock-side
>  	 * sender by adjusting SO_RCVBUF? */
>  }
> @@ -1799,6 +1807,7 @@ static void tcp_seq_init(const struct ctx *c, struct tcp_tap_conn *conn,
>  	ns = (now->tv_sec * 1000000000 + now->tv_nsec) >> 5;
>  
>  	conn->seq_to_tap = ((uint32_t)(hash >> 32) ^ (uint32_t)hash) + ns;
> +	conn->seq_wnd_edge_from_tap = conn->seq_to_tap;
>  }
>  
>  /**
> @@ -2208,13 +2217,12 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>   */
>  static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  {
> -	uint32_t wnd_scaled = conn->wnd_from_tap << conn->ws_from_tap;
>  	int fill_bufs, send_bufs = 0, last_len, iov_rem = 0;
>  	int sendlen, len, dlen, v4 = CONN_V4(conn);
> +	uint32_t already_sent, max_send, seq;
>  	int s = conn->sock, i, ret = 0;
>  	struct msghdr mh_sock = { 0 };
>  	uint16_t mss = MSS_GET(conn);
> -	uint32_t already_sent, seq;
>  	struct iovec *iov;
>  
>  	/* How much have we read/sent since last received ack ? */
> @@ -2228,19 +2236,24 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  		tcp_set_peek_offset(s, 0);
>  	}
>  
> -	if (!wnd_scaled || already_sent >= wnd_scaled) {
> +	/* How much are we still allowed to send within current window ? */
> +	max_send = conn->seq_wnd_edge_from_tap - conn->seq_to_tap;
> +	if (SEQ_LE(max_send, 0)) {

Although the maths probably works out correctly, I dislike using
SEQ_LE on sequence differences here, rather that using SEQ_LE directly
on seq_wnd_edge_from_tap and seq_to_tap.

> +		flow_trace(conn, "Window full: right edge: %u, sent: %u",
> +			   conn->seq_wnd_edge_from_tap, conn->seq_to_tap);
> +		conn->seq_wnd_edge_from_tap = conn->seq_to_tap;

So, here we pull seq_wnd_edge_from_tap back in line with seq_to_tap.
Which might be before even the "current" window of seq_ack_to_tap +
wnd_scaled.  Which means there's a pretty brief window in which
seq_wnd_edge_from_tap will actually be beyond the latest window.  It's
not clear to me why that brief window is important - or why getting
more data from the socket side would be relevant to finishing that
window.

>  		conn_flag(c, conn, STALLED);
>  		conn_flag(c, conn, ACK_FROM_TAP_DUE);
>  		return 0;
>  	}
>  
>  	/* Set up buffer descriptors we'll fill completely and partially. */
> -	fill_bufs = DIV_ROUND_UP(wnd_scaled - already_sent, mss);
> +	fill_bufs = DIV_ROUND_UP(max_send,  mss);
>  	if (fill_bufs > TCP_FRAMES) {
>  		fill_bufs = TCP_FRAMES;
>  		iov_rem = 0;
>  	} else {
> -		iov_rem = (wnd_scaled - already_sent) % mss;
> +		iov_rem = max_send % mss;
>  	}
>  
>  	/* Prepare iov according to kernel capability */
> @@ -2347,6 +2360,7 @@ err:
>   *
>   * Return: count of consumed packets
>   */
> +

Spurious whitespace change.

>  static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>  			      const struct pool *p, int idx)
>  {
> @@ -2950,7 +2964,7 @@ void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events)
>  		if (events & (EPOLLRDHUP | EPOLLHUP))
>  			conn_event(c, conn, SOCK_FIN_RCVD);
>  
> -		if (events & EPOLLIN)
> +		if (events & EPOLLIN && conn->wnd_from_tap)

Hrm.  If we don't even enter tcp_data_from_sock() when there's no
window, doesn't that mean we won't hit the handling for the max_send <
0 case, we won't set STALLED, won't switch the epoll flags for the
socket to edge triggered mode and will therefore just busy loop on
EPOLLIN socket events until the window re-opens.

>  			tcp_data_from_sock(c, conn);
>  
>  		if (events & EPOLLOUT)
> diff --git a/tcp_conn.h b/tcp_conn.h
> index d280b22..5cbad2a 100644
> --- a/tcp_conn.h
> +++ b/tcp_conn.h
> @@ -30,6 +30,7 @@
>   * @wnd_to_tap:		Sending window advertised to tap, unscaled (as sent)
>   * @seq_to_tap:		Next sequence for packets to tap
>   * @seq_ack_from_tap:	Last ACK number received from tap
> + * @seq_wnd_edge_from_tap: Right edge of last non-zero window from tap
>   * @seq_from_tap:	Next sequence for packets from tap (not actually sent)
>   * @seq_ack_to_tap:	Last ACK number sent to tap
>   * @seq_init_from_tap:	Initial sequence number from tap
> @@ -101,6 +102,7 @@ struct tcp_tap_conn {
>  
>  	uint32_t	seq_to_tap;
>  	uint32_t	seq_ack_from_tap;
> +	uint32_t	seq_wnd_edge_from_tap;
>  	uint32_t	seq_from_tap;
>  	uint32_t	seq_ack_to_tap;
>  	uint32_t	seq_init_from_tap;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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