time to bleed by Joe Damato

technical ramblings from a wanna-be unix dinosaur

Archive for the ‘profiling’ tag

memprof: A Ruby level memory profiler

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What is memprof and why do I care?

memprof is a Ruby gem which supplies memory profiler functionality similar to bleak_house without patching the Ruby VM. You just install the gem, call a function or two, and off you go.

Where do I get it?

memprof is available on gemcutter, so you can just:

gem install memprof

Feel free to browse the source code at: http://github.com/ice799/memprof.

How do I use it?

Using memprof is simple. Before we look at some examples, let me explain more precisely what memprof is measuring.

memprof is measuring the number of objects created and not destroyed during a segment of Ruby code. The ideal use case for memprof is to show you where objects that do not get destroyed are being created:

  • Objects are created and not destroyed when you create new classes. This is a good thing.
  • Sometimes garbage objects sit around until garbage_collect has had a chance to run. These objects will go away.
  • Yet in other cases you might be holding a reference to a large chain of objects without knowing it. Until you remove this reference, the entire chain of objects will remain in memory taking up space.

memprof will show objects created in all cases listed above.

OK, now Let’s take a look at two examples and their output.

A simple program with an obvious memory “leak”:

require 'memprof'

@blah = Hash.new([])

Memprof.start
100.times {
  @blah[1] << "aaaaa"
}

1000.times {
   @blah[2] << "bbbbb"
}
Memprof.stats
Memprof.stop

This program creates 1100 objects which are not destroyed during the start and stop sections of the file because references are held for each object created.

Let's look at the output from memprof:

   1000 test.rb:11:String
    100 test.rb:7:String

In this example memprof shows the 1100 created, broken up by file, line number, and type.

Let's take a look at another example:

require 'memprof'
Memprof.start
require "stringio"
StringIO.new
Memprof.stats

This simple program is measuring the number of objects created when requiring stringio.

Let's take a look at the output:

    108 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:__node__
     14 test2.rb:3:String
      2 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Class
      1 test2.rb:4:StringIO
      1 test2.rb:4:String
      1 test2.rb:3:Array
      1 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Enumerable

This output shows an internal Ruby interpreter type __node__ was created (these represent code), as well as a few Strings and other objects. Some of these objects are just garbage objects which haven't had a chance to be recycled yet.

What if nudge the garbage_collector along a little bit just for our example? Let's add the following two lines of code to our previous example:

GC.start
Memprof.stats

We're now nudging the garbage collector and outputting memprof stats information again. This should show fewer objects, as the garbage collector will recycle some of the garbage objects:

    108 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:__node__
      2 test2.rb:3:String
      2 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Class
      1 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Enumerable

As you can see above, a few Strings and other objects went away after the garbage collector ran.

Which Rubies and systems are supported?

  • Only unstripped binaries are supported. To determine if your Ruby binary is stripped, simply run: file `which ruby`. If it is, consult your package manager's documentation. Most Linux distributions offer a package with an unstripped Ruby binary.
  • Only x86_64 is supported at this time. Hopefully, I'll have time to add support for i386/i686 in the immediate future.
  • Linux Ruby Enterprise Edition (1.8.6 and 1.8.7) is supported.
  • Linux MRI Ruby 1.8.6 and 1.8.7 built with --disable-shared are supported. Support for --enable-shared binaries is coming soon.
  • Snow Leopard support is experimental at this time.
  • Ruby 1.9 support coming soon.

How does it work?

If you've been reading my blog over the last week or so, you'd have noticed two previous blog posts (here and here) that describe some tricks I came up with for modifying a running binary image in memory.

memprof is a combination of all those tricks and other hacks to allow memory profiling in Ruby without the need for custom patches to the Ruby VM. You simply require the gem and off you go.

memprof works by inserting trampolines on object allocation and deallocation routines. It gathers metadata about the objects and outputs this information when the stats method is called.

What else is planned?

Myself, Jake Douglas, and Aman Gupta have lots of interesting ideas for new features. We don't want to ruin the surprise, but stay tuned. More cool stuff coming really soon :)

Thanks for reading and don't forget to subscribe (via RSS or e-mail) and follow me on twitter.

