linux-kernel - Re: [PATCH v3 1/2] pidfd: show pids for nested pid namespaces in fdinfo

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Date:   Fri, 11 Oct 2019 17:30:03 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Christian Kellner <ckellner@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Christian Kellner <christian@...lner.me>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [PATCH v3 1/2] pidfd: show pids for nested pid namespaces in fdinfo

On Fri, Oct 11, 2019 at 5:17 PM Christian Brauner
<christian.brauner@...ntu.com> wrote:
>
> On Fri, Oct 11, 2019 at 04:55:59PM +0200, Jann Horn wrote:
> > On Fri, Oct 11, 2019 at 2:23 PM Christian Kellner <ckellner@...hat.com> wrote:
> > > The fdinfo file for a process file descriptor already contains the
> > > pid of the process in the callers namespaces. Additionally, if pid
> > > namespaces are configured, show the process ids of the process in
> > > all nested namespaces in the same format as in the procfs status
> > > file, i.e. "NSPid:\t%d\%d...". This allows the easy identification
> > > of the processes in nested namespaces.
> > [...]
> > >  #ifdef CONFIG_PROC_FS
> > > +static inline void print_pidfd_nspid(struct seq_file *m, struct pid *pid,
> > > +                                    struct pid_namespace *ns)
> >
> > `ns` is the namespace of the PID namespace of the procfs instance
> > through which the file descriptor is being viewed.
> >
> > > +{
> > > +#ifdef CONFIG_PID_NS
> > > +       int i;
> > > +
> > > +       seq_puts(m, "\nNSpid:");
> > > +       for (i = ns->level; i <= pid->level; i++) {
> >
> > ns->level is the level of the PID namespace associated with the procfs
> > instance through which the file descriptor is being viewed. pid->level
> > is the level of the PID associated with the pidfd.
> >
> > > +               ns = pid->numbers[i].ns;
> > > +               seq_put_decimal_ull(m, "\t", pid_nr_ns(pid, ns));
> > > +       }
> > > +#endif
> > > +}
> >
> > I think you assumed that `ns` is always going to contain `pid`.
> > However, that's not the case. Consider the following scenario:
> >
> >  - the init_pid_ns has two child PID namespaces, A and B (each with
> > its own mount namespace and procfs instance)
> >  - process P1 lives in A
> >  - process P2 lives in B
> >  - P1 opens a pidfd for itself
> >  - P1 passes the pidfd to P2 (e.g. via a unix domain socket)
> >  - P2 reads /proc/self/fdinfo/$pidfd
> >
> > Now the loop will print the ID of P1 in A. I don't think that's what
> > you intended? You might want to bail out if "pid_nr_ns(pid, ns) == 0",
> > or something like that.
>
> I assumed the same thing happens when you pass around an fd for
> /proc/self/status and that's why I didn't object to this behavior.

I don't see how /proc/$pid/status is relevant. In the
/proc/$pid/status case, the output is the list of PIDs starting at the
PID namespace the procfs is associated with; and the process is always
contained in that namespace, which also means that the first PID
listed is the one in the PID namespace of the procfs instance. In the
pidfd case, the process is not necessarily contained in that
namespace, and the output doesn't make sense.