Question

Iffy buffer

Asked by: CEHJ

I'm doubtful about the use of the buffer in the code below. Also i'm getting some runtime errors:

"substr outside of string at /bk/goose/tmptmp/sss.working-copy.pl line 203.
Use of uninitialized value $buff in ord at /bk/goose/tmptmp/sss.working-copy.pl line 203."

195     if($success) {
196         $t = sysread($client, $buff, 3);
197 
198         printf("Before line %d\n", 199);
199         printf("%d bytes were read\n", $t);
200         my $char1 = substr($buff, 0, 1);
201         printf("char1 was %d\n", ord($char1));
202         if(substr($buff, 0, 1) eq "\x05") {
203             if(ord(substr($buff, 2, 1)) == 0) { # reserved
204                 my($host, $raw_host) = socks_get_host($client);
205                 if(!$host) {
206                     # CJ
207                     print "Could not get host\n";
208                     return;
209                 }
210                 else {
211                     print "Got host\n";
212                 }
213                 my($port, $raw_port) = socks_get_port($client);
214                 if(!$port) {
215                     # CJ
216                     print "Could not get port\n";
217                     return;
218                 }
219                 else {
220                     print "Got port\n";
221                 }
222                 $ord = ord(substr($buff, 1, 1));
223                 $buff = "\x05\x00\x00".$raw_host.$raw_port;
224                 syswrite($client, $buff, length($buff));
225                 socks_do($ord, $client, $host, $port);
226             }
227         }
228     }

                                  
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Asked On
2009-09-18 at 08:07:20ID24743498
Topic

Perl Programming Language

Participating Experts
2
Points
500
Comments
14

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Answers

 

by: nemws1Posted on 2009-09-18 at 08:41:14ID: 25367118

What if you add this immediately after the the if() in line 195:

my $buff = "";

                                              
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by: CEHJPosted on 2009-09-18 at 08:57:36ID: 25367298

Yes, i tried that earlier, but i'll try it again

 

by: CEHJPosted on 2009-09-18 at 09:00:14ID: 25367327

The warnings/errors remain

 

by: CEHJPosted on 2009-09-18 at 09:11:23ID: 25367434

I'm going to post the original (not my) code, rather than my debugging working copy. The errors/warnings here refer to line 189

185         if($success) {
186          $t = sysread($client, $buff, 3);
187 
188          if(substr($buff, 0, 1) eq "\x05") {
189            if(ord(substr($buff, 2, 1)) == 0) { # reserved
190                  my($host, $raw_host) = socks_get_host($client);
191                  if(!$host) { return; }
192                  my($port, $raw_port) = socks_get_port($client);
193                  if(!$port) { return; }
194                  $ord = ord(substr($buff, 1, 1));
195                  $buff = "\x05\x00\x00".$raw_host.$raw_port;
196                  syswrite($client, $buff, length($buff));
197                  socks_do($ord, $client, $host, $port);
198            }
199          }
200         }
201         else { syswrite($client, "\x05\xFF", 2); }
202 
203         $client->close();
                                              
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by: nemws1Posted on 2009-09-18 at 09:22:03ID: 25367543

Let's do some debugging.  On line 187 add:

print STDERR "Bufflen: $t\n";
foreach my $char (split(//, $buff)) {
  print STDERR "Buffchar: $char - ", ord($char), "\n";
}

And re-run.  It looks like you're not getting enough data into $buff and then substr() is barfing.  You should actually check for this.  Something like this (again, line 187 is a good spot).

if ($t < 3) {
    print STDERR "Didn't receive enough data into buffer from sysread\n";
    exit;
} else {
.....
}

 

by: CEHJPosted on 2009-09-18 at 09:26:07ID: 25367571

That all sounds pretty plausible. I'm actually connecting to the above (socks proxy) with a Java client and i'm reading stuff that looks like the right stuff at the wrong offset. Just trying your suggestion

 

by: nemws1Posted on 2009-09-18 at 09:30:24ID: 25367614

Well, the foreach..split should spit out all the decimal numbers for all your bytes - you should be able to figure out how off you are in your offset! ;-)

 

by: CEHJPosted on 2009-09-18 at 09:34:39ID: 25367649

Bufflen: 1
Buffchar:  - 5
substr outside of string at /tmp/tsss.pl line 192.
Use of uninitialized value $buff in ord at /tmp/tsss.pl line 192.

 

by: nemws1Posted on 2009-09-18 at 09:39:54ID: 25367686

Yup - you're only getting one byte (5, which is the same in hex & decimal) back.  You're not getting enough info - the sysread() is asking for 3 bytes but only getting one. ;-(

 

by: CEHJPosted on 2009-09-18 at 11:26:01ID: 25368575

In that case, sysread with a length parameter must be a non-blocking call? I haven't checked the docs yet. A better option would probably be to ensure blocking probably?

 

by: nemws1Posted on 2009-09-18 at 11:47:13ID: 25368751

According to the docs...  it *attempts* to read LENGTH bytes.  It suggests "Use sysread() and check for a return value for 0 to decide whether you're done".  Maybe you should try this right *after* your sysread:

      my $buff2 = ""; $t2 = sysread($client, $buff2, 3); print "T2: $t2\n";

 

by: CEHJPosted on 2009-09-18 at 12:42:10ID: 25369308

Hmm. Pity there's no foce blocking option.

I know the problem is as i stated at  http:#25368575 btw as i employed a kludge 'from the other end' : namely i cause all calls to write at the client to be buffered and only actually written to the stream when i call flush(). That causes the server to behave properly, but of course this should not be necessary

 

by: Adam314Posted on 2009-09-22 at 09:41:06ID: 25394735

It will attempt to read the specified number of bytes, but read what is available and return the number of bytes read.
Depending on the type of stream, it may block if nothing is available.

If you know you need a certain number of bytes, keep reading until you get that many.

my $BytesRead = 0;
my $FullBuffer = '';
while($BytesRead < 3) {
    my ($ReadNow, $BufferNow);
    $ReadNow = sysread($client, $BufferNow, 3 - $BytesRead);
    $BytesRead += $ReadNow;
    $FullBuffer .= $BufferNow;
}

                                              
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by: CEHJPosted on 2009-09-22 at 11:54:13ID: 31630609

Thanks folks

20120131-EE-VQP-002

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