linux-kernel - [PATCH 1/3] tracing/user_events: Allow events to persist for perfmon

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Date:   Fri,  1 Sep 2023 20:43:30 +0000
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     rostedt@...dmis.org, mhiramat@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        ast@...nel.org, dcook@...ux.microsoft.com
Subject: [PATCH 1/3] 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.

Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>
---
 include/uapi/linux/user_events.h | 11 ++++++++++-
 kernel/trace/trace_events_user.c | 28 ++++++++++++++--------------
 2 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/include/uapi/linux/user_events.h b/include/uapi/linux/user_events.h
index 2984aae4a2b4..f74f3aedd49c 100644
--- a/include/uapi/linux/user_events.h
+++ b/include/uapi/linux/user_events.h
@@ -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 */
diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 6f046650e527..af663de93492 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -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.
@@ -1888,10 +1876,16 @@ 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;
 
+	/* Persistent events require CAP_PERFMON / CAP_SYS_ADMIN */
+	if (reg_flags & USER_EVENT_REG_PERSIST) {
+		if (!perfmon_capable())
+			return -EPERM;
+	}
+
 	/* Prevent dyn_event from racing */
 	mutex_lock(&event_mutex);
 	user = find_user_event(group, name, &key);
@@ -2024,6 +2018,12 @@ static int delete_user_event(struct user_event_group *group, char *name)
 	if (!user_event_last_ref(user))
 		return -EBUSY;
 
+	/* Persistent events require CAP_PERFMON / CAP_SYS_ADMIN */
+	if (user->reg_flags & USER_EVENT_REG_PERSIST) {
+		if (!perfmon_capable())
+			return -EPERM;
+	}
+
 	return destroy_user_event(user);
 }
 
-- 
2.34.1