proc/fd: In proc_fd_link use fget_task

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.

Simplifying proc_fd_link is a little bit tricky.  It is necessary to
know that there is a reference to fd_f	 ile while path_get is running.
This reference can either be guaranteed to exist either by locking the
fdtable as the code currently does or by taking a reference on the
file in question.

Use fget_task to remove the need for get_files_struct and
to take a reference to file in question.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-8-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-6-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 81882a1..d58960f 100644 (file)
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -146,29 +146,22 @@ 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);
+               fd_file = fget_task(task, fd);
                if (fd_file) {
                        *path = fd_file->f_path;
                        path_get(&fd_file->f_path);
                        ret = 0;
+                       fput(fd_file);
                }
-               spin_unlock(&files->file_lock);
-               put_files_struct(files);
+               put_task_struct(task);
        }
 
        return ret;