trace

README.md

@@ -108,6 +108,7 @@ Tools:
 - tools/[tcpconnect](tools/tcpconnect.py): Trace TCP active connections (connect()). [Examples](tools/tcpconnect_example.txt).
 - tools/[tcpconnlat](tools/tcpconnlat.py): Trace TCP active connection latency (connect()). [Examples](tools/tcpconnlat_example.txt).
 - tools/[tcpretrans](tools/tcpretrans.py): Trace TCP retransmits and TLPs. [Examples](tools/tcpretrans_example.txt).
+- tools/[trace](tools/trace.py): Trace arbitrary functions, with filters. [Examples](tools/trace_example.txt)
 - tools/[vfscount](tools/vfscount.py) tools/[vfscount.c](tools/vfscount.c): Count VFS calls. [Examples](tools/vfscount_example.txt).
 - tools/[vfsstat](tools/vfsstat.py) tools/[vfsstat.c](tools/vfsstat.c): Count some VFS calls, with column output. [Examples](tools/vfsstat_example.txt).
 - tools/[wakeuptime](tools/wakeuptime.py): Summarize sleep to wakeup time by waker kernel stack. [Examples](tools/wakeuptime_example.txt).

man/man8/trace.8

+.TH trace 8  "2016-02-18" "USER COMMANDS"
+.SH NAME
+trace \- Trace a function and print its arguments or return value, optionally evaluating a filter. Uses Linux eBPF/bcc.
+.SH SYNOPSIS
+.B trace [-h] [-p PID] [-v] [-Z STRING_SIZE] [-S] [-M MAX_EVENTS] [-o] probe [probe ...]
+.SH DESCRIPTION
+trace probes functions you specify and displays trace messages if a particular
+condition is met. You can control the message format to display function 
+arguments and return values. 
+
+Since this uses BPF, only the root user can use this tool.
+.SH REQUIREMENTS
+CONFIG_BPF and bcc.
+.SH OPTIONS
+.TP
+\-h
+Print usage message.
+.TP
+\-p PID
+Trace only functions in the process PID.
+.TP
+\-v
+Display the generated BPF program, for debugging purposes.
+.TP
+\-z STRING_SIZE
+When collecting string arguments (of type char*), collect up to STRING_SIZE 
+characters. Longer strings will be truncated.
+.TP
+\-S
+If set, trace messages from trace's own process. By default, this is off to
+avoid tracing storms -- for example, if you trace the write system call, and
+consider that trace is writing to the standard output.
+.TP
+\-M MAX_EVENTS
+Print up to MAX_EVENTS trace messages and then exit.
+.TP
+\-o
+Print times relative to the beginning of the trace (offsets), in seconds. The
+default is to print absolute time.
+.TP
+probe [probe ...]
+One or more probes that attach to functions, filter conditions, and print
+information. See PROBE SYNTAX below.
+.SH PROBE SYNTAX
+The general probe syntax is as follows:
+
+.B [{p,r}]:[library]:function [(predicate)] ["format string"[, arguments]]
+.TP
+.B [{p,r}]
+Probe type \- "p" for function entry, "r" for function return. The default
+probe type is "p".
+.TP
+.B [library]
+Library containing the probe.
+Specify the full path to the .so or executable file where the function to probe
+resides. Alternatively, you can specify just the lib name: for example, "c"
+refers to libc. If no library name is specified, the kernel is assumed. Also,
+you can specify an executable name (without a full path) if it is in the PATH.
+For example, "bash".
+.TP
+.B function
+The function to probe.
+.TP
+.B [(predicate)]
+The filter applied to the captured data. Only if the filter evaluates as true,
+the trace message will be printed. The filter can use any valid C expression
+that refers to the argument values: arg1, arg2, etc., or to the return value
+retval in a return probe. If necessary, use C cast operators to coerce the
+arguments to the desired type. For example, if arg1 is of type int, use the
+expression ((int)arg1 < 0) to trace only invocations where arg1 is negative.
+Note that only arg1-arg6 are supported, and only if the function is using the
+standard x86_64 convention where the first six arguments are in the RDI, RSI, 
+RDX, RCX, R8, R9 registers. If no predicate is specified, all function 
+invocations are traced.
