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Date: Tue, 6 Feb 2024 10:55:19 -0700
From: Tycho Andersen <tycho@tycho.pizza>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Tycho Andersen <tandersen@netflix.com>
Subject: Re: [PATCH] pidfd: getfd should always report ESRCH if a task is
 exiting
Message-ID: <ZcJyhwr+uW73iuIX@tycho.pizza>
References: <20240206164308.62620-1-tycho@tycho.pizza>
 <20240206173722.GA3593@redhat.com>
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In-Reply-To: <20240206173722.GA3593@redhat.com>

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