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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

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)

Thank you!


> 
> Thanks,
> 
> Mathieu
> 
> 
> > 
> > Reported-by: don <zds100@gmail.com>
> > Closes: https://lore.kernel.org/all/20240530215718.aeec973a1d0bf058d39cb1e3@kernel.org/
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> >   Changes in v2:
> >    - Fix build errors with CONFIG_MODULES=y.
> > ---
> >   kernel/trace/trace_fprobe.c |   46 ++++++++++++++++++++++++++++++++++++-------
> >   1 file changed, 38 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_fprobe.c b/kernel/trace/trace_fprobe.c
> > index 62e6a8f4aae9..1d8a983e1edc 100644
> > --- a/kernel/trace/trace_fprobe.c
> > +++ b/kernel/trace/trace_fprobe.c
> > @@ -385,6 +385,7 @@ static struct trace_fprobe *alloc_trace_fprobe(const char *group,
> >   					       const char *event,
> >   					       const char *symbol,
> >   					       struct tracepoint *tpoint,
> > +					       struct module *mod,
> >   					       int maxactive,
> >   					       int nargs, bool is_return)
> >   {
> > @@ -405,6 +406,7 @@ static struct trace_fprobe *alloc_trace_fprobe(const char *group,
> >   		tf->fp.entry_handler = fentry_dispatcher;
> >   
> >   	tf->tpoint = tpoint;
> > +	tf->mod = mod;
> >   	tf->fp.nr_maxactive = maxactive;
> >   
> >   	ret = trace_probe_init(&tf->tp, event, group, false, nargs);
> > @@ -895,8 +897,23 @@ static struct notifier_block tracepoint_module_nb = {
> >   struct __find_tracepoint_cb_data {
> >   	const char *tp_name;
> >   	struct tracepoint *tpoint;
> > +	struct module *mod;
> >   };
> >   
> > +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)) {
> > +			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);
> >   	traceprobe_finish_parse(&ctx);
> >   	trace_probe_log_clear();
> >   	kfree(new_argv);
> > 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>