+.TP
+.B ["format string"[, arguments]]
+A printf-style format string that will be used for the trace message. You can
+use the following format specifiers: %s, %d, %u, %lld, %llu, %hd, %hu, %c,
+%x, %llx -- with the same semantics as printf's. Make sure to pass the exact
+number of arguments as there are placeholders in the format string. The
+format specifier replacements may be any C expressions, and may refer to the
+same special keywords as in the predicate (arg1, arg2, etc.).
+
+The predicate expression and the format specifier replacements for printing
+may also use the following special keywords: $pid, $tgid to refer to the 
+current process' pid and tgid; $uid, $gid to refer to the current user's
+uid and gid; $cpu to refer to the current processor number.
+.SH EXAMPLES
+.TP
+Trace all invocations of the open system call with the name of the file being opened:
+#
+.B trace '::do_sys_open """%s"", arg2'
+.TP
+Trace all invocations of the read system call where the number of bytes requested is greater than 20,000:
+#
+.B trace '::sys_read (arg3 > 20000) """read %d bytes"", arg3'
+.TP
+Trace all malloc calls and print the size of the requested allocation:
+#
+.B trace ':c:malloc """size = %d"", arg1'
+.TP
+Trace returns from the readline function in bash and print the return value as a string:
+#
+.B trace 'r:bash:readline """%s"", retval' 
+.SH SOURCE
+This is from bcc.
+.IP
+https://github.com/iovisor/bcc
+.PP
+Also look in the bcc distribution for a companion _examples.txt file containing
+example usage, output, and commentary for this tool.
+.SH OS
+Linux
+.SH STABILITY
+Unstable - in development.
+.SH AUTHOR
+Sasha Goldshtein

tools/trace.py

+#!/usr/bin/env python
+#
+# trace         Trace a function and print a trace message based on its
+#               parameters, with an optional filter.
+#
+# USAGE: trace [-h] [-p PID] [-v] [-Z STRING_SIZE] [-S] [-M MAX_EVENTS] [-o]
+#              probe [probe ...]
+# Licensed under the Apache License, Version 2.0 (the "License")
+# Copyright (C) 2016 Sasha Goldshtein.
+
+from bcc import BPF
+from time import sleep, strftime
+import argparse
+import re
+import ctypes as ct
+import os
+import traceback
+import sys
+
+class Time(object):
+        # BPF timestamps come from the monotonic clock. To be able to filter
+        # and compare them from Python, we need to invoke clock_gettime.
+        # Adapted from http://stackoverflow.com/a/1205762
+        CLOCK_MONOTONIC_RAW = 4         # see <linux/time.h>
+
+        class timespec(ct.Structure):
+                _fields_ = [
+                        ('tv_sec', ct.c_long),
+                        ('tv_nsec', ct.c_long)
+                ]
+
+        librt = ct.CDLL('librt.so.1', use_errno=True)
+        clock_gettime = librt.clock_gettime
+        clock_gettime.argtypes = [ct.c_int, ct.POINTER(timespec)]
+
+        @staticmethod
+        def monotonic_time():
+                t = Time.timespec()
+                if Time.clock_gettime(
+                        Time.CLOCK_MONOTONIC_RAW, ct.pointer(t)) != 0:
+                        errno_ = ct.get_errno()
+                        raise OSError(errno_, os.strerror(errno_))
+                return t.tv_sec * 1e9 + t.tv_nsec
+
+class Probe(object):
+        probe_count = 0
+        max_events = None
+        event_count = 0
+        first_ts = 0
+        use_localtime = True
+
+        @classmethod
+        def configure(cls, args):
+                cls.max_events = args.max_events
+                cls.use_localtime = not args.offset
+                cls.first_ts = Time.monotonic_time()
+
+        def __init__(self, probe, string_size):
+                self.raw_probe = probe
+                self.string_size = string_size
+                Probe.probe_count += 1
+                self._parse_probe()
+                self.probe_num = Probe.probe_count
+                self.probe_name = "probe_%s_%d" % \
+                                (self.function, self.probe_num)
+
+        def __str__(self):
+                return "%s:%s`%s FLT=%s ACT=%s/%s" % (self.probe_type,
+                        self.library, self.function, self.filter,
+                        self.types, self.values)
+
+        def is_default_action(self):
+                return self.python_format == ""
+
+        def _bail(self, error):
+                raise ValueError("error parsing probe '%s': %s" %
+                                 (self.raw_probe, error))
+
+        def _parse_probe(self):
+                text = self.raw_probe
+
+                # Everything until the first space is the probe specifier
+                first_space = text.find(' ')
+                spec = text[:first_space] if first_space >= 0 else text
+                self._parse_spec(spec)
+                if first_space >= 0:
