Re: [PATCH] pidfd: getfd should always report ESRCH if a task is exiting
From: Oleg Nesterov <oleg@redhat.com>
Date: 2024-02-06 17:38:46
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On 02/06, Tycho Andersen wrote:
From: Tycho Andersen <redacted> 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:
quoted hunk ↗ jump to hunk
--- 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.