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

[Konstantin Khlebnikov]

On 28.09.2015 19:57, Eric W. Biederman wrote:
> Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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.

Sorry for late response. This completely fell out of my mind after LinuxCon.

> 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.

Yep proc interface is bad. /proc/$pid/stat is almost impossible to
parse without flaws because task could set second field "comm" into
any string and fake ppid  - for example ") Z 1". /proc/$pid/status
is better but it has more information and thus slower.

This trick for distant getppid looks cheap useful:
in this interface space of negative pids is free for use.

> > 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.

Fd which pins pids isn't a good idea.

I think it's better to refer (but not hold) task rather than pid.
For example inode of taskfd will hold small buffer for task exit
status: task holds reference to its own taskfd inode and populates
status when exits. Here will be no zombies and delayed reaping.

Something like:

task_fd = clonefd()
                     ...
select(...)
                     exit(...)
pread(task_fd, &status_rusage_etc, sizeof, 0);
close(task_fd);

Task pid also could be part of structure in that fd. Potentially it
could provide the same information as /proc/$pid/... in effective
binary format: we can read only required fields of structure and
kernel can skip unneeded calculations.

> 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

--
Konstantin
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