[PATCH 08/17] proc/fd: In proc_fd_link use fcheck_task

[Eric W. Biederman]

When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Using fcheck_task instead of get_files_struct simplifies proc_fd_link by
removing unnecessary locking, and reference counting.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@xxxxxxxxxx
Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/proc/fd.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 4048a87c51ee..abfdcb21cc79 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -141,29 +141,23 @@ static const struct dentry_operations tid_fd_dentry_operations = {
 
 static int proc_fd_link(struct dentry *dentry, struct path *path)
 {
-	struct files_struct *files = NULL;
 	struct task_struct *task;
 	int ret = -ENOENT;
 
 	task = get_proc_task(d_inode(dentry));
 	if (task) {
-		files = get_files_struct(task);
-		put_task_struct(task);
-	}
-
-	if (files) {
 		unsigned int fd = proc_fd(d_inode(dentry));
 		struct file *fd_file;
 
-		spin_lock(&files->file_lock);
-		fd_file = fcheck_files(files, fd);
+		rcu_read_lock();
+		fd_file = fcheck_task(task, fd);
 		if (fd_file) {
 			*path = fd_file->f_path;
 			path_get(&fd_file->f_path);
 			ret = 0;
 		}
-		spin_unlock(&files->file_lock);
-		put_files_struct(files);
+		rcu_read_unlock();
+		put_task_struct(task);
 	}
 
 	return ret;
-- 
2.25.0