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Date:   Fri, 24 Mar 2023 08:18:24 +0800
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Beau Belgrave <beaub@linux.microsoft.com>
Cc:     rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        dcook@linux.microsoft.com, alanau@linux.microsoft.com,
        brauner@kernel.org, akpm@linux-foundation.org,
        ebiederm@xmission.com, keescook@chromium.org, tglx@linutronix.de,
        linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v8 11/11] tracing/user_events: Limit global user_event
 count
Message-Id: <20230324081824.b917c2944da217e5239e1223@kernel.org>
In-Reply-To: <20230221211143.574-12-beaub@linux.microsoft.com>
References: <20230221211143.574-1-beaub@linux.microsoft.com>
        <20230221211143.574-12-beaub@linux.microsoft.com>
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Hi Beau,

On Tue, 21 Feb 2023 13:11:43 -0800
Beau Belgrave <beaub@linux.microsoft.com> 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.

sysctl is good to me, but would we really need the kernel parameter?
The user_events starts using when user-space is up, so I think setting
the limit with sysctl is enough.

BTW, Vlastimil tried to add 'sysctl.*' kernel parameter support(*). If we
need a kernel cmdline support, I think this is more generic way. But it
seems the discussion has been stopped.

(*) https://patchwork.kernel.org/project/linux-mm/patch/20200427180433.7029-2-vbabka@suse.cz/

Thank you,

> 
> Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> ---
>  kernel/trace/trace_events_user.c | 59 ++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 222f2eb59c7c..6a5ebe243999 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
> @@ -1241,6 +1248,8 @@ static int destroy_user_event(struct user_event *user)
>  {
>  	int ret = 0;
>  
> +	lockdep_assert_held(&event_mutex);
> +
>  	/* Must destroy fields before call removal */
>  	user_event_destroy_fields(user);
>  
> @@ -1257,6 +1266,11 @@ static int destroy_user_event(struct user_event *user)
>  	kfree(EVENT_NAME(user));
>  	kfree(user);
>  
> +	if (current_user_events > 0)
> +		current_user_events--;
> +	else
> +		pr_alert("BUG: Bad current_user_events\n");
> +
>  	return ret;
>  }
>  
> @@ -1744,6 +1758,11 @@ 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)
> @@ -1755,6 +1774,7 @@ 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);
>  
> @@ -2386,6 +2406,43 @@ 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;
> @@ -2415,6 +2472,8 @@ 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;
>  }
>  
> -- 
> 2.25.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>