[RFC PATCH 14/17] perf: Add ioctl(REATTACH) for detached events

[Alexander Shishkin]

This adds an ioctl command to demote a detached event to a 'normal' one
that gets destroyed when its file descriptor is closed. It can still be
used to mmap the buffers, but not very useful otherwise.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 include/uapi/linux/perf_event.h |  1 +
 kernel/events/core.c            | 32 +++++++++++++++++++++++++++++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 4cdd4fab9d..ae54bd496d 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -435,6 +435,7 @@ struct perf_event_attr {
 #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
 #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
 #define PERF_EVENT_IOC_PAUSE_OUTPUT	_IOW('$', 9, __u32)
+#define PERF_EVENT_IOC_REATTACH		_IO ('$', 10)
 
 enum perf_event_ioc_flags {
 	PERF_IOC_FLAG_GROUP		= 1U << 0,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f0b77b33b4..fbee221d19 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4532,7 +4532,19 @@ EXPORT_SYMBOL_GPL(perf_event_release_kernel);
  */
 static int perf_release(struct inode *inode, struct file *file)
 {
-	perf_event_release_kernel(file->private_data);
+	struct perf_event *event = file->private_data;
+
+	/*
+	 * For a DETACHED event, perf_release() can't have the last reference,
+	 * because we grabbed one extra in the sys_perf_event_open, IOW it is
+	 * always put_event(). In order for it to be the last reference, we'd
+	 * first need to ioctl(REATTACH) on this event, which would drop the
+	 * PERF_ATTACH_DETACHED attach state.
+	 */
+	if (event->attach_state & PERF_ATTACH_DETACHED)
+		put_event(event);
+	else
+		perf_event_release_kernel(file->private_data);
 	return 0;
 }
 
@@ -4885,6 +4897,11 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 	void (*func)(struct perf_event *);
 	u32 flags = arg;
 
+	if (event->attach_state & PERF_ATTACH_DETACHED &&
+	    cmd != PERF_EVENT_IOC_REATTACH &&
+	    cmd != PERF_EVENT_IOC_ID)
+		return -EINVAL;
+
 	switch (cmd) {
 	case PERF_EVENT_IOC_ENABLE:
 		func = _perf_event_enable;
@@ -4948,6 +4965,19 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 		rcu_read_unlock();
 		return 0;
 	}
+	case PERF_EVENT_IOC_REATTACH:
+		/*
+		 * DETACHED state is serialized on ctx::mutex
+		 */
+		if (!is_detached_event(event))
+			return -EINVAL;
+
+		event->attach_state &= ~PERF_ATTACH_DETACHED;
+		tracefs_remove(event->dent);
+		event->dent = NULL;
+		put_event(event); /* can't be last */
+
+		return 0;
 	default:
 		return -ENOTTY;
 	}
-- 
2.14.1