[PATCH 04/17] kcmp: In kcmp_epoll_target use fget_task

Eric W. Biederman
2020-08-17T22:10:01Z

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@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
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