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Date: Thu, 31 Mar 2022 19:10:05 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Jakob Koschel <jakobkoschel@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
Mike Rapoport <rppt@...nel.org>,
"Brian Johannesmeyer" <bjohannesmeyer@...il.com>,
Cristiano Giuffrida <c.giuffrida@...nl>,
"Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH] ftrace: remove check of list iterator against head past
the loop body
On Fri, 1 Apr 2022 00:37:52 +0200
Jakob Koschel <jakobkoschel@...il.com> wrote:
Hi Jakob,
The patch looks fine, I have some small nits though.
First, the subject should actually be:
Subject: [PATCH] tracing: Remove check of list iterator against head past the loop body
Changes that only deal specifically with the function probes are labeled as
"ftrace:", but the more generic changes that touches files outside of
ftrace.c and fgraph.c should be "tracing:". Also, Linus prefers to have the
next part of the subject start with a capital letter: "Remove" instead of
"remove".
> When list_for_each_entry() completes the iteration over the whole list
> without breaking the loop, the iterator value will be a bogus pointer
> computed based on the head element.
>
> While it is safe to use the pointer to determine if it was computed
> based on the head element, either with list_entry_is_head() or
> &pos->member == head, using the iterator variable after the loop should
> be avoided.
>
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer to point to the found element [1].
>
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
> kernel/trace/ftrace.c | 20 ++++++++++++--------
> kernel/trace/trace_eprobe.c | 14 ++++++++------
> kernel/trace/trace_events.c | 12 ++++++------
> 3 files changed, 26 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 4f1d2f5e7263..096f5a83358d 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -4560,8 +4560,8 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
> struct ftrace_probe_ops *probe_ops,
> void *data)
> {
> + struct ftrace_func_probe *probe = NULL, *iter;
> struct ftrace_func_entry *entry;
> - struct ftrace_func_probe *probe;
> struct ftrace_hash **orig_hash;
> struct ftrace_hash *old_hash;
> struct ftrace_hash *hash;
> @@ -4580,11 +4580,13 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
>
> mutex_lock(&ftrace_lock);
> /* Check if the probe_ops is already registered */
> - list_for_each_entry(probe, &tr->func_probes, list) {
> - if (probe->probe_ops == probe_ops)
> + list_for_each_entry(iter, &tr->func_probes, list) {
> + if (iter->probe_ops == probe_ops) {
> + probe = iter;
> break;
> + }
> }
> - if (&probe->list == &tr->func_probes) {
> + if (!probe) {
> probe = kzalloc(sizeof(*probe), GFP_KERNEL);
> if (!probe) {
> mutex_unlock(&ftrace_lock);
> @@ -4704,7 +4706,7 @@ unregister_ftrace_function_probe_func(char *glob, struct trace_array *tr,
> {
> struct ftrace_ops_hash old_hash_ops;
> struct ftrace_func_entry *entry;
> - struct ftrace_func_probe *probe;
> + struct ftrace_func_probe *probe = NULL, *iter;
Can you move this to the first declaration to keep the nice "upside-down
x-mas tree" look.
> struct ftrace_glob func_g;
> struct ftrace_hash **orig_hash;
> struct ftrace_hash *old_hash;
> @@ -4732,11 +4734,13 @@ unregister_ftrace_function_probe_func(char *glob, struct trace_array *tr,
>
> mutex_lock(&ftrace_lock);
> /* Check if the probe_ops is already registered */
> - list_for_each_entry(probe, &tr->func_probes, list) {
> - if (probe->probe_ops == probe_ops)
> + list_for_each_entry(iter, &tr->func_probes, list) {
> + if (iter->probe_ops == probe_ops) {
> + probe = iter;
> break;
> + }
> }
> - if (&probe->list == &tr->func_probes)
> + if (!probe)
> goto err_unlock_ftrace;
>
> ret = -EINVAL;
> diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
> index 541aa13581b9..63e901a28425 100644
> --- a/kernel/trace/trace_eprobe.c
> +++ b/kernel/trace/trace_eprobe.c
> @@ -650,7 +650,7 @@ static struct trace_event_functions eprobe_funcs = {
> static int disable_eprobe(struct trace_eprobe *ep,
> struct trace_array *tr)
> {
> - struct event_trigger_data *trigger;
> + struct event_trigger_data *trigger = NULL, *iter;
> struct trace_event_file *file;
> struct eprobe_data *edata;
>
> @@ -658,14 +658,16 @@ static int disable_eprobe(struct trace_eprobe *ep,
> if (!file)
> return -ENOENT;
>
> - list_for_each_entry(trigger, &file->triggers, list) {
> - if (!(trigger->flags & EVENT_TRIGGER_FL_PROBE))
> + list_for_each_entry(iter, &file->triggers, list) {
> + if (!(iter->flags & EVENT_TRIGGER_FL_PROBE))
> continue;
> - edata = trigger->private_data;
> - if (edata->ep == ep)
> + edata = iter->private_data;
> + if (edata->ep == ep) {
> + trigger = iter;
> break;
> + }
> }
> - if (list_entry_is_head(trigger, &file->triggers, list))
> + if (!trigger)
> return -ENODEV;
>
> list_del_rcu(&trigger->list);
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index e11e167b7809..fe3dc157e635 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -2281,7 +2281,7 @@ event_subsystem_dir(struct trace_array *tr, const char *name,
> struct trace_event_file *file, struct dentry *parent)
> {
> struct trace_subsystem_dir *dir;
> - struct event_subsystem *system;
> + struct event_subsystem *system, *iter;
And move this above dir as well, for the same reason.
Thanks,
-- Steve
> struct dentry *entry;
>
> /* First see if we did not already create this dir */
> @@ -2295,13 +2295,13 @@ event_subsystem_dir(struct trace_array *tr, const char *name,
> }
>
> /* Now see if the system itself exists. */
> - list_for_each_entry(system, &event_subsystems, list) {
> - if (strcmp(system->name, name) == 0)
> + system = NULL;
> + list_for_each_entry(iter, &event_subsystems, list) {
> + if (strcmp(iter->name, name) == 0) {
> + system = iter;
> break;
> + }
> }
> - /* Reset system variable when not found */
> - if (&system->list == &event_subsystems)
> - system = NULL;
>
> dir = kmalloc(sizeof(*dir), GFP_KERNEL);
> if (!dir)
>
> base-commit: f82da161ea75dc4db21b2499e4b1facd36dab275
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