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

[Jonathan Kowalski <bl0pbl33p@gmail.com>]

On Sat, Mar 30, 2019 at 5:24 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@brauner.io> wrote:
> >
> >
> > To clarify, what the Android guys really wanted to be part of the api is
> > a way to get race-free access to metadata associated with a given pidfd.
> > And the idea was that *if and only if procfs is mounted* you could do:
> >
> > int pidfd = pidfd_open(1234, 0);
> >
> > int procfd = open("/proc", O_RDONLY | O_CLOEXEC);
> > int procpidfd = ioctl(pidfd, PIDFD_TO_PROCFD, procfd);
>
> And my claim is that this is three system calls - one of them very
> hacky - to just do
>
>     int pidfd = open("/proc/%d", O_PATH);
>
> and you're done. It acts as the pidfd _and_ the way to get the
> associated status files etc.
>
> So there is absolutely zero advantage to going through pidfd_open().
>
> No. No. No.
>
> So the *only* reason for "pidfd_open()" is if you don't have /proc in
> the first place. In which case the whole PIDFD_TO_PROCFD is bogus.
>
> Yeah, yeah, if you want to avoid going through the pathname
> translation, that's one thing, but if that's your aim, then you again
> should also just admit that PIDFD_TO_PROCFD is disgusting and wrong,
> and you're basically saying "ok, I'm not going to do /proc at all".
>
> So I'm ok with the whole "simpler, faster, no-proc pidfd", but then it
> really has to be *SIMPLER* and *NO PROCFS*.
>

(Resending because accidently it wasn't a reply-all)

If you go with pidfd_open, that should also mean you remove the
ability to be able to use /proc/<PID> dir fds in pidfd_send_signal.

Otherwise the semantics are hairy: I can only pidfd_open a task
reachable from my active namespace, but somehow also be able to open a
pidfd if I happen to see someone's /proc in my mount namespace and
have the access to open it?

> PIDFD_TO_PROCFD violates *everything*.
>
>                       Linus