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

[Beau Belgrave <beaub@linux.microsoft.com>]

On Mon, Dec 05, 2022 at 04:33:34PM -0500, Mathieu Desnoyers wrote:
> 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 ?
> 

All callers of destroy_user_event hold the event_mutex, since it removes
the call from the system. This is the same for when the
current_user_events get incremented.

Maybe add a lock_dep statement here to make it clear?

Thanks,
-Beau

> 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