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From:   Jann Horn <jannh@google.com>
Date:   Thu, 16 May 2019 16:03:27 +0200
Message-ID: <CAG48ez05OtBi_yX+071TrrfK3zKOn9h1kFyPr5rttiqQAZ0sEA@mail.gmail.com>
Subject: Re: [PATCH 1/2] pid: add pidfd_open()
To:     Christian Brauner <christian@brauner.io>,
        Daniel Colascione <dancol@google.com>
Cc:     Oleg Nesterov <oleg@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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On Thu, May 16, 2019 at 3:08 PM Christian Brauner <christian@brauner.io> wrote:
> On Wed, May 15, 2019 at 10:45:06AM -0700, Daniel Colascione wrote:
> > On Wed, May 15, 2019 at 3:04 AM Christian Brauner <christian@brauner.io> wrote:
> > >
> > > This adds the pidfd_open() syscall. It allows a caller to retrieve pollable
> > > pidfds for a process which did not get created via CLONE_PIDFD, i.e. for a
> > > process that is created via traditional fork()/clone() calls that is only
> > > referenced by a PID:
[...]
> > > +/**
> > > + * pidfd_open() - Open new pid file descriptor.
> > > + *
> > > + * @pid:   pid for which to retrieve a pidfd
> > > + * @flags: flags to pass
> > > + *
> > > + * This creates a new pid file descriptor with the O_CLOEXEC flag set for
> > > + * the process identified by @pid. Currently, the process identified by
> > > + * @pid must be a thread-group leader. This restriction currently exists
> > > + * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
> > > + * be used with CLONE_THREAD) and pidfd polling (only supports thread group
> > > + * leaders).
> > > + *
> > > + * Return: On success, a cloexec pidfd is returned.
> > > + *         On error, a negative errno number will be returned.
> > > + */
> > > +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> > > +{
[...]
> > > +       if (pid <= 0)
> > > +               return -EINVAL;
> >
> > WDYT of defining pid == 0 to mean "open myself"?
>
> I'm torn. It be a nice shortcut of course but pid being 0 is usually an
> indicator for child processes. So unless the getpid() before
> pidfd_open() is an issue I'd say let's leave it as is. If you really
> want the shortcut might -1 be better?

Joining the bikeshed painting club: Please don't allow either 0 or -1
as shortcut for "self". James Forshaw found an Android security bug a
while back (https://bugs.chromium.org/p/project-zero/issues/detail?id=727)
that passed a PID to getpidcon(), except that the PID was 0
(placeholder for oneway binder transactions), and then the service
thought it was talking to itself. You could pick some other number and
provide a #define for that, but I think pidfd_open(getpid(), ...)
makes more sense.