[PATCH RFC v3 2/2] pidns: introduce syscall getvpid

[Eric W. Biederman]

Konstantin Khlebnikov <khlebnikov at yandex-team.ru> writes:

> If pid is negative then getvpid() returns pid of parent task for -pid.

Now that I am noticing this.  I don't think I have seen any discussion
about justifying a syscall getting another processes parent pid.  My
apologies if I just missed it.

Why do we want the the parent pid?  We can we usefully do with it?
Is proc really that bad of an interface?

Fetching a parent pid feels like a separate logical operation
from pid translation.  Which makes me a bit uneasy about this
part of the conversation.

> Examples:
> getvpid(pid, ns, -1)      - get pid in our pid namespace
> getvpid(pid, -1, ns)      - get pid in container
> getvpid(pid, -1, ns) > 0  - is pid is reachable from container?
> getvpid(1, ns1, ns2) > 0  - is ns1 inside ns2?
> getvpid(1, ns1, ns2) == 0 - is ns1 outside ns2?
> getvpid(1, ns, -1)        - get init task of pid-namespace
> getvpid(-1, ns, -1)       - get reaper of init task in parent pid-namespace
> getvpid(-pid, -1, -1)     - get ppid by pid

As I step back and pay attention to this case I am half wondering if
perhaps what would be most useful is a file descriptor that refers
to a pid and an updated set of system calls that takes pid file
descriptors instead of pids.

Something like:

    getpidfd(int pidnsfd, pid_t pid);
    
    waitfd(int pidfd, int *status, int options, struct rusage *rusage);
    
    killfd(int pidfd, int sig);
    
    clonefd(...);
    
And perhaps:
    pid_nr_ns(int pidnsfd, int pidfd);

    parentfd(int pidfd);

Eric