Commit 5dbd04ed authored by Beau Belgrave's avatar Beau Belgrave Committed by Steven Rostedt (Google)
Browse files

tracing/user_events: Allow events to persist for perfmon_capable users 

There are several scenarios that have come up where having a user_event
persist even if the process that registered it exits. The main one is
having a daemon create events on bootup that shouldn't get deleted if
the daemon has to exit or reload. Another is within OpenTelemetry
exporters, they wish to potentially check if a user_event exists on the
system to determine if exporting the data out should occur. The
user_event in this case must exist even in the absence of the owning
process running (such as the above daemon case).

Expose the previously internal flag USER_EVENT_REG_PERSIST to user
processes. Upon register or delete of events with this flag, ensure the
user is perfmon_capable to prevent random user processes with access to
tracefs from creating events that persist after exit.

Link: https://lkml.kernel.org/r/20230912180704.1284-2-beaub@linux.microsoft.com



Signed-off-by: default avatarBeau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent bdf4fb62

include/uapi/linux/user_events.h

+10 −1
Original line number Diff line number Diff line
@@ -17,6 +17,15 @@ 
/* Create dynamic location entry within a 32-bit value */

#define DYN_LOC(offset, size) ((size) << 16 | (offset))



/* List of supported registration flags */

enum user_reg_flag {

	/* Event will not delete upon last reference closing */

	USER_EVENT_REG_PERSIST		= 1U << 0,



	/* This value or above is currently non-ABI */

	USER_EVENT_REG_MAX		= 1U << 1,

};



/*

 * Describes an event registration and stores the results of the registration.

 * This structure is passed to the DIAG_IOCSREG ioctl, callers at a minimum
@@ -33,7 +42,7 @@ struct user_reg { 
	/* Input: Enable size in bytes at address */

	__u8	enable_size;



	/* Input: Flags for future use, set to 0 */

	/* Input: Flags to use, if any */

	__u16	flags;



	/* Input: Address to update when enabled */

kernel/trace/trace_events_user.c

+22 −14
Original line number Diff line number Diff line
@@ -49,18 +49,6 @@ 
#define EVENT_STATUS_PERF BIT(1)

#define EVENT_STATUS_OTHER BIT(7)



/*

 * User register flags are not allowed yet, keep them here until we are

 * ready to expose them out to the user ABI.

 */

enum user_reg_flag {

	/* Event will not delete upon last reference closing */

	USER_EVENT_REG_PERSIST		= 1U << 0,



	/* This value or above is currently non-ABI */

	USER_EVENT_REG_MAX		= 1U << 1,

};



/*

 * Stores the system name, tables, and locks for a group of events. This

 * allows isolation for events by various means.
@@ -220,6 +208,17 @@ static u32 user_event_key(char *name) 
	return jhash(name, strlen(name), 0);

}



static bool user_event_capable(u16 reg_flags)

{

	/* Persistent events require CAP_PERFMON / CAP_SYS_ADMIN */

	if (reg_flags & USER_EVENT_REG_PERSIST) {

		if (!perfmon_capable())

			return false;

	}



	return true;

}



static struct user_event *user_event_get(struct user_event *user)

{

	refcount_inc(&user->refcnt);
@@ -1811,6 +1810,9 @@ static int user_event_free(struct dyn_event *ev) 
	if (!user_event_last_ref(user))

		return -EBUSY;



	if (!user_event_capable(user->reg_flags))

		return -EPERM;



	return destroy_user_event(user);

}
@@ -1926,10 +1928,13 @@ static int user_event_parse(struct user_event_group *group, char *name, 
	int argc = 0;

	char **argv;



	/* User register flags are not ready yet */

	if (reg_flags != 0 || flags != NULL)

	/* Currently don't support any text based flags */

	if (flags != NULL)

		return -EINVAL;



	if (!user_event_capable(reg_flags))

		return -EPERM;



	/* Prevent dyn_event from racing */

	mutex_lock(&event_mutex);

	user = find_user_event(group, name, &key);
@@ -2062,6 +2067,9 @@ static int delete_user_event(struct user_event_group *group, char *name) 
	if (!user_event_last_ref(user))

		return -EBUSY;



	if (!user_event_capable(user->reg_flags))

		return -EPERM;



	return destroy_user_event(user);

}