Written by Joe Damato

December 11th, 2009 at 5:59 am

Hot patching inlined functions with x86_64 asm metaprogramming

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Disclaimer

The tricks, techniques, and ugly hacks in this article are PLATFORM SPECIFIC, DANGEROUS, and NOT PORTABLE.

This article will make reference to information in my previous article Rewrite your Ruby VM at runtime to hot patch useful features so be sure to check it out if you find yourself lost during this article.

Also, this might not qualify as metaprogramming in the traditional definition1, but this article will show how to generate assembly at runtime that works well with the particular instructions generated for a binary. In other words, the assembly is constructed based on data collected from the binary at runtime. When I explained this to Aman, he called it assembly metaprogramming.

TLDR

This article expands on a previous article by showing how to hook functions which are inlined by the compiler. This technique can be applied to other binaries, but the binary in question is Ruby Enterprise Edition 1.8.7. The use case is to build a memory profiler without requiring patches to the VM, but just a Ruby gem.

It’s on GitHub

The memory profiler is NOT DONE, yet. It will be soon. Stay tuned.

The code described here is incorporated into a Ruby Gem which can be found on github: http://github.com/ice799/memprof specifically at: http://github.com/ice799/memprof/blob/master/ext/memprof.c#L202-318

Overview of the plan of attack

The plan of attack is relatively straight forward:

  1. Find the inlined code.
  2. Overwrite part of it to redirect to a stub.
  3. Call out to a handler from the stub.
  4. Make sure the return path is sane.

As simple as this seems, implementing these steps is actually a bit tricky.

Finding pieces of inlined code

Before finding pieces of inlined code, let’s first examine the C code we want to hook. I’m going to be showing how to hook the inline function add_freelist.

The code for add_freelist is short:

static inline void
add_freelist(p)
    RVALUE *p;
{
    if (p->as.free.flags != 0)
        p->as.free.flags = 0;
    if (p->as.free.next != freelist)
        p->as.free.next = freelist;
    freelist = p;
}

There is one really important feature of this code which stands out almost immediately. freelist has (at least) compilation unit scope. This is awesome because freelist serves as a marker when searching for assembly instructions to overwrite. Since the freelist has compilation unit scope, it’ll live at some static memory location.

If we find writes to this static memory location, we find our inline function code.

Let’s take a look at the instructions generated from this C code (unrelated instructions snipped out):

  437f21:       48 c7 00 00 00 00 00    movq   $0x0,(%rax)
   . . . . .
  437f2c:       48 8b 05 65 de 2d 00    mov    0x2dde65(%rip),%rax  # 715d98 [freelist]
   . . . . .
  437f48:       48 89 05 49 de 2d 00    mov    %rax,0x2dde49(%rip)  # 715d98 [freelist]

The last instruction above updates freelist, it is the instruction generated for the C statement freelist = p;.

As you can see from the instruction, the destination is freelist. This makes it insanely easy to locate instances of this inline function. Just need to write a piece of C code which scans the binary image in memory, searching for mov instructions where the destination is freelist and I’ve found the inlined instances of add_freelist.

Why not insert a trampoline by overwriting that last mov instruction?

Overwriting with a jmp

The mov instruction above is 7 bytes wide. As long as the instruction we’re going to implant is 7 bytes or thinner, everything is good to go. Using a callq is out of the question because we can’t ensure the stack is 16-byte aligned as per the x86_64 ABI2. As it turns out, a jmp instruction that uses a 32bit displacement from the instruction pointer only requires 5 bytes. We’ll be able to implant the instruction that’s needed, and even have room to spare.

I created a struct to encapsulate this short 7 byte trampoline. 5 bytes for the jmp, 2 bytes for NOPs. Let’s take a look:

  struct tramp_inline tramp = {
    .jmp           = {'\xe9'},
    .displacement  = 0,
    .pad           = {'\x90', '\x90'},
  };

Let’s fill in the displacement later, after actually finding the instruction that’s going to get overwritten.