+                        text = text[first_space:].lstrip()
+                else:
+                        text = ""
+
+                # If we now have a (, wait for the balanced closing ) and that
+                # will be the predicate
+                self.filter = None
+                if len(text) > 0 and text[0] == "(":
+                        balance = 1
+                        for i in range(1, len(text)):
+                                if text[i] == "(":
+                                        balance += 1
+                                if text[i] == ")":
+                                        balance -= 1
+                                if balance == 0:
+                                        self._parse_filter(text[:i+1])
+                                        text = text[i+1:]
+                                        break
+                        if self.filter is None:
+                                self._bail("unmatched end of predicate")
+
+                if self.filter is None:
+                        self.filter = "1"
+
+                # The remainder of the text is the printf action
+                self._parse_action(text.lstrip())
+
+        def _parse_spec(self, spec):
+                parts = spec.split(":")
+                # Two special cases: 'func' means 'p::func', 'lib:func' means
+                # 'p:lib:func'. Other combinations need to provide an empty
+                # value between delimiters, e.g. 'r::func' for a kretprobe on
+                # the function func.
+                if len(parts) == 1:
+                        parts = ["p", "", parts[0]]
+                elif len(parts) == 2:
+                        parts = ["p", parts[0], parts[1]]
+                if len(parts[0]) == 0:
+                        self.probe_type = "p"
+                elif parts[0] in ["p", "r"]:
+                        self.probe_type = parts[0]
+                else:
+                        self._bail("expected '', 'p', or 'r', got '%s'" %
+                                   parts[0])
+                self.library = parts[1]
+                self.function = parts[2]
+
+        def _parse_filter(self, filt):
+                self.filter = self._replace_args(filt)
+
+        def _parse_types(self, fmt):
+                for match in re.finditer(
+                                r'[^%]%(s|u|d|llu|lld|hu|hd|x|llx|c)', fmt):
+                        self.types.append(match.group(1))
+                fmt = re.sub(r'([^%]%)(u|d|llu|lld|hu|hd)', r'\1d', fmt)
+                fmt = re.sub(r'([^%]%)(x|llx)', r'\1x', fmt)
+                self.python_format = fmt.strip('"')
+
+        def _parse_action(self, action):
+                self.values = []
+                self.types = []
+                self.python_format = ""
+                if len(action) == 0:
+                        return
+
+                parts = action.split(',')
+                self.raw_format = parts[0]
+                self._parse_types(self.raw_format)
+                for part in parts[1:]:
+                        part = self._replace_args(part)
+                        self.values.append(part)
+
+        aliases = {
+                "retval": "ctx->ax",
+                "arg1": "ctx->di",
+                "arg2": "ctx->si",
+                "arg3": "ctx->dx",
+                "arg4": "ctx->cx",
+                "arg5": "ctx->r8",
+                "arg6": "ctx->r9",
+                "$uid": "(unsigned)(bpf_get_current_uid_gid() & 0xffffffff)",
+                "$gid": "(unsigned)(bpf_get_current_uid_gid() >> 32)",
+                "$pid": "(unsigned)(bpf_get_current_pid_tgid() & 0xffffffff)",
+                "$tgid": "(unsigned)(bpf_get_current_pid_tgid() >> 32)",
+                "$cpu": "bpf_get_smp_processor_id()"
+        }
+
+        def _replace_args(self, expr):
+                for alias, replacement in Probe.aliases.items():
+                        expr = expr.replace(alias, replacement)
+                return expr
+
+        p_type = { "u": ct.c_uint, "d": ct.c_int,
+                   "llu": ct.c_ulonglong, "lld": ct.c_longlong,
+                   "hu": ct.c_ushort, "hd": ct.c_short,
+                   "x": ct.c_uint, "llx": ct.c_ulonglong,
+                   "c": ct.c_ubyte }
+
+        def _generate_python_field_decl(self, idx, fields):
+                field_type = self.types[idx]
+                if field_type == "s":
+                        ptype = ct.c_char * self.string_size
+                else:
+                        ptype = Probe.p_type[field_type]
+                fields.append(("v%d" % idx, ptype))
+
+        def _generate_python_data_decl(self):
+                self.python_struct_name = "%s_%d_Data" % \
+                                (self.function, self.probe_num)
+                fields = [
