Check for NULL result from bpf_attach_kprobe

The check for NULL in the return value from C functions was comparing to
None incorrectly, causing an error check to pass improperly. Simply
check the truthiness of the return instead seems to be more resilient.

Add a test as well.

Fixes: #568
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

src/python/bcc/__init__.py

@@ -171,7 +171,7 @@ class BPF(object):
                 self.module = lib.bpf_module_create_c(src_file.encode("ascii"),
                         self.debug, cflags_array, len(cflags_array))
 
-        if self.module == None:
+        if not self.module:
             raise Exception("Failed to compile BPF module %s" % src_file)
 
         # If any "kprobe__" prefixed functions were defined, they will be
@@ -195,7 +195,7 @@ class BPF(object):
         if func_name in self.funcs:
             return self.funcs[func_name]
 
-        if lib.bpf_function_start(self.module, func_name.encode("ascii")) == None:
+        if not lib.bpf_function_start(self.module, func_name.encode("ascii")):
             raise Exception("Unknown program %s" % func_name)
 
         log_buf = ct.create_string_buffer(65536) if self.debug else None
@@ -227,7 +227,7 @@ class BPF(object):
         """
         Return the eBPF bytecodes for the specified function as a string
         """
-        if lib.bpf_function_start(self.module, func_name.encode("ascii")) == None:
+        if not lib.bpf_function_start(self.module, func_name.encode("ascii")):
             raise Exception("Unknown program %s" % func_name)
 
         start, = lib.bpf_function_start(self.module, func_name.encode("ascii")),
@@ -373,7 +373,7 @@ class BPF(object):
                 desc.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)
-        if res == None:
+        if not res:
             raise Exception("Failed to attach BPF to kprobe")
         open_kprobes[ev_name] = res
         return self
@@ -421,7 +421,7 @@ class BPF(object):
                 desc.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)
-        if res == None:
+        if not res:
             raise Exception("Failed to attach BPF to kprobe")
         open_kprobes[ev_name] = res
         return self
@@ -481,7 +481,7 @@ class BPF(object):
                 desc.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)
-        if res == None:
+        if not res:
             raise Exception("Failed to attach BPF to uprobe")
         open_uprobes[ev_name] = res
         return self
@@ -525,7 +525,7 @@ class BPF(object):
                 desc.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)
-        if res == None:
+        if not res:
             raise Exception("Failed to attach BPF to uprobe")
         open_uprobes[ev_name] = res
         return self

tests/python/test_probe_count.py

@@ -34,6 +34,14 @@ class TestProbeQuota(TestCase):
         with self.assertRaises(Exception):
             self.b.attach_kprobe(event_re=".*", fn_name="count")
 
+class TestProbeNotExist(TestCase):
+    def setUp(self):
+        self.b = BPF(text="""int count(void *ctx) { return 0; }""")
+
+    def test_not_exist(self):
+        with self.assertRaises(Exception):
+            b.attach_kprobe(event="___doesnotexist", fn_name="count")
+
 
 if __name__ == "__main__":
     main()