So, to find the instruction that’ll be overwritten, just look for a mov opcode and check that the destination is freelist:

    /* make sure it is a mov instruction */
    if (byte[1] == '\x89') {

      /* Read the REX byte to make sure it is a mov that we care about */
      if ( (byte[0] == '\x48') ||
          (byte[0] == '\x4c') ) {

        /* Grab the target of the mov. REMEMBER: in this case the target is 
         * a 32bit displacment that gets added to RIP (where RIP is the adress of
         * the next instruction).
         */
        mov_target = *(uint32_t *)(byte + 3);

        /* Sanity check. Ensure that the displacement from freelist to the next
         * instruction matches the mov_target. If so, we know this mov is
         * updating freelist.
         */
        if ( (freelist - (void *)(byte+7) ) == mov_target) {

At this point we’ve definitely found a mov instruction with freelist as the destination. Let’s calculate the displacement to the stage 2 trampoline for our jmp instruction and write the instruction into memory.

/* Setup the stage 1 trampoline. Calculate the displacement to
 * the stage 2 trampoline from the next instruction.
 *
 * REMEMBER!!!! The next instruction will be NOP after our stage 1
 * trampoline is written. This is 5 bytes into the structure, even
 * though the original instruction we overwrote was 7 bytes.
 */
 tramp.displacement = (uint32_t)(destination - (void *)(byte+5));

/* Figure out what page the stage 1 tramp is gonna be written to, mark
 * it WRITE, write the trampoline in, and then remove WRITE permission.
 */
 aligned_addr = page_align(byte);
 mprotect(aligned_addr, (void *)byte - aligned_addr + 10,
               PROT_READ|PROT_WRITE|PROT_EXEC);
 memcpy(byte, &tramp, sizeof(struct tramp_inline));
 mprotect(aligned_addr, (void *)byte - aligned_addr + 10,
              PROT_READ|PROT_EXEC);

Cool, all that’s left is to build the stage 2 trampoline which will set everything up for the C level handler.

An assembly stub to set the stage for our C handler

So, what does the assembly need to do to call the C handler? Quite a bit actually so let’s map it out, step by step:

  1. Replicate the instruction which was overwritten so that the object is actually added to the freelist.
  2. Save the value of rdi register. This register is where the first argument to a function lives and will store the obj that was added to the freelist for the C handler to do analysis on.
  3. Load the object being added to the freelist into rdi
  4. Save the value of rbx so that we can use the register as an operand for an absolute indirect callq instruction.
  5. Save rbp and rsp to allow a way to undo the stack alignment later.
  6. Align the stack to a 16-byte boundary to comply with the x86_64 ABI.
  7. Move the address of the handler into rbx
  8. Call the handler through rbx.
  9. Restore rbp, rsp, rdi, rbx.
  10. Jump back to the instruction after the instruction which was overwritten.

To accomplish this let’s build out a structure with as much set up as possible and fill in the displacement fields later. This “base” struct looks like this:

  struct inline_tramp_tbl_entry inline_ent = {
    .rex     = {'\x48'},
    .mov     = {'\x89'},
    .src_reg = {'\x05'},
    .mov_displacement = 0,

    .frame = {
      .push_rdi = {'\x57'},
      .mov_rdi = {'\x48', '\x8b', '\x3d'},
      .rdi_source_displacement = 0,
      .push_rbx = {'\x53'},
      .push_rbp = {'\x55'},
      .save_rsp = {'\x48', '\x89', '\xe5'},
      .align_rsp = {'\x48', '\x83', '\xe4', '\xf0'},
      .mov = {'\x48', '\xbb'},
      .addr = error_tramp,
      .callq = {'\xff', '\xd3'},
      .leave = {'\xc9'},
      .rbx_restore = {'\x5b'},
      .rdi_restore = {'\x5f'},
    },

    .jmp  = {'\xe9'},
    .jmp_displacement = 0,
  };

So, what’s left to do:

  1. Copy the REX and source register bytes of the instruction which was overwritten to replicate it.
  2. Calculate the displacement to freelist to fully generate the overwritten mov.
  3. Calculate the displacement to freelist so that it can be stored in rdi as an argument to the C handler.
  4. Fill in the absolute address for the handler.
  5. Calculate the displacement to the instruction after the stage 1 trampoline in order to jmp back to resume execution as normal.

