Re: [PATCH RFC 06/10] pidfs: allow to retrieve exit information

[Oleg Nesterov <oleg@redhat.com>]

On 02/28, Christian Brauner wrote:
>
> Some tools like systemd's jounral need to retrieve the exit and cgroup
> information after a process has already been reaped.
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

But unless I am totally confused do_exit() calls pidfd_exit() even
before exit_notify(), the exiting task is not even zombie yet. It
will reaped only when it passes exit_notify() and its parent does
wait().



And what about the multi-threaded case? Suppose the main thread
does sys_exit(0) and it has alive sub-threads.

In this case pidfd_info() will report kinfo.exit_code = 0.
And this is probably fine if (file->f_flags & PIDFD_THREAD) != 0.

But what if this file was created without PIDFD_THREAD? If another
thread does exit_group(1) after that, the process's exit code is
1 << 8, but it can't be retrieved.



Finally, sys_execve(). Suppose we have a main thread L and a
sub-thread T.

T execs and kill the leader L. L exits and populates
pidfs_i(inode)->exit_info.

T calls exchange_tids() in de_thread() and becomes the new leader
with the same (old) pid.

Now, T is very much alive, but pidfs_i(inode)->exit_info != NULL.

Or I am totally confused?



> +	exit_info = READ_ONCE(pidfs_i(inode)->exit_info);
> +	if (exit_info) {
> +		/*
> +		 * TODO: Oleg, I didn't see a reason for putting
> +		 * retrieval of the exit status of a task behind some
> +		 * form of permission check.

Neither me.

Oleg.