+                        ("timestamp_ns", ct.c_ulonglong),
+                        ("pid", ct.c_uint),
+                        ("comm", ct.c_char * 16)       # TASK_COMM_LEN
+                ]
+                for i in range(0, len(self.types)):
+                        self._generate_python_field_decl(i, fields)
+                return type(self.python_struct_name, (ct.Structure,),
+                            dict(_fields_=fields))
+
+        c_type = { "u": "unsigned int", "d": "int",
+                   "llu": "unsigned long long", "lld": "long long",
+                   "hu": "unsigned short", "hd": "short",
+                   "x": "unsigned int", "llx": "unsigned long long",
+                   "c": "char" }
+        fmt_types = c_type.keys()
+
+        def _generate_field_decl(self, idx):
+                field_type = self.types[idx]
+                if field_type == "s":
+                        return "char v%d[%d];\n" % (idx, self.string_size)
+                if field_type in Probe.fmt_types:
+                        return "%s v%d;\n" % (Probe.c_type[field_type], idx)
+                self._bail("unrecognized format specifier %s" % field_type)
+
+        def _generate_data_decl(self):
+                # The BPF program will populate values into the struct
+                # according to the format string, and the Python program will
+                # construct the final display string.
+                self.events_name = "%s_events" % self.probe_name
+                self.struct_name = "%s_data_t" % self.probe_name
+
+                data_fields = ""
+                for i, field_type in enumerate(self.types):
+                        data_fields += "        " + \
+                                       self._generate_field_decl(i)
+
+                text = """
+struct %s
+{
+        u64 timestamp_ns;
+        u32 pid;
+        char comm[TASK_COMM_LEN];
+%s
+};
+
+BPF_PERF_OUTPUT(%s);
+"""
+                return text % (self.struct_name, data_fields, self.events_name)
+
+        def _generate_field_assign(self, idx):
+                field_type = self.types[idx]
+                expr = self.values[idx]
+                if field_type == "s":
+                        return """
+        if (%s != 0) {
+                bpf_probe_read(&__data.v%d, sizeof(__data.v%d), (void *)%s);
+        }
+"""                     % (expr, idx, idx, expr)
+                        # return ("bpf_probe_read(&__data.v%d, " + \
+                        # "sizeof(__data.v%d), (char*)%s);\n") % (idx, idx, expr)
+                        # return ("__builtin_memcpy(&__data.v%d, (void *)%s, " + \
+                        #        "sizeof(__data.v%d));\n") % (idx, expr, idx)
+                if field_type in Probe.fmt_types:
+                        return "        __data.v%d = (%s)%s;\n" % \
+                                        (idx, Probe.c_type[field_type], expr)
+                self._bail("unrecognized field type %s" % field_type)
+
+        def generate_program(self, pid, include_self):
+                data_decl = self._generate_data_decl()
+                self.pid = pid
+                # kprobes don't have built-in pid filters, so we have to add
+                # it to the function body:
+                if len(self.library) == 0 and pid != -1:
+                        pid_filter = """
+        u32 __pid = bpf_get_current_pid_tgid();
+        if (__pid != %d) { return 0; }
+"""             % pid
+                elif not include_self:
+                        pid_filter = """
+        u32 __pid = bpf_get_current_pid_tgid();
+        if (__pid == %d) { return 0; }
+"""             % os.getpid()
+                else:
+                        pid_filter = ""
+
+                data_fields = ""
+                for i, expr in enumerate(self.values):
+                        data_fields += self._generate_field_assign(i)
+
+                text = """
+int %s(struct pt_regs *ctx)
+{
+        %s
+        if (!(%s)) return 0;
+
+        struct %s __data = {0};
+        __data.timestamp_ns = bpf_ktime_get_ns();
+        __data.pid = bpf_get_current_pid_tgid();
+        bpf_get_current_comm(&__data.comm, sizeof(__data.comm));
+%s
+        %s.perf_submit(ctx, &__data, sizeof(__data));
+        return 0;
+}
+"""
+                text = text % (self.probe_name, pid_filter,
+                               self.filter, self.struct_name,
+                               data_fields, self.events_name)
+
+                return data_decl + "\n" + text
+
+        @classmethod
+        def _time_off_str(cls, timestamp_ns):
+                return "%.6f" % (1e-9 * (timestamp_ns - cls.first_ts))
+
+        def print_event(self, cpu, data, size):
+                # Cast as the generated structure type and display
+                # according to the format string in the probe.
