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Re: [Qemu-devel] [PATCH v2 2/3] filter-rewriter: fix memory leak for con

From: Hailiang Zhang
Subject: Re: [Qemu-devel] [PATCH v2 2/3] filter-rewriter: fix memory leak for connection in connection_track_table
Date: Mon, 27 Feb 2017 11:11:52 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 
On 2017/2/23 12:16, Jason Wang wrote:



On 2017年02月22日 16:51, Hailiang Zhang wrote:

On 2017/2/22 16:45, Hailiang Zhang wrote:

On 2017/2/22 16:07, Jason Wang wrote:



On 2017年02月22日 11:46, zhanghailiang wrote:

After a net connection is closed, we didn't clear its releated
resources
in connection_track_table, which will lead to memory leak.


Not a real leak but would lead reset of hash table if too many closed
connections.



Yes, you are right, there will be lots of stale connection data in
hash table
if we don't remove it while it is been closed. Which



Ok, so let's come up with a better title of the patch.



OK.





Let't track the state of net connection, if it is closed, its related
resources will be cleared up.


The issue is the state were tracked partially, do we need a full state
machine here?



Not, IMHO, we only care about the last state of it, because, we will
do nothing
even if we track the intermedial states.


Well, you care at least syn state too. Without a complete state machine,
it's very hard to track even partial state I believe. And you will fail
to track some state transition for sure which makes the code fragile.



Agree, but here things are a little different. There are some extreme cases
that we may can't track the complete process of closing connection.
For example (I have explained that in the bellow, it seems that you didn't
got it ;) ).
If VM is running before we want to make it goes into COLO FT state,
there maybe some connections exist already, in extreme case, VM is going into
COLO state while some connections are in half closing state, we can only track
the bellow half closing state in filter-rewriter and colo compare object.





Signed-off-by: zhanghailiang <address@hidden>
---
     net/colo.h            |  4 +++
     net/filter-rewriter.c | 70
+++++++++++++++++++++++++++++++++++++++++++++------
     2 files changed, 67 insertions(+), 7 deletions(-)

diff --git a/net/colo.h b/net/colo.h
index 7c524f3..cd9027f 100644
--- a/net/colo.h
+++ b/net/colo.h
@@ -18,6 +18,7 @@
     #include "slirp/slirp.h"
     #include "qemu/jhash.h"
     #include "qemu/timer.h"
+#include "slirp/tcp.h"

     #define HASHTABLE_MAX_SIZE 16384

@@ -69,6 +70,9 @@ typedef struct Connection {
          * run once in independent tcp connection
          */
         int syn_flag;
+
+    int tcp_state; /* TCP FSM state */
+    tcp_seq fin_ack_seq; /* the seq of 'fin=1,ack=1' */
     } Connection;

     uint32_t connection_key_hash(const void *opaque);
diff --git a/net/filter-rewriter.c b/net/filter-rewriter.c
index c4ab91c..7e7ec35 100644
--- a/net/filter-rewriter.c
+++ b/net/filter-rewriter.c
@@ -60,9 +60,9 @@ static int is_tcp_packet(Packet *pkt)
     }

     /* handle tcp packet from primary guest */
-static int handle_primary_tcp_pkt(NetFilterState *nf,
+static int handle_primary_tcp_pkt(RewriterState *rf,
                                       Connection *conn,
-                                  Packet *pkt)
+                                  Packet *pkt, ConnectionKey *key)
     {
         struct tcphdr *tcp_pkt;

@@ -97,15 +97,45 @@ static int
handle_primary_tcp_pkt(NetFilterState *nf,
             tcp_pkt->th_ack = htonl(ntohl(tcp_pkt->th_ack) +
conn->offset);

             net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+        /*
+         * Case 1:
+         * The *server* side of this connect is VM, *client* tries
to close
+         * the connection.
+         *
+         * We got 'ack=1' packets from client side, it acks
'fin=1, ack=1'
+         * packet from server side. From this point, we can ensure
that there
+         * will be no packets in the connection, except that, some
errors
+         * happen between the path of 'filter object' and vNIC, if
this rare
+         * case really happen, we can still create a new connection,
+         * So it is safe to remove the connection from
connection_track_table.
+         *
+         */
+        if ((conn->tcp_state == TCPS_LAST_ACK) &&
+            (ntohl(tcp_pkt->th_ack) == (conn->fin_ack_seq + 1))) {
+            fprintf(stderr, "Remove conn "


Can this even compile?



Oops, i forgot to remove it, will remove it in next version.


+ g_hash_table_remove(rf->connection_track_table, key);
+        }
+    }
+    /*
+     * Case 2:
+     * The *server* side of this connect is VM, *server* tries to
close
+     * the connection.
+     *
+     * We got 'fin=1, ack=1' packet from client side, we need to
+     * record the seq of 'fin=1, ack=1' packet.
+     */
+    if ((tcp_pkt->th_flags & (TH_ACK | TH_FIN)) == (TH_ACK |
TH_FIN)) {
+        conn->fin_ack_seq = htonl(tcp_pkt->th_seq);
+        conn->tcp_state = TCPS_LAST_ACK;
         }

         return 0;
     }

     /* handle tcp packet from secondary guest */
-static int handle_secondary_tcp_pkt(NetFilterState *nf,
+static int handle_secondary_tcp_pkt(RewriterState *rf,
                                         Connection *conn,
-                                    Packet *pkt)
+                                    Packet *pkt, ConnectionKey *key)
     {
         struct tcphdr *tcp_pkt;

