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

[Tycho Andersen <tycho@tycho.pizza>]

On Tue, Feb 06, 2024 at 06:37:22PM +0100, Oleg Nesterov wrote:
> > --- 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);
> 
> 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?

Yes, you're absolutely right. Let me resend.

Tycho