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

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Message-ID: <20240206180607.GB3593@redhat.com>
Date: Tue, 6 Feb 2024 19:06:07 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Tycho Andersen <tycho@...ho.pizza>
Cc: Christian Brauner <brauner@...nel.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Tycho Andersen <tandersen@...flix.com>
Subject: 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:
> >
> > From: Tycho Andersen <tandersen@...flix.com>
> >
> > 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,
> 
> > Let's check PF_EXITING, and just report ESRCH if so.
> 
> agreed, we can pretend that the task has already exited,
> 
> But:
> 
> > --- 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.