Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sun, Mar 31, 2019 at 02:03:25PM -0700, Linus Torvalds wrote:

On Sun, Mar 31, 2019 at 1:38 PM Andy Lutomirski [off-list ref] wrote:

quoted

openat(fd to pidfd’s proc directory, “status”, ...);

And we want a non-utterly-crappy way to do this.  The ioctl is certainly ugly, but it *works*.

It's beyond clunky. It's a disgrace.

If people really want equivalency between open("/proc/%d") and some
new pidfd_open(), then just *make* the two equivalent.

I don't think that we want or can make them equivalent since that would
mean we depend on procfs. If userspace really wants to turn a pidfd into
an fd for /proc/<pid> then they can be burdened to do so by parsing out
the pid relative to their procfs pid namespace from the pidfds fdinfo:

int pidfd = pidfd_open(pid, 0);
int pid = parse_fdinfo("/proc/self/fdinfo/<pidfd>");
int procpidfd = open("/proc/<pid>", ...);

/* Test if process still exists by sending signal 0 through our pidfd. */
int ret = pidfd_send_signal(pid, 0, NULL, PIDFD_SIGNAL_THREAD);
if (ret < 0 && errno == ESRCH) {
        /* pid has been recycled and procpidfd refers to another process */
}

it's race free and no ioctl() is needed.

No effing crappy ioctl idiocy to create one from the other. Just make
the damn things be the exact same thing (and then if we extend clone()
to return one, make that return the same exact thing too).

Btw, we have several clone bits left:

 - if we don't have CLONE_PARENT set, the low 8 bits are still available

 - ignoring that, wehave bit #12 free: It used to be CLONE_IDLETASK
long long ago, but it was always kernel-only so it's never been
exposed as a user space bit.

That's good to know.