+                event = ct.cast(data, ct.POINTER(self.python_struct)).contents
+                values = map(lambda i: getattr(event, "v%d" % i),
+                             range(0, len(self.values)))
+                msg = self.python_format % tuple(values)
+                time = strftime("%H:%M:%S") if Probe.use_localtime else \
+                       Probe._time_off_str(event.timestamp_ns)
+                print("%-8s %-6d %-12s %-16s %s" % \
+                    (time[:8], event.pid, event.comm[:12], self.function, msg))
+
+                Probe.event_count += 1
+                if Probe.max_events is not None and \
+                   Probe.event_count >= Probe.max_events:
+                        exit()
+
+        def attach(self, bpf, verbose):
+                if len(self.library) == 0:
+                        self._attach_k(bpf)
+                else:
+                        self._attach_u(bpf)
+                self.python_struct = self._generate_python_data_decl()
+                bpf[self.events_name].open_perf_buffer(self.print_event)
+
+        def _attach_k(self, bpf):
+                if self.probe_type == "r":
+                        bpf.attach_kretprobe(event=self.function,
+                                             fn_name=self.probe_name)
+                elif self.probe_type == "p":
+                        bpf.attach_kprobe(event=self.function,
+                                          fn_name=self.probe_name)
+
+        def _attach_u(self, bpf):
+                libpath = BPF.find_library(self.library)
+                if libpath is None:
+                        # This might be an executable (e.g. 'bash')
+                        with os.popen("/usr/bin/which %s 2>/dev/null" %
+                                      self.library) as w:
+                                libpath = w.read().strip()
+                if libpath is None or len(libpath) == 0:
+                        self._bail("unable to find library %s" % self.library)
+
+                if self.probe_type == "r":
+                        bpf.attach_uretprobe(name=libpath,
+                                             sym=self.function,
+                                             fn_name=self.probe_name,
+                                             pid=self.pid)
+                else:
+                        bpf.attach_uprobe(name=libpath,
+                                          sym=self.function,
+                                          fn_name=self.probe_name,
+                                          pid=self.pid)
+
+class Tool(object):
+        examples = """
+EXAMPLES:
+
+trace ::do_sys_open
+        Trace the open syscall and print a default trace message when entered
+trace '::do_sys_open "%s", arg2'
+        Trace the open syscall and print the filename being opened
+trace '::sys_read (arg3 > 20000) "read %d bytes", arg3'
+        Trace the read syscall and print a message for reads >20000 bytes
+trace 'r::do_sys_return "%llx", retval'
+        Trace the return from the open syscall and print the return value
+trace ':c:open (arg2 == 42) "%s %d", arg1, arg2'
+        Trace the open() call from libc only if the flags (arg2) argument is 42
+trace ':c:malloc "size = %d", arg1'
+        Trace malloc calls and print the size being allocated
+trace 'r:c:malloc (retval) "allocated = %p", retval
+        Trace returns from malloc and print non-NULL allocated buffers
+"""
+
+        def __init__(self):
+                parser = argparse.ArgumentParser(description=
+                        "Attach to functions and print trace messages.",
+                        formatter_class=argparse.RawDescriptionHelpFormatter,
+                        epilog=Tool.examples)
+                parser.add_argument("-p", "--pid", type=int,
+                  help="id of the process to trace (optional)")
+                parser.add_argument("-v", "--verbose", action="store_true",
+                  help="print resulting BPF program code before executing")
+                parser.add_argument("-Z", "--string-size", type=int,
+                  default=80, help="maximum size to read from strings")
+                parser.add_argument("-S", "--include-self", action="store_true",
+                  help="do not filter trace's own pid from the trace")
+                parser.add_argument("-M", "--max-events", type=int,
+                  help="number of events to print before quitting")
+                parser.add_argument("-o", "--offset", action="store_true",
+                  help="use relative time from first traced message")
