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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 25 Mar 2019 16:42:14 -0700
Message-ID: <CALCETrV-52TXKbAim06Fr7vvFeOvk0SczneeV_Rtqr6-LN7-GQ@mail.gmail.com>
Subject: Re: pidfd design
To:     Daniel Colascione <dancol@google.com>
Cc:     Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sultan Alsawaf <sultan@kerneltoast.com>,
        Tim Murray <timmurray@google.com>,
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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On Mon, Mar 25, 2019 at 1:23 PM Daniel Colascione <dancol@google.com> wrote:
>
> On Mon, Mar 25, 2019 at 1:14 PM Jann Horn <jannh@google.com> wrote:
> >
> > On Mon, Mar 25, 2019 at 8:44 PM Andy Lutomirski <luto@kernel.org> wrote:

> > One ioctl on procfs roots to translate pidfds into that procfs,
> > subject to both the normal lookup permission checks and only working
> > if the pidfd has a translation into the procfs:
> >
> > int proc_root_fd = open("/proc", O_RDONLY);
> > int proc_dir_fd = ioctl(proc_root_fd, PROC_PIDFD_TO_PROCFSFD, pidfd);
> >
> > And one ioctl on procfs directories to translate from PGIDs and PIDs to pidfds:
> >
> > int proc_pgid_fd = open("/proc/self", O_RDONLY);
> > int self_pg_pidfd = ioctl(proc_pgid_fd, PROC_PROCFSFD_TO_PIDFD, 0);
> > int proc_pid_fd = open("/proc/thread-self", O_RDONLY);
> > int self_p_pidfd = ioctl(proc_pid_fd, PROC_PROCFSFD_TO_PIDFD, 0);
> >

This sounds okay to me.  Or we could make it so that a procfs
directory fd also works as a pidfd, but that seems more likely to be
problematic than just allowing two-way translation like this

> >
> > And then, as you proposed, the new sys_clone() can just return a
> > pidfd, and you can convert it into a procfs fd yourself if you want.
>
> I think that's the consensus we reached on the other thread. The
> O_DIRECTORY open on /proc/self/fd/mypidfd seems like it'd work well
> enough.

I must have missed this particular email.

IMO, if /proc/self/fd/mypidfd allows O_DIRECTORY open to work, then it
really ought to do function just like /proc/self/fd/mypidfd/. and
/proc/self/fd/mypidfd/status should work.  And these latter two
options seem nutty.

Also, this O_DIRECTORY thing is missing the entire point of the ioctl
interface -- it doesn't require procfs access.

--Andy