Re: selftests: user_events: ftrace_test - RIP: 0010:tracing_update_buffers (kernel/trace/trace.c:6470)

[Beau Belgrave]

On Tue, Oct 31, 2023 at 12:07:02PM -0400, Steven Rostedt wrote:
> On Tue, 31 Oct 2023 10:45:51 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > @@ -1404,7 +1424,7 @@ event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
> >  		ret = -ENODEV;
> >  		mutex_lock(&event_mutex);
> >  		file = event_file_data(filp);
> > -		if (likely(file)) {
> > +		if (likely(file && !(file->flags & EVENT_FILE_FL_FREED))) {
> >  			printk("update file = %px\n", file);
> >  			printk("update tr = %px\n", file->tr);
> >  			ret = tracing_update_buffers(file->tr);
> 
> Well, this won't apply because I still had debugging in it when I added
> these changes.
> 
> Here's a better version:
> 
Thanks!

I ended up testing the official patch [1] instead of this one along with
patches [2][3] in the next branch.

My printk's are now good, everything looks back to normal for my repro.

I added my Tested-by onto the official patch thread.

Thanks,
-Beau

1. https://lore.kernel.org/linux-trace-kernel/20231031122453.7a48b923@xxxxxxxxxxxxxxxxxx/
2. https://lore.kernel.org/linux-trace-kernel/20231030114047.759c7bdf@xxxxxxxxxxxxxxxxxx
3. https://lore.kernel.org/linux-trace-kernel/20231028164650.4f5ea18a@xxxxxxxxxxxxxxxxxxxx/

> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index 12207dc6722d..696f8dc4aa53 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -492,6 +492,7 @@ enum {
>  	EVENT_FILE_FL_TRIGGER_COND_BIT,
>  	EVENT_FILE_FL_PID_FILTER_BIT,
>  	EVENT_FILE_FL_WAS_ENABLED_BIT,
> +	EVENT_FILE_FL_FREED_BIT,
>  };
>  
>  extern struct trace_event_file *trace_get_event_file(const char *instance,
> @@ -630,6 +631,7 @@ extern int __kprobe_event_add_fields(struct dynevent_cmd *cmd, ...);
>   *  TRIGGER_COND  - When set, one or more triggers has an associated filter
>   *  PID_FILTER    - When set, the event is filtered based on pid
>   *  WAS_ENABLED   - Set when enabled to know to clear trace on module removal
> + *  FREED         - File descriptor is freed, all fields should be considered invalid
>   */
>  enum {
>  	EVENT_FILE_FL_ENABLED		= (1 << EVENT_FILE_FL_ENABLED_BIT),
> @@ -643,6 +645,7 @@ enum {
>  	EVENT_FILE_FL_TRIGGER_COND	= (1 << EVENT_FILE_FL_TRIGGER_COND_BIT),
>  	EVENT_FILE_FL_PID_FILTER	= (1 << EVENT_FILE_FL_PID_FILTER_BIT),
>  	EVENT_FILE_FL_WAS_ENABLED	= (1 << EVENT_FILE_FL_WAS_ENABLED_BIT),
> +	EVENT_FILE_FL_FREED		= (1 << EVENT_FILE_FL_FREED_BIT),
>  };
>  
>  struct trace_event_file {
> @@ -671,6 +674,7 @@ struct trace_event_file {
>  	 * caching and such. Which is mostly OK ;-)
>  	 */
>  	unsigned long		flags;
> +	atomic_t		ref;	/* ref count for opened files */
>  	atomic_t		sm_ref;	/* soft-mode reference counter */
>  	atomic_t		tm_ref;	/* trigger-mode reference counter */
>  };
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 2539cfc20a97..9aebf904ff97 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4978,6 +4978,20 @@ int tracing_open_file_tr(struct inode *inode, struct file *filp)
>  	if (ret)
>  		return ret;
>  
> +	mutex_lock(&event_mutex);
> +
> +	/* Fail if the file is marked for removal */
> +	if (file->flags & EVENT_FILE_FL_FREED) {
> +		trace_array_put(file->tr);
> +		ret = -ENODEV;
> +	} else {
> +		event_file_get(file);
> +	}
> +
> +	mutex_unlock(&event_mutex);
> +	if (ret)
> +		return ret;
> +
>  	filp->private_data = inode->i_private;
>  
>  	return 0;
> @@ -4988,6 +5002,7 @@ int tracing_release_file_tr(struct inode *inode, struct file *filp)
>  	struct trace_event_file *file = inode->i_private;
>  
>  	trace_array_put(file->tr);
> +	event_file_put(file);
>  
>  	return 0;
>  }
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 0e1405abf4f7..b7f4ea25a194 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1669,6 +1669,9 @@ extern void event_trigger_unregister(struct event_command *cmd_ops,
>  				     char *glob,
>  				     struct event_trigger_data *trigger_data);
>  
> +extern void event_file_get(struct trace_event_file *file);
> +extern void event_file_put(struct trace_event_file *file);
> +
>  /**
>   * struct event_trigger_ops - callbacks for trace event triggers
>   *
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index f9e3e24d8796..f29e815ca5b2 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -990,13 +990,35 @@ static void remove_subsystem(struct trace_subsystem_dir *dir)
>  	}
>  }
>  
> +void event_file_get(struct trace_event_file *file)
> +{
> +	atomic_inc(&file->ref);
> +}
> +
> +void event_file_put(struct trace_event_file *file)
> +{
> +	if (WARN_ON_ONCE(!atomic_read(&file->ref))) {
> +		if (file->flags & EVENT_FILE_FL_FREED)
> +			kmem_cache_free(file_cachep, file);
> +		return;
> +	}
> +
> +	if (atomic_dec_and_test(&file->ref)) {
> +		/* Count should only go to zero when it is freed */
> +		if (WARN_ON_ONCE(!(file->flags & EVENT_FILE_FL_FREED)))
> +			return;
> +		kmem_cache_free(file_cachep, file);
> +	}
> +}
> +
>  static void remove_event_file_dir(struct trace_event_file *file)
>  {
>  	eventfs_remove_dir(file->ei);
>  	list_del(&file->list);
>  	remove_subsystem(file->system);
>  	free_event_filter(file->filter);
> -	kmem_cache_free(file_cachep, file);
> +	file->flags |= EVENT_FILE_FL_FREED;
> +	event_file_put(file);
>  }
>  
>  /*
> @@ -1369,7 +1391,7 @@ event_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
>  		flags = file->flags;
>  	mutex_unlock(&event_mutex);
>  
> -	if (!file)
> +	if (!file || flags & EVENT_FILE_FL_FREED)
>  		return -ENODEV;
>  
>  	if (flags & EVENT_FILE_FL_ENABLED &&
> @@ -1403,7 +1425,7 @@ event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
>  		ret = -ENODEV;
>  		mutex_lock(&event_mutex);
>  		file = event_file_data(filp);
> -		if (likely(file)) {
> +		if (likely(file && !(file->flags & EVENT_FILE_FL_FREED))) {
>  			ret = tracing_update_buffers(file->tr);
>  			if (ret < 0) {
>  				mutex_unlock(&event_mutex);
> @@ -1683,7 +1705,7 @@ event_filter_read(struct file *filp, char __user *ubuf, size_t cnt,
>  
>  	mutex_lock(&event_mutex);
>  	file = event_file_data(filp);
> -	if (file)
> +	if (file && !(file->flags & EVENT_FILE_FL_FREED))
>  		print_event_filter(file, s);
>  	mutex_unlock(&event_mutex);
>  
> @@ -2902,6 +2924,7 @@ trace_create_new_event(struct trace_event_call *call,
>  	atomic_set(&file->tm_ref, 0);
>  	INIT_LIST_HEAD(&file->triggers);
>  	list_add(&file->list, &tr->events);
> +	event_file_get(file);
>  
>  	return file;
>  }
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 33264e510d16..0c611b281a5b 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -2349,6 +2349,9 @@ int apply_event_filter(struct trace_event_file *file, char *filter_string)
>  	struct event_filter *filter = NULL;
>  	int err;
>  
> +	if (file->flags & EVENT_FILE_FL_FREED)
> +		return -ENODEV;
> +
>  	if (!strcmp(strstrip(filter_string), "0")) {
>  		filter_disable(file);
>  		filter = event_filter(file);