Re: [PATCH 0/4] pid: add pidctl()

[Daniel Colascione]

On Mon, Mar 25, 2019 at 2:55 PM Jonathan Kowalski <bl0pbl33p@xxxxxxxxx> wrote:
>
> On Mon, Mar 25, 2019 at 9:43 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Mar 25, 2019 at 10:19:26PM +0100, Jann Horn wrote:
> > > On Mon, Mar 25, 2019 at 10:11 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > But often you don't just want to wait for a single thing to happen;
> > > you want to wait for many things at once, and react as soon as any one
> > > of them happens. This is why the kernel has epoll and all the other
> > > "wait for event on FD" APIs. If waiting for a process isn't possible
> > > with fd-based APIs like epoll, users of this API have to spin up
> > > useless helper threads.
> >
> > This is true. I almost forgot about the polling requirement, sorry. So then a
> > new syscall it is.. About what to wait for, that can be a separate parameter
> > to pidfd_wait then.
> >
>
> This introduces a time window where the process changes state between
> "get pidfd" and "setup waitfd", it would be simpler if the pidfd
> itself supported .poll and on termination the exit status was made
> readable from the file descriptor.

I don't see a race here. Process state moves in one direction, so
there's no race. If you want the poll to end when a process exits and
the process exits before you poll, the poll should finish immediately.
If the process exits before you even create the polling FD, whatever
creates the polling FD can fail with ESRCH, which is what usually
happens when you try to do anything with a process that's gone. Either
way, whatever's trying to set up the poll knows the state of the
process and there's no possibility of a missed wakeup.

> Further, in the clone4 patchset, there was a mechanism to autoreap
> such a process so that it does not interfere with waiting a process
> does normally. How do you intend to handle this case if anyone except
> the parent is wanting to *wait* on the process (a second process,
> without reaping, so as to not disrupt any waiting in the parent), and
> for things like libraries that finally can manage their own set of
> process when pidfd_clone is added, by excluding this process from the
> process's normal wait handling logic.

I think that discussion is best had around pidfd_clone. pidfd waiting
functionality shouldn't affect wait* in any way nor affect a zombie
transition or reaping.

> Moreover, should anyone except the parent process be allowed to open a
> readable pidfd (or waitfd), without additional capabilities?

That's a separate discussion. See
https://lore.kernel.org/lkml/CAKOZueussVwABQaC+O9fW+MZayccvttKQZfWg0hh-cZ+1ykXig@xxxxxxxxxxxxxx/,
where we talked about permissions extensively. Anyone can hammer on
/proc today or hammer on kill(pid, 0) to learn about a process
exiting, so anyone should be able to wait for a process to die --- it
just automates what anyone can do manually. What's left is the
question of who should be able to learn a process's exit code. As I
mentioned in the linked email, process exit codes are public on
FreeBSD, and the simplest option is to make them public in Linux too.