Re: [PATCH] pidfd: getfd should always report ESRCH if a task is exiting

Sorry for noise, forgot to mention...

On 02/06, Oleg Nesterov wrote:

On 02/06, Tycho Andersen wrote:

quoted

From: Tycho Andersen <redacted>

We can get EBADF from __pidfd_fget() if a task is currently exiting, which
might be confusing.

agreed, because EBADF looks as if the "fd" argument was wrong,

quoted

Let's check PF_EXITING, and just report ESRCH if so.

agreed, we can pretend that the task has already exited,

But:

quoted

--- a/kernel/pid.c
+++ b/kernel/pid.c

@@ -688,7 +688,7 @@ static int pidfd_getfd(struct pid *pid, int fd)
 	int ret;
 
 	task = get_pid_task(pid, PIDTYPE_PID);
-	if (!task)
+	if (!task || task->flags & PF_EXITING)
 		return -ESRCH;

This looks racy. Suppose that pidfd_getfd() races with the exiting task.

It is possible that this task sets PF_EXITING and does exit_files()
after the "task->flags & PF_EXITING" check above and before pidfd_getfd()
does __pidfd_fget(), in this case pidfd_getfd() still returns the same
EBADF we want to avoid.

Perhaps we can change pidfd_getfd() to do

	if (IS_ERR(file))
		return (task->flags & PF_EXITING) ? -ESRCH : PTR_ERR(file);

Or we can check task->files != NULL rather than PF_EXITING.

To me this looks even better, but looks more confusing without a comment.
OTOH, imo this needs a comment anyway ;)

instead?

This needs a comment to explain the PF_EXITING check. And perhaps another
comment to explain that we can't miss PF_EXITING if the target task has
already passed exit_files, both exit_files() and fget_task() take the same
task_lock(task).

What do you think?

Oleg.