Messages in this thread | | | Date | Tue, 6 Feb 2024 18:37:22 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] pidfd: getfd should always report ESRCH if a task is exiting |
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On 02/06, Tycho Andersen wrote: > > From: Tycho Andersen <tandersen@netflix.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.
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