Add field support to trace_print and friends

Add trace_readline_fields helper to parse the output of trace_pipe
Add field parsing support to trace_print. Addresses iovisor#149.
Fix typo in trace_open s/trace/tracefile/
Make nonblocking=False the default in trace_readline
Use IOError vs BlockingIOError for greater compatibility (untested)

Fixes: iovisor#149
Signed-off-by: Brenden Blanco <[email protected]>

examples/hello_world.py

@@ -6,14 +6,13 @@
 # sudo ./hello_world.py"
 
 from bpf import BPF
-from subprocess import call
 
 prog = """
 int hello(void *ctx) {
  bpf_trace_printk("Hello, World!\\n");
  return 0;
-};
+}
 """
 b = BPF(text=prog)
 b.attach_kprobe(event="sys_clone", fn_name="hello")
-b.trace_print()
+b.trace_print(fmt="{1} {5}")

src/python/bpf/__init__.py

@@ -389,7 +389,7 @@ def attach_kprobe(self, event="", fn_name="", pid=0, cpu=-1, group_fd=-1):
  if res < 0:
  raise Exception("Failed to attach BPF to kprobe")
  open_kprobes[ev_name] = res
- return res
+ return self
 
  @staticmethod
  def detach_kprobe(event):
@@ -412,7 +412,7 @@ def attach_kretprobe(self, event="", fn_name="", pid=-1, cpu=0, group_fd=-1):
  if res < 0:
  raise Exception("Failed to attach BPF to kprobe")
  open_kprobes[ev_name] = res
- return res
+ return self
 
  @staticmethod
  def detach_kretprobe(event):
@@ -437,14 +437,33 @@ def trace_open(nonblocking=False):
  if not tracefile:
  tracefile = open("%s/trace_pipe" % TRACEFS)
  if nonblocking:
- fd = trace.fileno()
+ fd = tracefile.fileno()
  fl = fcntl.fcntl(fd, fcntl.F_GETFL)
  fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
  return tracefile
 
  @staticmethod
- def trace_readline(nonblocking=True):
- """trace_readline(nonblocking=True)
+ def trace_readline_fields(nonblocking=False):
+ """trace_readline_fields(nonblocking=False)
+
+ Read from the kernel debug trace pipe and return a tuple of the
+ fields (task, pid, cpu, flags, timestamp, msg) or None if no
+ line was read (nonblocking=True)
+ """
+ line = BPF.trace_readline(nonblocking)
+ if line:
+ task = line[:16].lstrip()
+ line = line[17:]
+ ts_end = line.find(":")
+ pid, cpu, flags, ts = line[:ts_end].split()
+ cpu = cpu[1:-1]
+ msg = line[ts_end + 4:]
+ return (task, int(pid), int(cpu), flags, float(ts), msg)
+ return
+
+ @staticmethod
+ def trace_readline(nonblocking=False):
+ """trace_readline(nonblocking=False)
 
  Read from the kernel debug trace pipe and return one line
  If nonblocking is False, this will block until ctrl-C is pressed.
@@ -454,17 +473,28 @@ def trace_readline(nonblocking=True):
 
  line = None
  try:
- line = trace.readline(128).rstrip()
- except BlockingIOError:
+ line = trace.readline(1024).rstrip()
+ except IOError:
  pass
+ except KeyboardInterrupt:
+ exit()
  return line
 
  @staticmethod
- def trace_print():
- try:
- while True:
+ def trace_print(fmt=None):
+ """trace_print(fmt=None)
+
+ Read from the kernel debug trace pipe and print on stdout.
+ If fmt is specified, apply as a format string to the output. See
+ trace_readline_fields for the members of the tuple
+ example: trace_print(fmt="pid {1}, msg = {5}")
+ """
+
+ while True:
+ if fmt:
+ fields = BPF.trace_readline_fields(nonblocking=False)
+ line = fmt.format(*fields)
+ else:
  line = BPF.trace_readline(nonblocking=False)
- print(line)
- sys.stdout.flush()
- except KeyboardInterrupt:
- exit()
+ print(line)
+ sys.stdout.flush()