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SubjectRe: [PATCH 2/4] clone: add CLONE_PIDFD
On 04/15, Christian Brauner wrote:
>
> > CLONE_PARENT_SETTID doesn't look very usefule, so what if we add
> >
> > if ((clone_flags & (CLONE_PIDFD|CLONE_PARENT_SETTID)) ==
> > (CLONE_PIDFD|CLONE_PARENT_SETTID))
> > return ERR_PTR(-EINVAL);
> >
> > at the start of copy_process() ?
> >
> > Then it can do
> >
> > if (clone_flags & CLONE_PIDFD) {
> > retval = pidfd_create(pid, &pidfdf);
> > if (retval < 0)
> > goto bad_fork_free_pid;
> > retval = put_user(retval, parent_tidptr)
> > if (retval < 0)
> > goto bad_fork_free_pid;
> > }
>
> Uhhh Oleg, that is nifty. I have to say I like that a lot. This would
> let us return the pid and the pidfd in one go and we can also start
> pidfd numbering at 0.

Christian, sorry if it was already discussed, but I can't force myself to
read all the previous discussions ;)

If we forget about CONFIG_PROC_FS, why do we really want to create a file?


Suppose we add a global u64 counter incremented by copy_process and reported
in /proc/$pid/status. Suppose that clone(CLONE_PIDFD) writes this counter to
*parent_tidptr. Let's denote this counter as UNIQ_PID.

Now, if you want to (say) safely kill a task and you have its UNIQ_PID, you
can do

kill_by_pid_uniq(int pid, u64 uniq_pid)
{
pidfd = open("/proc/$pid", O_DIRECTORY);

status = openat(pidfd, "status");
u64 this_uniq_pid = ... read UNIQ_PID from status ...;

if (uniq_pid != this_uniq_pid)
return;

pidfd_send_signal(pidfd);
}

Why else do we want pidfd?

Oleg.

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