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

From: Jann Horn
Date: Fri Oct 11 2019 - 10:56:30 EST

On Fri, Oct 11, 2019 at 2:23 PM Christian Kellner <ckellner@xxxxxxxxxx> 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.
> +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.