Doing that is relatively straight-forward. Let’s take a look at the C snippets that make this happen:

/* Before the stage 1 trampoline gets written, we need to generate
 * the code for the stage 2 trampoline. Let's copy over the REX byte
 * and the byte which mentions the source register into the stage 2
 * trampoline.
 */
inl_tramp_st2 = inline_tramp_table + entry;
inl_tramp_st2->rex[0] = byte[0];
inl_tramp_st2->src_reg[0] = byte[2];

. . . . . 

/* Finish setting up the stage 2 trampoline. */

/* calculate the displacement to freelist from the next instruction.
 *
 * This is used to replicate the original instruction we overwrote.
 */
inl_tramp_st2->mov_displacement = freelist - (void *)&(inl_tramp_st2->frame);

/* fill in the displacement to freelist from the next instruction.
 *
 * This is to arrange for the new value in freelist to be in %rdi, and as such
 * be the first argument to the C handler. As per the amd64 ABI.
 */
inl_tramp_st2->frame.rdi_source_displacement = freelist - 
                                          (void *)&(inl_tramp_st2->frame.push_rbx);

/* jmp back to the instruction after stage 1 trampoline was inserted 
 *
 * This can be 5 or 7, it doesn't matter. If its 5, we'll hit our 2
 * NOPS. If its 7, we'll land directly on the next instruction.
 */
inl_tramp_st2->jmp_displacement = (uint32_t)((void *)(byte + 7) -
                                         (void *)(inline_tramp_table + entry + 1));

/* write the address of our C level trampoline in to the structure */
inl_tramp_st2->frame.addr = freelist_tramp;

Awesome.

We’ve successfully patched the binary in memory, inserted an assembly stub which was generated at runtime, called a hook function, and ensured that execution can resume normally.

So, what’s the status on that memory profiler?

Almost done, stay tuned for more updates coming SOON.

Conclusion

  • Hackery like this is unmaintainable, unstable, stupid, but also fun to work on and think about.
  • Being able to hook add_freelist like this provides the last tool needed to implement a version of bleak_house (a Ruby memory profiler) without patching the Ruby VM.
  • x86_64 instruction set is a painful instruction set.
  • Use the GNU assembler (gas) instead of trying to generate opcodes by reading the Intel instruction set PDFs if you value your sanity.

Thanks for reading and don’t forget to subscribe (via RSS or e-mail) and follow me on twitter.

References

  1. http://en.wikipedia.org/wiki/Metaprogramming []
  2. x86_64 ABI []

Written by Joe Damato

December 10th, 2009 at 5:59 am

Debugging Ruby: Understanding and Troubleshooting the VM and your Application

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Download the PDF here.

Debugging Ruby

Written by Aman Gupta

December 2nd, 2009 at 8:30 pm

Extending ltrace to make your Ruby/Python/Perl/PHP apps faster

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A few days ago, Aman (@tmm1) was complaining to me about a slow running process:

I want to see what is happening in userland and trace calls to extensions. Why doesn’t ltrace work for Ruby processes? I want to figure out which MySQL queries are causing my app to be slow.

It turns out that ltrace did not have support for libraries loaded with libdl. This is a problem for languages like Ruby, Python, PHP, Perl, and others because in many cases extensions, libraries, and plugins for these languages are loaded by the VM using libdl. This means that ltrace is somewhat useless for tracking down performance issues in dynamic languages.

A couple late nights of hacking and I managed to finagle libdl support in ltrace. Since most people probably don’t care about the technical details of how it was implemented, I’ll start with showing how to use the patch I wrote and what sort of output you can expect. This patch has made tracking down slow queries (among other things) really easy and I hope others will find this useful.

How to use ltrace:

After you’ve applied my patch (below) and rebuilt ltrace, let’s say you’d like to trace MySQL queries and have ltrace tell you when the query was executed and how long it took. There are two steps:

  1. Give ltrace info so it can pretty print – echo “int mysql_real_query(addr,string,ulong);” > custom.conf
  2. Tell ltrace you want to hear about mysql_real_query: ltrace -F custom.conf -ttTgx mysql_real_query -p <pid>

Here’s what those arguments mean:

  • -F use a custom config file when pretty-printing (default: /etc/ltrace.conf, add your stuff there to avoid -F if you wish).
  • -tt print the time (including microseconds) when the call was executed
  • -T time the call and print how long it took
  • -x tells ltrace the name of the function you care about
  • -g avoid placing breakpoints on all library calls except the ones you specify with -x. This is optional, but it makes ltrace produce much less output and is a lot easier to read if you only care about your one function.