@@ -133,8 +163,34 @@ static int
handle_secondary_tcp_pkt(NetFilterState *nf,
             tcp_pkt->th_seq = htonl(ntohl(tcp_pkt->th_seq) -
conn->offset);

             net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+        /*
+         * Case 2:
+         * The *server* side of this connect is VM, *server* tries
to close
+         * the connection.
+         *
+         * We got 'ack=1' packets from server side, it acks
'fin=1, ack=1'
+         * packet from client side. Like Case 1, there should be
no packets
+         * in the connection from now know, But the difference
here is
+         * if the packet is lost, We will get the resent
'fin=1,ack=1' packet.
+         * TODO: Fix above case.
+         */
+        if ((conn->tcp_state == TCPS_LAST_ACK) &&
+            (ntohl(tcp_pkt->th_ack) == (conn->fin_ack_seq + 1))) {
+ g_hash_table_remove(rf->connection_track_table, key);
+        }
+    }
+    /*
+     * Case 1:
+     * The *server* side of this connect is VM, *client* tries to
close
+     * the connection.
+     *
+     * We got 'fin=1, ack=1' packet from server side, we need to
+     * record the seq of 'fin=1, ack=1' packet.
+     */
+    if ((tcp_pkt->th_flags & (TH_ACK | TH_FIN)) == (TH_ACK |
TH_FIN)) {
+        conn->fin_ack_seq = ntohl(tcp_pkt->th_seq);
+        conn->tcp_state = TCPS_LAST_ACK;


I thought the tcp_state should store the state of TCP from the view of
secondary VM? So TCPS_LAST_ACK is wrong and bring lots of confusion.
And
the handle of active close needs more states here. E.g if connection is
in FIN_WAIT_2, the connection is only half closed, remote peer can
still
send packet to us unless we receive a FIN.



Yes, i know what you mean, actually, here, we try to only track the last
two steps for closing a connection, that is 'fin=1,ack=1,seq=2,ack=u+1'

                                              ^
'FIN=1,ACK=1,seq=w,ack=u+1'


and 'ack=1,seq=u+1,ack=w+1', because if we get a 'fin=1,ack=1', we can

      ^ 'ACK=1,seq=u+1,ack=w+1'                     ^ 'FIN=1，ACK=1'


ensure that the 'fin=1,seq=u' packet has been posted.


                   ^ 'FIN=1,seq=u'


That's just the case I'm saying, the transition above is in fact:

secondary(ESTABLISHED)
secondary(FIN_WAIT_1): ->  FIN,seq=w,ack=u+1   -> :remote
secondary(FIN_WAIT_2): <-  seq=u+1,ack=w+1     <- :remote

So we are in fact in FIN_WAIT_2, which means the connection is only half
closed, but your patch will treat this as fully closed connection and
will remove the connection from the hashtable.



Er, here we track the last two states 'FIN=1, ACK=1' and  'ACK=1' ( which asks
the 'FIN=1,ACK=1' packet, We will remove the connection while got the 'ACK=1'
packet, so  is it enough ?


What's more I don't think we can decide passive or active close by:


+    if ((tcp_pkt->th_flags & (TH_ACK | TH_FIN)) == (TH_ACK | TH_FIN)) {

Since both cases will send FIN,ACK for sure.



I didn't quite understand, here we have tracked the closing request no matter
it is from the server side (passive close ?) or client side ( active close ?).
You can refer to the comment in codes, 'Case 1' and 'Case 2' comments.

Here, it seems that we can't track one case which both sides send the closing
requests at the same time, in that case, there are only 'FIN=1' and 'ACK=1'
packets.


Thanks.
Hailiang




Another reason is we may can't track the 'fin=1,seq=u' packet while
we start COLO while one connection is closing, which the
'fin=1,seq=u' packet
has been posted.

Actually, here, if we start COLO while one connection is closing,
which the
'fin=1,ack=1' has been posted, we can only track 'ack=1' packet. In this


  ^ 'FIN=1,ACK=1'

Sorry for the typo. :)


case, the connection will be left in hash table for ever though it is
harmless.
Any ideas for this case ?


Sorry I don't follow the question.



For the above codes question, i'd like to change tcp_state to
tap_closing_wait,
is it OK ?


You mean "tcp_closing_wait". I think we need first figure out if we can
track the state correctly first.

Thanks



Thanks.
Hailiang


Thanks


         }
-
         return 0;
     }

@@ -178,7 +234,7 @@ static ssize_t
colo_rewriter_receive_iov(NetFilterState *nf,

             if (sender == nf->netdev) {
                 /* NET_FILTER_DIRECTION_TX */
-            if (!handle_primary_tcp_pkt(nf, conn, pkt)) {
+            if (!handle_primary_tcp_pkt(s, conn, pkt, &key)) {
qemu_net_queue_send(s->incoming_queue, sender, 0,
                     (const uint8_t *)pkt->data, pkt->size, NULL);
                     packet_destroy(pkt, NULL);
@@ -191,7 +247,7 @@ static ssize_t
colo_rewriter_receive_iov(NetFilterState *nf,
                 }
             } else {
                 /* NET_FILTER_DIRECTION_RX */
-            if (!handle_secondary_tcp_pkt(nf, conn, pkt)) {
+            if (!handle_secondary_tcp_pkt(s, conn, pkt, &key)) {
qemu_net_queue_send(s->incoming_queue, sender, 0,
                     (const uint8_t *)pkt->data, pkt->size, NULL);
                     packet_destroy(pkt, NULL);



.







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