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

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Message-ID: <20240206173722.GA3593@redhat.com>
Date: Tue, 6 Feb 2024 18:37:22 +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

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);

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.