PHP

Test script

mysql_connect("localhost", "root");
while(true){
    mysql_query("SELECT sleep(1)");
}

ltrace output

22:31:50.507523 zend_hash_find(0x025dc3a0, "mysql_query", 12) = 0 <0.000029>
22:31:50.507781 mysql_real_query(0x027bc540, "SELECT sleep(1)", 15) = 0 <1.000600>
22:31:51.508531 zend_hash_find(0x025dc3a0, "mysql_query", 12) = 0 <0.000025>
22:31:51.508675 mysql_real_query(0x027bc540, "SELECT sleep(1)", 15) = 0 <1.000926>

ltrace command

ltrace -ttTg -x zend_hash_find -x mysql_real_query -p [pid of script above]

Python

Test script

import MySQLdb
db = MySQLdb.connect("localhost", "root", "", "test")
cursor = db.cursor()
sql = """SELECT sleep(1)"""
while True:
	cursor.execute(sql)
	data = cursor.fetchone()
db.close()

ltrace output

22:24:39.104786 PyEval_SaveThread() = 0x21222e0 <0.000029>
22:24:39.105020 PyEval_SaveThread() = 0x21222e0 <0.000024>
22:24:39.105210 PyEval_SaveThread() = 0x21222e0 <0.000024>
22:24:39.105303 mysql_real_query(0x021d01d0, "SELECT sleep(1)", 15) = 0 <1.002083>
22:24:40.107553 PyEval_SaveThread() = 0x21222e0 <0.000026>
22:24:40.107713 PyEval_SaveThread()= 0x21222e0 <0.000024>
22:24:40.107909 PyEval_SaveThread() = 0x21222e0 <0.000025>
22:24:40.108013 mysql_real_query(0x021d01d0, "SELECT sleep(1)", 15) = 0 <1.001821>

ltrace command

ltrace -ttTg -x PyEval_SaveThread -x mysql_real_query -p [pid of script above]

Perl

Test script

#!/usr/bin/perl
use DBI;

$dsn = "DBI:mysql:database=test;host=localhost";
$dbh = DBI->connect($dsn, "root", "");
$drh = DBI->install_driver("mysql");
@databases = DBI->data_sources("mysql");
$sth = $dbh->prepare("SELECT SLEEP(1)");

while (1) {
  $sth->execute;
}

ltrace output

22:42:11.194073 Perl_push_scope(0x01bd3010) =  <0.000028>
22:42:11.194299 mysql_real_query(0x01bfbf40, "SELECT SLEEP(1)", 15) = 0 <1.000876>
22:42:12.195302 Perl_push_scope(0x01bd3010) =  <0.000024>
22:42:12.195408 mysql_real_query(0x01bfbf40, "SELECT SLEEP(1)", 15) = 0 <1.000967>

ltrace command

ltrace -ttTg -x mysql_real_query -x Perl_push_scope -p [pid of script above]

Ruby

Test script

require 'rubygems'
require 'sequel'

DB = Sequel.connect('mysql://root@localhost/test')

while true
  p DB['select sleep(1)'].select.first
  GC.start
end

snip of ltrace output

22:10:00.195814 garbage_collect()  = 0 <0.022194>
22:10:00.218438 mysql_real_query(0x02740000, "select sleep(1)", 15) = 0 <1.001100>
22:10:01.219884 garbage_collect() = 0 <0.021401>
22:10:01.241679 mysql_real_query(0x02740000, "select sleep(1)", 15) = 0 <1.000812>

ltrace command used:

ltrace -ttTg -x garbage_collect -x mysql_real_query -p [pid of script above]

Where to get it

How ltrace works normally

ltrace works by setting software breakpoints on entries in a process’ Procedure Linkage Table (PLT).

