title: Systemtap does't show all the local variables in a kernel function
body: <p>I'm trying to debug some code in the kernel using systemtap. I need to print the value of local variables in that function but it looks like systemtap can only see the function arguments and not the local variables defined in that function. Here is my probe script. </p>
  
  <pre><code>probe kernel.function("tcp_write_xmit") {
      if( execname() == "bw_client"){
              printf(
                 "tcp_write_xmit skb len %d\n",
                 $skb
              );
      }
  }
  </code></pre>
  
  <p>When I run this, I get the following error </p>
  
  <pre><code>semantic error: failed to retrieve location attribute for 'skb'
  [man error::dwarf] (dieoffset: 0x5bd30b4): identifier 
  '$skb' at /home/cca-user/systaptest/txprobe.stp:37:6
  source:                    $skb
                             ^
  Pass 2: analysis failed.  [man error::pass2]
  </code></pre>
  
  <p>However the function <code>tcp_write_xmit</code> clearly has <code>skb</code></p>
  
  <p>Using the <code>-L</code> option to print available variables gives me 5 variables which are the function arguments but none of the local variables are seen. </p>
  
  <pre><code>root@i-sahmed-node2: /mnt/linux-3.13.0 # stap -L
  'kernel.function("tcp_write_xmit")'
  kernel.function("tcp_write_xmit@/build/buildd/linux-3.13.0/net/ipv4
  /tcp_output.c:1832") $sk:struct sock* $mss_now:unsigned int $nonagle:int  
  $push_one:int $gfp:gfp_t
  </code></pre>
  
  <p>Here is the kernel and the systemtap version that I am running</p>
  
  <pre><code>root@i-sahmed-node2: /mnt/linux-3.13.0 # stap --version
  Systemtap translator/driver (version 2.3/0.158, Debian version 2.3-1ubuntu1 (trusty))
  Copyright (C) 2005-2013 Red Hat, Inc. and others
  This is free software; see the source for copying conditions.
  enabled features: AVAHI LIBSQLITE3 NSS TR1_UNORDERED_MAP NLS
  
  root@i-sahmed-node2: /mnt/linux-3.13.0 # uname -a
  Linux i-sahmed-node2 3.13.0-53-generic #89-Ubuntu SMP Wed May 20 10:34:39 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
  </code></pre>
  
  <p>Any Ideas? </p>