+                parser.add_argument(metavar="probe", dest="probes", nargs="+",
+                  help="probe specifier (see examples)")
+                self.args = parser.parse_args()
+
+        def _create_probes(self):
+                Probe.configure(self.args)
+                self.probes = []
+                for probe_spec in self.args.probes:
+                        self.probes.append(Probe(
+                                probe_spec, self.args.string_size))
+
+        def _generate_program(self):
+                self.program = """
+#include <linux/ptrace.h>
+#include <linux/sched.h>        /* For TASK_COMM_LEN */
+
+"""
+                for probe in self.probes:
+                        self.program += probe.generate_program(
+                                self.args.pid or -1, self.args.include_self)
+
+                if self.args.verbose:
+                        print(self.program)
+
+        def _attach_probes(self):
+                self.bpf = BPF(text=self.program)
+                for probe in self.probes:
+                        if self.args.verbose:
+                                print(probe)
+                        probe.attach(self.bpf, self.args.verbose)
+
+        def _main_loop(self):
+                all_probes_trivial = all(map(Probe.is_default_action,
+                                             self.probes))
+
+                # Print header
+                print("%-8s %-6s %-12s %-16s %s" % \
+                      ("TIME", "PID", "COMM", "FUNC",
+                      "-" if not all_probes_trivial else ""))
+
+                while True:
+                        self.bpf.kprobe_poll()
+
+        def run(self):
+                try:
+                        self._create_probes()
+                        self._generate_program()
+                        self._attach_probes()
+                        self._main_loop()
+                except:
+                        if self.args.verbose:
+                                traceback.print_exc()
+                        else:
+                                print(sys.exc_value)
+
+if __name__ == "__main__":
+       Tool().run()

tools/trace_example.txt

+Demonstrations of trace.
+
+
+trace probes functions you specify and displays trace messages if a particular
+condition is met. You can control the message format to display function 
+arguments and return values. 
+
+For example, suppose you want to trace all commands being exec'd across the
+system:
+
+# trace '::sys_execve "%s", arg1'
+TIME     PID    COMM         FUNC             -
+05:11:51 4402   bash         sys_execve       /usr/bin/man
+05:11:51 4411   man          sys_execve       /usr/local/bin/less
+05:11:51 4411   man          sys_execve       /usr/bin/less
+05:11:51 4410   man          sys_execve       /usr/local/bin/nroff
+05:11:51 4410   man          sys_execve       /usr/bin/nroff
+05:11:51 4409   man          sys_execve       /usr/local/bin/tbl
+05:11:51 4409   man          sys_execve       /usr/bin/tbl
+05:11:51 4408   man          sys_execve       /usr/local/bin/preconv
+05:11:51 4408   man          sys_execve       /usr/bin/preconv
+05:11:51 4415   nroff        sys_execve       /usr/bin/locale
+05:11:51 4416   nroff        sys_execve       /usr/bin/groff
+05:11:51 4418   groff        sys_execve       /usr/bin/grotty
+05:11:51 4417   groff        sys_execve       /usr/bin/troff
+^C
+
+The ::sys_execve syntax specifies that you want an entry probe (which is the
+default), in a kernel function (which is the default) called sys_execve. Next,
+the format string to print is simply "%s", which prints a string. Finally, the
+value to print is the first argument to the sys_execve function, which happens
+to be the command that is exec'd. The above trace was generated by executing
+"man ls" in a separate shell. As you see, man executes a number of additional
+programs to finally display the man page.
+
+Next, suppose you are looking for large reads across the system. Let's trace
+the read system call and inspect the third argument, which is the number of
+bytes to be read:
+
+# trace '::sys_read (arg3 > 20000) "read %d bytes", arg3'
+TIME     PID    COMM         FUNC             -
+05:18:23 4490   dd           sys_read         read 1048576 bytes
+05:18:23 4490   dd           sys_read         read 1048576 bytes
+05:18:23 4490   dd           sys_read         read 1048576 bytes
+05:18:23 4490   dd           sys_read         read 1048576 bytes
+^C
+
+During the trace, I executed "dd if=/dev/zero of=/dev/null bs=1M count=4".
