The anatomy of “Hello World” python program in bcc

The bcc gives a simple Hello World program analysis, but I want to give a more detailed anatomy of it:

from bcc import BPF

BPF(text='int kprobe__sys_clone(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }').trace_print()

(1)

from bcc import BPF  

bcc is a package, and it will be installed in Python‘s site-packages directory. Take my server as an example:

>>> import bcc
>>> bcc.__path__
['/usr/lib/python3.6/site-packages/bcc']

BPF is a class defined in __init__.py file of bcc:

......
class BPF(object):
# From bpf_prog_type in uapi/linux/bpf.h
    SOCKET_FILTER = 1
    KPROBE = 2
......

(2) Divide

BPF(text='int kprobe__sys_clone(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }').trace_print()

into 2 parts:

BPF(text='int kprobe__sys_clone(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }')

and

trace_print()

a)

BPF(text='int kprobe__sys_clone(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }')

This statement will create a BPF object, and the code is simplified as following:

......
from .libbcc import lib, _CB_TYPE, bcc_symbol, _SYM_CB_TYPE 
......
class BPF(object):
......
    def __init__(self, src_file="", hdr_file="", text=None, cb=None, debug=0,
        cflags=[], usdt_contexts=[]):
        ......

        if text:
            self.module = lib.bpf_module_create_c_from_string(text.encode("ascii"),
            self.debug, cflags_array, len(cflags_array))
        else:
            ......

        self._trace_autoload()

Check libbcc.py:

lib = ct.CDLL("libbcc.so.0", use_errno=True)

We can see lib module is indeed libbcc dynamic library, and bpf_module_create_c_from_string is implemented as this:

void * bpf_module_create_c_from_string(const char *text, unsigned flags, const char *cflags[], int ncflags) {
    auto mod = new ebpf::BPFModule(flags);
    if (mod->load_string(text, cflags, ncflags) != 0) {
        delete mod;
        return nullptr;
    }
    return mod;
}

We won’t delve it too much, and just know it returns a ebpf module is enough.

For self._trace_autoload() part, we can check _trace_autoload method definition:

def _trace_autoload(self):
    for i in range(0, lib.bpf_num_functions(self.module)):
        func_name = str(lib.bpf_function_name(self.module, i).decode())
        if func_name.startswith("kprobe__"):
            fn = self.load_func(func_name, BPF.KPROBE)
            self.attach_kprobe(event=fn.name[8:], fn_name=fn.name)
        elif func_name.startswith("kretprobe__"):
            fn = self.load_func(func_name, BPF.KPROBE)
            self.attach_kretprobe(event=fn.name[11:], fn_name=fn.name)
        elif func_name.startswith("tracepoint__"):
            fn = self.load_func(func_name, BPF.TRACEPOINT)
            tp = fn.name[len("tracepoint__"):].replace("__", ":")
            self.attach_tracepoint(tp=tp, fn_name=fn.name)

Based on above code, we can see bcc now supports kprobe, kretprobe and tracepoint. For Hello World example, it actually execute attach_kprobemethod:

def attach_kprobe(self, event="", fn_name="", event_re="",
        pid=-1, cpu=0, group_fd=-1):

    ......

    event = str(event)
    self._check_probe_quota(1)
    fn = self.load_func(fn_name, BPF.KPROBE)
    ev_name = "p_" + event.replace("+", "_").replace(".", "_")
    res = lib.bpf_attach_kprobe(fn.fd, 0, ev_name.encode("ascii"),
            event.encode("ascii"), pid, cpu, group_fd,
            self._reader_cb_impl, ct.cast(id(self), ct.py_object))
    res = ct.cast(res, ct.c_void_p)
    ......
    self._add_kprobe(ev_name, res)
    return self

Finally, it will call libbcc‘s bpf_attach_kprobe function. Until now, the kprobe is attached successfully, and attach_kprobe will return a BPF object.

(b)

BPF(...).trace_print()

The trace_print method is not too hard:

def trace_print(self, fmt=None):
    ......
    try:
        while True:
            if fmt:
                fields = self.trace_fields(nonblocking=False)
                if not fields: continue
                line = fmt.format(*fields)
            else:
                line = self.trace_readline(nonblocking=False)
            print(line)
            sys.stdout.flush()
    except KeyboardInterrupt:
        exit()

For fmt=None case, it just calls trace_readline(), which read from trace pipe, and prints it on the stdout:

print(line)
sys.stdout.flush()

That’s all, and hope you enjoy bcc and ebpf!