[PATCH v2 04/24] kcmp: In kcmp_epoll_target use fget_task

[Eric W. Biederman]

Use the helper fget_task and simplify the code.

As well as simplifying the code this removes one unnecessary increment of
struct files_struct.  This unnecessary increment of files_struct.count can
result in exec unnecessarily unsharing files_struct and breaking posix
locks, and it can result in fget_light having to fallback to fget reducing
performance.

Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
v1: https://lkml.kernel.org/r/20200817220425.9389-4-ebiederm@xxxxxxxxxxxx
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 kernel/kcmp.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index b3ff9288c6cc..87c48c0104ad 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -107,7 +107,6 @@ static int kcmp_epoll_target(struct task_struct *task1,
 {
 	struct file *filp, *filp_epoll, *filp_tgt;
 	struct kcmp_epoll_slot slot;
-	struct files_struct *files;
 
 	if (copy_from_user(&slot, uslot, sizeof(slot)))
 		return -EFAULT;
@@ -116,23 +115,12 @@ static int kcmp_epoll_target(struct task_struct *task1,
 	if (!filp)
 		return -EBADF;
 
-	files = get_files_struct(task2);
-	if (!files)
+	filp_epoll = fget_task(task2, slot.efd);
+	if (!filp_epoll)
 		return -EBADF;
 
-	spin_lock(&files->file_lock);
-	filp_epoll = fcheck_files(files, slot.efd);
-	if (filp_epoll)
-		get_file(filp_epoll);
-	else
-		filp_tgt = ERR_PTR(-EBADF);
-	spin_unlock(&files->file_lock);
-	put_files_struct(files);
-
-	if (filp_epoll) {
-		filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
-		fput(filp_epoll);
-	}
+	filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
+	fput(filp_epoll);
 
 	if (IS_ERR(filp_tgt))
 		return PTR_ERR(filp_tgt);
-- 
2.25.0