Re: [PATCH v2 2/3] tracing/fprobe: Support raw tracepoint events on modules

Steven Rostedt
2024-06-04T15:12:52Z

On Tue, 4 Jun 2024 08:49:55 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> On Mon, 3 Jun 2024 15:50:55 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
> > On 2024-06-01 04:22, Masami Hiramatsu (Google) wrote:
> > > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > >
> > > Support raw tracepoint event on module by fprobe events.
> > > Since it only uses for_each_kernel_tracepoint() to find a tracepoint,
> > > the tracepoints on modules are not handled. Thus if user specified a
> > > tracepoint on a module, it shows an error.
> > > This adds new for_each_module_tracepoint() API to tracepoint subsystem,
> > > and uses it to find tracepoints on modules.
> >
> > Hi Masami,
> >
> > Why prevent module unload when a fprobe tracepoint is attached to a
> > module ? This changes the kernel's behavior significantly just for the
> > sake of instrumentation.
>
> I don't prevent module unloading all the time, just before registering
> tracepoint handler (something like booking a ticket :-) ).
> See the last hunk of this patch, it puts the module before exiting
> __trace_fprobe_create().
>
> >
> > As an alternative, LTTng-modules attach/detach to/from modules with the
> > coming/going notifiers, so the instrumentation gets removed when a
> > module is unloaded rather than preventing its unload by holding a module
> > reference count. I would recommend a similar approach for fprobe.
>
> Yes, since tracepoint subsystem provides a notifier API to notify the
> tracepoint is gone, fprobe already uses it to find unloading and
> unregister the target function. (see __tracepoint_probe_module_cb)
>

Ah, it only prevents module unloading in __trace_fprobe_create()

> +static void __find_tracepoint_module_cb(struct tracepoint *tp, void *priv)
> +{
> + struct __find_tracepoint_cb_data *data = priv;
> +
> + if (!data->tpoint && !strcmp(data->tp_name, tp->name)) {
> + data->tpoint = tp;
> + data->mod = __module_text_address((unsigned long)tp->probestub);
> + if (!try_module_get(data->mod)) {

Here it gets the module. Should only happen once, as it sets data->tpoint.

> + data->tpoint = NULL;
> + data->mod = NULL;
> + }
> + }
> +}
> +
> static void __find_tracepoint_cb(struct tracepoint *tp, void *priv)
> {
> struct __find_tracepoint_cb_data *data = priv;
> @@ -905,14 +922,28 @@ static void __find_tracepoint_cb(struct tracepoint *tp, void *priv)
> data->tpoint = tp;
> }
>
> -static struct tracepoint *find_tracepoint(const char *tp_name)
> +/*
> + * Find a tracepoint from kernel and module. If the tracepoint is in a module,
> + * this increments the module refcount to prevent unloading until the
> + * trace_fprobe is registered to the list. After registering the trace_fprobe
> + * on the trace_fprobe list, the module refcount is decremented because
> + * tracepoint_probe_module_cb will handle it.
> + */
> +static struct tracepoint *find_tracepoint(const char *tp_name,
> + struct module **tp_mod)
> {
> struct __find_tracepoint_cb_data data = {
> .tp_name = tp_name,
> + .mod = NULL,
> };
>
> for_each_kernel_tracepoint(__find_tracepoint_cb, &data);
>
> + if (!data.tpoint && IS_ENABLED(CONFIG_MODULES)) {
> + for_each_module_tracepoint(__find_tracepoint_module_cb, &data);
> + *tp_mod = data.mod;
> + }
> +
> return data.tpoint;
> }
>
> @@ -996,6 +1027,7 @@ static int __trace_fprobe_create(int argc, const char *argv[])
> char abuf[MAX_BTF_ARGS_LEN];
> char *dbuf = NULL;
> bool is_tracepoint = false;
> + struct module *tp_mod = NULL;
> struct tracepoint *tpoint = NULL;
> struct traceprobe_parse_context ctx = {
> .flags = TPARG_FL_KERNEL | TPARG_FL_FPROBE,
> @@ -1080,7 +1112,7 @@ static int __trace_fprobe_create(int argc, const char *argv[])
>
> if (is_tracepoint) {
> ctx.flags |= TPARG_FL_TPOINT;
> - tpoint = find_tracepoint(symbol);
> + tpoint = find_tracepoint(symbol, &tp_mod);
> if (!tpoint) {
> trace_probe_log_set_index(1);
> trace_probe_log_err(0, NO_TRACEPOINT);
> @@ -1110,8 +1142,8 @@ static int __trace_fprobe_create(int argc, const char *argv[])
> goto out;
>
> /* setup a probe */
> - tf = alloc_trace_fprobe(group, event, symbol, tpoint, maxactive,
> - argc, is_return);
> + tf = alloc_trace_fprobe(group, event, symbol, tpoint, tp_mod,
> + maxactive, argc, is_return);
> if (IS_ERR(tf)) {
> ret = PTR_ERR(tf);
> /* This must return -ENOMEM, else there is a bug */
> @@ -1119,10 +1151,6 @@ static int __trace_fprobe_create(int argc, const char *argv[])
> goto out; /* We know tf is not allocated */
> }
>
> - if (is_tracepoint)
> - tf->mod = __module_text_address(
> - (unsigned long)tf->tpoint->probestub);
> -
> /* parse arguments */
> for (i = 0; i < argc && i < MAX_TRACE_ARGS; i++) {
> trace_probe_log_set_index(i + 2);
> @@ -1155,6 +1183,8 @@ static int __trace_fprobe_create(int argc, const char *argv[])
> }
>
> out:
> + if (tp_mod)
> + module_put(tp_mod);

And on the way out, it puts it.

-- Steve

> traceprobe_finish_parse(&ctx);
> trace_probe_log_clear();
> kfree(new_argv);