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From "Christian Czezatke (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-3286) xid wrap-around causes connection loss/segfault when hitting predefined XIDs
Date Thu, 21 Feb 2019 22:01:00 GMT
Christian Czezatke created ZOOKEEPER-3286:
---------------------------------------------

             Summary: xid wrap-around causes connection loss/segfault when hitting predefined
XIDs
                 Key: ZOOKEEPER-3286
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3286
             Project: ZooKeeper
          Issue Type: Bug
          Components: c client
    Affects Versions: 3.5.4
         Environment: CentOS 7.2
            Reporter: Christian Czezatke
             Fix For: 3.5.5
         Attachments: c-client-xid.diff

*Description:*

The get_xid functions in mt_adaptor.c/st_adaptor.c return a 32 bit signed integer that is
initialized to the current unix epoch timestamp on startup.

This counter will eventually wrap around, which is not a problem per se, since the client
does not expect XID values to monotonically increase: It just verifies that replies to operations
come back in order by checking the XID of a request received against the next XID expected.
(zookeeper.c:zookeeper_process).

However, after a wrap-around the XID values will eventually collide with the reserved XIDs
ad defined in zk_adaptor.h:
 * The first collision will be with SET_WATCHES_XID (-8): The reply to the request that happens
to get tagged with -8 will be misinterpreted as a reply to SET_WATCHES. This causes the client
to see a connection timeout.
 * The next collision will be with AUTH_XID (-4): At that point the client will segfault,
when mis-interpreting the reply:

#0  0x0000000000407645 in auth_completion_func (zh=0x61d010, rc=0) at src/zookeeper.c:1823
 #1  zookeeper_process (zh=zh@entry=0x61d010, events=<optimized out>) at src/zookeeper.c:2896
 #2  0x000000000040c34c in do_io (v=0x61d010) at src/mt_adaptor.c:451
 #3  0x00007ffff7bc8dc5 in start_thread () from /lib64/libpthread.so.0
 #4  0x00007ffff75f573d in clone () from /lib64/libc.so.6

I hit this with a busy C client that runs for a very long time (months). Also, when a client
spins in a tight loop trying to submit more operations even for a short period of time after
a connection loss the xid values will increment very fast.

 

*Proposed patch:*

To avoid introducing any additional locking, this can be solved by just masking out the MSB
in the xid returned by get_xid. Effectively this prevents the returned XID from ever going
negative.

To avoid a race when the static xid variable hits -1 eventually after a wrap, around, I propose
to not initialize xid with the result of time(0) on startup. This is not needed. This also
means that the get_xid function in mt_adapter.c no longer needs to be flagged as constructor.

 Proposed patch is attached.

 

I ran into this on zookeeper 3.5.4 but other versions are likely affected as well.



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