[PATCH 18/19] convert kernel/events/core.c

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

a questionable trick in perf_event_open(2) - deliberate call of
fdget(-1), expecting it to yield empty.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
 kernel/events/core.c | 47 +++++++++++++++-----------------------------
 1 file changed, 16 insertions(+), 31 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index bc4910442642..0cb3ecdaecae 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5866,18 +5866,9 @@ EXPORT_SYMBOL_GPL(perf_event_period);
 
 static const struct file_operations perf_fops;
 
-static inline int perf_fget_light(int fd, struct fd *p)
+static inline bool is_perf_file(struct fd f)
 {
-	struct fd f = fdget(fd);
-	if (!fd_file(f))
-		return -EBADF;
-
-	if (fd_file(f)->f_op != &perf_fops) {
-		fdput(f);
-		return -EBADF;
-	}
-	*p = f;
-	return 0;
+	return !fd_empty(f) && fd_file(f)->f_op == &perf_fops;
 }
 
 static int perf_event_set_output(struct perf_event *event,
@@ -5925,20 +5916,16 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 
 	case PERF_EVENT_IOC_SET_OUTPUT:
 	{
-		int ret;
 		if (arg != -1) {
 			struct perf_event *output_event;
-			struct fd output;
-			ret = perf_fget_light(arg, &output);
-			if (ret)
-				return ret;
+			CLASS(fd, output)(arg);
+			if (!is_perf_file(output))
+				return -EBADF;
 			output_event = fd_file(output)->private_data;
-			ret = perf_event_set_output(event, output_event);
-			fdput(output);
+			return perf_event_set_output(event, output_event);
 		} else {
-			ret = perf_event_set_output(event, NULL);
+			return perf_event_set_output(event, NULL);
 		}
-		return ret;
 	}
 
 	case PERF_EVENT_IOC_SET_FILTER:
@@ -12434,7 +12421,6 @@ SYSCALL_DEFINE5(perf_event_open,
 	struct perf_event_attr attr;
 	struct perf_event_context *ctx;
 	struct file *event_file = NULL;
-	struct fd group = EMPTY_FD;
 	struct task_struct *task = NULL;
 	struct pmu *pmu;
 	int event_fd;
@@ -12505,10 +12491,12 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (event_fd < 0)
 		return event_fd;
 
+	CLASS(fd, group)(group_fd);	// group_fd == -1 => empty
 	if (group_fd != -1) {
-		err = perf_fget_light(group_fd, &group);
-		if (err)
+		if (!is_perf_file(group)) {
+			err = -EBADF;
 			goto err_fd;
+		}
 		group_leader = fd_file(group)->private_data;
 		if (flags & PERF_FLAG_FD_OUTPUT)
 			output_event = group_leader;
@@ -12520,7 +12508,7 @@ SYSCALL_DEFINE5(perf_event_open,
 		task = find_lively_task_by_vpid(pid);
 		if (IS_ERR(task)) {
 			err = PTR_ERR(task);
-			goto err_group_fd;
+			goto err_fd;
 		}
 	}
 
@@ -12787,12 +12775,11 @@ SYSCALL_DEFINE5(perf_event_open,
 	mutex_unlock(&current->perf_event_mutex);
 
 	/*
-	 * Drop the reference on the group_event after placing the
-	 * new event on the sibling_list. This ensures destruction
-	 * of the group leader will find the pointer to itself in
-	 * perf_group_detach().
+	 * File reference in group guarantees that group_leader has been
+	 * kept alive until we place the new event on the sibling_list.
+	 * This ensures destruction of the group leader will find
+	 * the pointer to itself in perf_group_detach().
 	 */
-	fdput(group);
 	fd_install(event_fd, event_file);
 	return event_fd;
 
@@ -12811,8 +12798,6 @@ SYSCALL_DEFINE5(perf_event_open,
 err_task:
 	if (task)
 		put_task_struct(task);
-err_group_fd:
-	fdput(group);
 err_fd:
 	put_unused_fd(event_fd);
 	return err;
-- 
2.39.2