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

Mathieu Desnoyers
2022-12-05T21:33:24Z

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.