examples: Add `mallocstacks`

This simple example script traces all calls to `malloc` in a process and
prints the callsite using the new `BPF_STACK_TRACE` table API.

examples/tracing/mallocstacks.py

+#!/usr/bin/python
+#
+# mallocstacks  Trace malloc() calls in a process and print the full
+#               stack trace for all callsites.
+#               For Linux, uses BCC, eBPF. Embedded C.
+#
+# This script is a basic example of the new Linux 4.6+ BPF_STACK_TRACE
+# table API.
+#
+# Copyright 2016 GitHub, Inc.
+# Licensed under the Apache License, Version 2.0 (the "License")
+
+from __future__ import print_function
+from bcc import BPF, ProcessSymbols
+from time import sleep
+import sys
+
+if len(sys.argv) < 2:
+    print("USAGE: mallocstacks PID")
+    exit()
+pid = int(sys.argv[1])
+
+# load BPF program
+b = BPF(text="""
+#include <uapi/linux/ptrace.h>
+
+BPF_HASH(calls, int);
+BPF_STACK_TRACE(stack_traces, 1024)
+
+int alloc_enter(struct pt_regs *ctx, size_t size) {
+    int key = stack_traces.get_stackid(ctx,
+        BPF_F_USER_STACK|BPF_F_REUSE_STACKID);
+    if (key < 0)
+        return 0;
+
+    u64 zero = 0, *val;
+    val = calls.lookup_or_init(&key, &zero);
+    (*val) += size;
+    return 0;
+};
+""")
+
+b.attach_uprobe(name="c", sym="malloc", fn_name="alloc_enter", pid=pid)
+print("Attaching to malloc in pid %d, Ctrl+C to quit." % pid)
+
+decoder = ProcessSymbols(pid)
+
+# sleep until Ctrl-C
+try:
+    sleep(99999999)
+except KeyboardInterrupt:
+    pass
+
+calls = b.get_table("calls")
+stack_traces = b.get_table("stack_traces")
+
+for k, v in reversed(sorted(calls.items(), key=lambda c: c[1].value)):
+    print("%d bytes allocated at:" % v.value)
+    for addr in stack_traces.walk(k.value):
+        print("\t%s (%x)" % (decoder.decode_addr(addr), addr))