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

Mathieu Desnoyers
2024-06-03T19:50:17Z

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.

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.

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.