What is a software breakpoint

A software breakpoint is just a series of bytes (0xcc on the x86 and x86_64) that raise a debug interrupt (interrupt 3 on the x86 and x86_64). When interrupt 3 is raised, the CPU executes a handler installed by the kernel. The kernel then sends a signal to the process that generated the interrupt. (Want to know more about how signals and interrupts work? Check out an earlier blog post: here)

What is a PLT and how does it work?

A PLT is a table of absolute addresses to functions. It is used because the link editor doesn’t know where functions in shared objects will be located. Instead, a table is created so that the program and the dynamic linker can work together to find and execute functions in shared objects. I’ve simplified the explanation a bit1, but at a high level:

  1. Program calls a function in a shared object, the link editor makes sure that the program jumps to a slot in the PLT.
  2. The program sets some data up for the dynamic linker and then hands control over to it.
  3. The dynamic linker looks at the info set up by the program and fills in the absolute address of the function that was called in the PLT.
  4. Then the dynamic linker calls the function.
  5. Subsequent calls to the same function jump to the same slot in the PLT, but every time after the first call the absolute address is already in the PLT (because when the dynamic linker is invoked the first time, it fills in the absolute address in the PLT).

Since all calls to library functions occur via the PLT, ltrace sets breakpoints on each PLT entry in a program.

Why ltrace didn’t work with libdl loaded libraries

Libraries loaded with libdl are loaded at run time and functions (and other symbols) are accessed by querying the dynamic linker (by calling dlsym()). The compiler and link editor don’t know anything about libraries loaded this way (they may not even exist!) and as such no PLT entries are created for them.

Since no PLT entries exist, ltrace can’t trace these functions.

What needed to be done to make ltrace libdl-aware

OK, so we understand the problem. ltrace only sets breakpoints on PLT entries and libdl loaded libraries don’t have PLT entries. How can this be fixed?

Luckily, the dynamic linker and ELF all work together to save your ass.

Executable and Linking Format (ELF) is a file format for executables, shared libraries, and more2. The file format can get a bit complicated, but all you really need to know is: ELF consists of different sections which hold different types of entries. There is a section called .dynamic which has an entry named DT_DEBUG. This entry stores the address of a debugging structure in the address space of the process. In Linux, this struct has type struct r_debug.

How to use struct r_debug to win the game

The debug structure is updated by the dynamic linker at runtime to reflect the current state of shared object loading. The structure contains 3 things that will help us in our quest:

  1. state – the current state of the mapping change taking place (begin add, begin delete, consistent)
  2. brk – the address of a function internal to the dynamic linker that will be called when the linker maps, unmaps, or has completed mapping a shared object.
  3. link map – Pointer to the start of a list of currently loaded objects. This list is called the link map and is represented as a struct link_map in Linux.

Tie it all together and bring it home

To add support for libdl loaded libraries to ltrace, the steps are:

  1. Find the address of the debug structure in the .dynamic section of the program.
  2. Set a software breakpoint on brk.
  3. When the dynamic linker updates the link map, it will trigger the software breakpoint.
  4. When the breakpoint is triggered, check state in the debug structure.
  5. If a new library has been added, walk the link map and figure out what was added.
  6. Search the added library’s symbol table for the symbols we care about.
  7. Set a software breakpoints on whatever is found.
  8. Steps 3-8 repeat.

That isn’t too hard all thanks to the dynamic linker providing a way for us to hook into its internal events.

Conclusion

  • Read the System V ABI for your CPU. It is filled with insanely useful information that can help you be a better programmer.
  • Use the source. A few times while hacking on this patch I looked through the source for GDB and glibc to help me figure out what was going on.
  • Understanding how things work at a low-level can help you build tools to solve your high-level problems.

Thanks for reading and don’t forget to subscribe (via RSS or e-mail) and follow me on twitter.

References

  1. System V Application Binary Interface AMD64 Architecture Processor Supplement, p 78 []
  2. Executable and Linking Format (ELF) Specification []

Written by Joe Damato

October 8th, 2009 at 4:59 am

Ruby Hoedown Slides

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Below are the slides for a talk that Aman Gupta and I gave at Ruby Hoedown

Download the PDF here

Thanks for reading and don’t forget to subscribe (via RSS or e-mail) and follow me on twitter.

Written by Joe Damato

August 29th, 2009 at 1:05 am