python: Print BPF syscall error if DEBUG_BPF is on but log_buf is empty.

Commit 759029fe provided an option to
store the output from BPF syscall into a buffer (and not to print it
systematically to standard output) on program load in libbpf.c.

But doing so, it only stores the content of attr.log_buf, while the
error string--resulting from a failed BPF syscall--is no more displayed
when the DEBUG_BPF flag is used in the Python script responsible for
converting and injecting the code.

This commit proposes a fix for this bug by printing the error message
(associated to the return value from the syscall) from the Python
caller, when all the following conditions are met:

- the syscall fails,
- the DEBUG_BPF flag has been provided, and
- log_buf is empty (has not been filled by kernel).

Note: when DEBUG_BPF is not provided, the error string is printed in the
C wrapper in libbpf.c (bpf_prog_load) anyway.

Fixes: 759029fe ("Add option for custom log string to bpf_prog_load")
Signed-off-by: Quentin Monnet <quentin.monnet@6wind.com>

src/python/bcc/__init__.py

@@ -207,10 +207,15 @@ class BPF(object):
                 lib.bpf_module_kern_version(self.module),
                 log_buf, ct.sizeof(log_buf) if log_buf else 0)
 
-        if self.debug & DEBUG_BPF:
+        if self.debug & DEBUG_BPF and log_buf.value:
             print(log_buf.value.decode(), file=sys.stderr)
 
         if fd < 0:
+            if self.debug & DEBUG_BPF and not log_buf.value:
+                errstr = os.strerror(ct.get_errno())
+                raise Exception("Failed to load BPF program %s: %s" %
+                                (func_name, errstr))
+            else:
                 raise Exception("Failed to load BPF program %s" % func_name)
 
         fn = BPF.Function(self, func_name, fd)