trace: Use correct argument index and tgid for filters

`trace` would use the incorrect argument index for USDT probes when
filtering specifically, e.g. `trace u:lib:tp (arg1 != 0) ...` would
actually use the type of the 2nd argument, and not the 1st argument
for the type of the filter variable in the generated program. This
could cause compilation errors or subtle bugs where the data would
be either extended or contracted to fit the wrong argument's type.

Additionally, `trace` would use the pid (thread id, `-L`) filter
with the `attach_uprobe` API, which expects a tgid (process id).
As a result, incorrect filtering would happen.

tools/trace.py

@@ -354,33 +354,35 @@ BPF_PERF_OUTPUT(%s);
 
         def _generate_usdt_filter_read(self):
             text = ""
-            if self.probe_type == "u":
-                    for arg, _ in Probe.aliases.items():
-                        if not (arg.startswith("arg") and
-                                (arg in self.filter)):
-                                continue
-                        arg_index = int(arg.replace("arg", ""))
-                        arg_ctype = self.usdt.get_probe_arg_ctype(
-                                self.usdt_name, arg_index)
-                        if not arg_ctype:
-                                self._bail("Unable to determine type of {} "
-                                           "in the filter".format(arg))
-                        text += """
+            if self.probe_type != "u":
+                    return text
+            for arg, _ in Probe.aliases.items():
+                    if not (arg.startswith("arg") and
+                            (arg in self.filter)):
+                            continue
+                    arg_index = int(arg.replace("arg", ""))
+                    arg_ctype = self.usdt.get_probe_arg_ctype(
+                            self.usdt_name, arg_index-1)
+                    if not arg_ctype:
+                            self._bail("Unable to determine type of {} "
+                                       "in the filter".format(arg))
+                    text += """
         {} {}_filter;
         bpf_usdt_readarg({}, ctx, &{}_filter);
-                        """.format(arg_ctype, arg, arg_index, arg)
-                        self.filter = self.filter.replace(
-                                arg, "{}_filter".format(arg))
+                    """.format(arg_ctype, arg, arg_index, arg)
+                    self.filter = self.filter.replace(
+                            arg, "{}_filter".format(arg))
             return text
 
         def generate_program(self, include_self):
                 data_decl = self._generate_data_decl()
-                # kprobes don't have built-in pid filters, so we have to add
-                # it to the function body:
-                if len(self.library) == 0 and Probe.pid != -1:
+                if Probe.pid != -1:
                         pid_filter = """
         if (__pid != %d) { return 0; }
                 """ % Probe.pid
+                # uprobes can have a built-in tgid filter passed to
+                # attach_uprobe, hence the check here -- for kprobes, we
+                # need to do the tgid test by hand:
                 elif len(self.library) == 0 and Probe.tgid != -1:
                         pid_filter = """
         if (__tgid != %d) { return 0; }
@@ -542,12 +544,12 @@ BPF_PERF_OUTPUT(%s);
                         bpf.attach_uretprobe(name=libpath,
                                              sym=self.function,
                                              fn_name=self.probe_name,
-                                             pid=Probe.pid)
+                                             pid=Probe.tgid)
                 else:
                         bpf.attach_uprobe(name=libpath,
                                           sym=self.function,
                                           fn_name=self.probe_name,
-                                          pid=Probe.pid)
+                                          pid=Probe.tgid)
 
 class Tool(object):
         examples = """