Add uprobe strlen histogram example

This example traces all calls to libc's strlen(). The program is attached as a
retprobe, therefore giving access to the resulting string length. The value is
kept in a log2 histogram that is printed to console once per second.

Example:
```
$ sudo ./strlen_hist.py
22:12:51
     strlen return:      : count     distribution
         0 -> 1          : 2041     |****************                        |
         2 -> 3          : 1120     |********                                |
         4 -> 7          : 3300     |**************************              |
         8 -> 15         : 4995     |****************************************|
        16 -> 31         : 2130     |*****************                       |
        32 -> 63         : 562      |****                                    |
^C
```

Signed-off-by: Brenden Blanco <[email protected]>

examples/tracing/strlen_hist.py

@@ -0,0 +1,59 @@
+#!/usr/bin/python
+
+#
+# strlen_hist.py Histogram of system-wide strlen return values
+#
+# A basic example of using uprobes along with a histogram to show
+# distributions.
+#
+# Runs until ctrl-c is pressed.
+#
+# Copyright (c) PLUMgrid, Inc.
+# Licensed under the Apache License, Version 2.0 (the "License")
+# 
+# Example output:
+# $ sudo ./strlen_hist.py
+# 22:12:52
+# strlen return: : count distribution
+# 0 -> 1 : 2106 |**************** |
+# 2 -> 3 : 1172 |********* |
+# 4 -> 7 : 3892 |****************************** |
+# 8 -> 15 : 5096 |****************************************|
+# 16 -> 31 : 2201 |***************** |
+# 32 -> 63 : 547 |**** |
+# 64 -> 127 : 106 | |
+# 128 -> 255 : 13 | |
+# 256 -> 511 : 27 | |
+# 512 -> 1023 : 6 | |
+# 1024 -> 2047 : 10 | |
+# ^C$
+#
+
+from __future__ import print_function
+import bcc
+import time
+
+text = """
+#include <uapi/linux/ptrace.h>
+BPF_HISTOGRAM(dist);
+int count(struct pt_regs *ctx) {
+ dist.increment(bpf_log2l(ctx->ax));
+ return 0;
+}
+"""
+
+b = bcc.BPF(text=text)
+sym="strlen"
+b.attach_uretprobe(name="c", sym=sym, fn_name="count")
+
+dist = b["dist"]
+
+try:
+ while True:
+ time.sleep(1)
+ print("%-8s\n" % time.strftime("%H:%M:%S"), end="")
+ dist.print_log2_hist(sym + " return:")
+ dist.clear()
+
+except KeyboardInterrupt:
+ pass