Re: [PATCH v5 11/11] tracing/user_events: Limit global user_event count

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2022-12-05 16:00, Beau Belgrave wrote:
> Operators want to be able to ensure enough tracepoints exist on the
> system for kernel components as well as for user components. Since there
> are only up to 64K events, by default allow up to half to be used by
> user events.
> 
> Add a boot parameter (user_events_max=%d) and a kernel sysctl parameter
> (kernel.user_events_max) to set a global limit that is honored among all
> groups on the system. This ensures hard limits can be setup to prevent
> user processes from consuming all event IDs on the system.
> 
> Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> ---
>   kernel/trace/trace_events_user.c | 57 ++++++++++++++++++++++++++++++++
>   1 file changed, 57 insertions(+)
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 36def244a755..754942ba92a1 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -20,6 +20,7 @@
>   #include <linux/types.h>
>   #include <linux/uaccess.h>
>   #include <linux/highmem.h>
> +#include <linux/init.h>
>   #include <linux/user_events.h>
>   #include "trace.h"
>   #include "trace_dynevent.h"
> @@ -61,6 +62,12 @@ struct user_event_group {
>   /* Group for init_user_ns mapping, top-most group */
>   static struct user_event_group *init_group;
>   
> +/* Max allowed events for the whole system */
> +static unsigned int max_user_events = 32768;
> +
> +/* Current number of events on the whole system */
> +static unsigned int current_user_events;
> +
>   /*
>    * Stores per-event properties, as users register events
>    * within a file a user_event might be created if it does not
> @@ -1247,6 +1254,11 @@ static int destroy_user_event(struct user_event *user)
>   	kfree(EVENT_NAME(user));
>   	kfree(user);
>   
> +	if (current_user_events > 0)
> +		current_user_events--;

What holds the user_events mutex that guarantees that non-atomic 
decrement is safe here ?

Thanks,

Mathieu

> +	else
> +		pr_alert("BUG: Bad current_user_events\n");
> +
>   	return ret;
>   }
>   
> @@ -1732,6 +1744,11 @@ static int user_event_parse(struct user_event_group *group, char *name,
>   
>   	mutex_lock(&event_mutex);
>   
> +	if (current_user_events >= max_user_events) {
> +		ret = -EMFILE;
> +		goto put_user_lock;
> +	}
> +
>   	ret = user_event_trace_register(user);
>   
>   	if (ret)
> @@ -1743,6 +1760,7 @@ static int user_event_parse(struct user_event_group *group, char *name,
>   	dyn_event_init(&user->devent, &user_event_dops);
>   	dyn_event_add(&user->devent, &user->call);
>   	hash_add(group->register_table, &user->node, key);
> +	current_user_events++;
>   
>   	mutex_unlock(&event_mutex);
>   
> @@ -2369,6 +2387,43 @@ static int create_user_tracefs(void)
>   	return -ENODEV;
>   }
>   
> +static int __init set_max_user_events(char *str)
> +{
> +	if (!str)
> +		return 0;
> +
> +	if (kstrtouint(str, 0, &max_user_events))
> +		return 0;
> +
> +	return 1;
> +}
> +__setup("user_events_max=", set_max_user_events);
> +
> +static int set_max_user_events_sysctl(struct ctl_table *table, int write,
> +				      void *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int ret;
> +
> +	mutex_lock(&event_mutex);
> +
> +	ret = proc_douintvec(table, write, buffer, lenp, ppos);
> +
> +	mutex_unlock(&event_mutex);
> +
> +	return ret;
> +}
> +
> +static struct ctl_table user_event_sysctls[] = {
> +	{
> +		.procname	= "user_events_max",
> +		.data		= &max_user_events,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= set_max_user_events_sysctl,
> +	},
> +	{}
> +};
> +
>   static int __init trace_events_user_init(void)
>   {
>   	int ret;
> @@ -2398,6 +2453,8 @@ static int __init trace_events_user_init(void)
>   	if (dyn_event_register(&user_event_dops))
>   		pr_warn("user_events could not register with dyn_events\n");
>   
> +	register_sysctl_init("kernel", user_event_sysctls);
> +
>   	return 0;
>   }
>   

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com