file: flush delayed work in delayed fput()

The fput() of file rcS might not have completed causing issues when
executing the file.

rcS is opened in do_populate_rootfs before executed. At the end of
do_populate_rootfs() flush_delayed_fput() is called. Now
do_populate_rootfs() assumes that all fput()s caused by
do_populate_rootfs() have completed.

But flush_delayed_fput() can only ensure that fput() on the current
delayed_fput_list has finished. Any file that has been removed from
delayed_fput_list asynchronously in the meantime might not have
completed causing the exec to fail.

do_populate_rootfs	delayed_fput_list	delayed_fput	execve
fput()			a
fput()			a->b
fput()			a->b->rcS
						__fput(a)
fput()			c
fput()			c->d
						__fput(b)
flush_delayed_fput
__fput(c)
__fput(d)
						__fput(b)
						__fput(b)	execve(rcS)

Ensure that all delayed work is done by calling flush_delayed_work() in
flush_delayed_fput() explicitly.

Signed-off-by: Chen Lin <[email protected]>
Signed-off-by: Shao Mingyin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: Yang Yang <[email protected]>
Cc: Yang Tao <[email protected]>
Cc: Xu Xin <[email protected]>
[brauner: rewrite commit message]
Signed-off-by: Christian Brauner <[email protected]>

Author: shao mingyin

fs/file_table.c

@@ -478,6 +478,8 @@ static void ____fput(struct callback_head *work)
 	__fput(container_of(work, struct file, f_task_work));
 }
 
+static DECLARE_DELAYED_WORK(delayed_fput_work, delayed_fput);
+
 /*
  * If kernel thread really needs to have the final fput() it has done
  * to complete, call this.  The only user right now is the boot - we
@@ -491,11 +493,10 @@ static void ____fput(struct callback_head *work)
 void flush_delayed_fput(void)
 {
 	delayed_fput(NULL);
+	flush_delayed_work(&delayed_fput_work);
 }
 EXPORT_SYMBOL_GPL(flush_delayed_fput);
 
-static DECLARE_DELAYED_WORK(delayed_fput_work, delayed_fput);
-
 void fput(struct file *file)
 {
 	if (file_ref_put(&file->f_ref)) {