+The individual reads are visible, with the custom format message printed for
+each read. The parenthesized expression "(arg3 > 20000)" is a filter that is
+evaluated for each invocation of the probe before printing anything.
+
+You can also trace user functions. For example, let's simulate the bashreadline
+script, which attaches to the readline function in bash and prints its return
+value, effectively snooping all bash shell input across the system:
+
+# trace 'r:bash:readline "%s", retval'
+TIME     PID    COMM         FUNC             -
+05:24:50 2740   bash         readline         echo hi!
+05:24:53 2740   bash         readline         man ls
+^C
+
+The special retval keywords stands for the function's return value, and can
+be used only in a retprobe, specified by the 'r' prefix. The next component
+of the probe is the library that contains the desired function. It's OK to
+specify executables too, as long as they can be found in the PATH. Or, you
+can specify the full path to the executable (e.g. "/usr/bin/bash").
+
+Multiple probes can be combined on the same command line. For example, let's
+trace failed read and write calls on the libc level:
+
+# trace 'r:c:read ((int)retval < 0) "read failed: %d", retval' \
+        'r:c:write ((int)retval < 0) "write failed: %d", retval'
+TIME     PID    COMM         FUNC             -
+05:31:57 3388   bash         write            write failed: -1
+05:32:00 3388   bash         write            write failed: -1
+^C
+
+Note that the retval variable must be cast to int before comparing to zero.
+The reason is that the default type for argN and retval is an unsigned 64-bit
+integer, which can never be smaller than 0.
+
+As a final example, let's trace open syscalls for a specific process. By 
+default, tracing is system-wide, but the -p switch overrides this:
+
+# trace -p 2740 '::do_sys_open "%s", arg2'
+TIME     PID    COMM         FUNC             -
+05:36:16 15872  ls           do_sys_open      /etc/ld.so.cache
+05:36:16 15872  ls           do_sys_open      /lib64/libselinux.so.1
+05:36:16 15872  ls           do_sys_open      /lib64/libcap.so.2
+05:36:16 15872  ls           do_sys_open      /lib64/libacl.so.1
+05:36:16 15872  ls           do_sys_open      /lib64/libc.so.6
+05:36:16 15872  ls           do_sys_open      /lib64/libpcre.so.1
+05:36:16 15872  ls           do_sys_open      /lib64/libdl.so.2
+05:36:16 15872  ls           do_sys_open      /lib64/libattr.so.1
+05:36:16 15872  ls           do_sys_open      /lib64/libpthread.so.0
+05:36:16 15872  ls           do_sys_open      /usr/lib/locale/locale-archive
+05:36:16 15872  ls           do_sys_open      /home/vagrant
+^C
+
+In this example, we traced the "ls ~" command as it was opening its shared
+libraries and then accessing the /home/vagrant directory listing.
+
+
+USAGE message:
+
+# trace -h
+usage: trace.py [-h] [-p PID] [-v] [-Z STRING_SIZE] [-S] [-M MAX_EVENTS] [-o]
+                probe [probe ...]
+
+Attach to functions and print trace messages.
+
+positional arguments:
+  probe                 probe specifier (see examples)
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -p PID, --pid PID     id of the process to trace (optional)
+  -v, --verbose         print resulting BPF program code before executing
+  -Z STRING_SIZE, --string-size STRING_SIZE
+                        maximum size to read from strings
+  -S, --include-self    do not filter trace's own pid from the trace
+  -M MAX_EVENTS, --max-events MAX_EVENTS
+                        number of events to print before quitting
+  -o, --offset          use relative time from first traced message
+
+EXAMPLES:
+
+trace ::do_sys_open
+        Trace the open syscall and print a default trace message when entered
+trace '::do_sys_open "%s", arg2'
+        Trace the open syscall and print the filename being opened
+trace '::sys_read (arg3 > 20000) "read %d bytes", arg3'
+        Trace the read syscall and print a message for reads >20000 bytes
+trace r::do_sys_return
+        Trace the return from the open syscall
+trace ':c:open (arg2 == 42) "%s %d", arg1, arg2'
+        Trace the open() call from libc only if the flags (arg2) argument is 42
+trace ':c:malloc "size = %d", arg1'
+        Trace malloc calls and print the size being allocated
+trace 'r:c:malloc (retval) "allocated = %p", retval
+        Trace returns from malloc and print non-NULL allocated buffers
+