Messages in this thread | | | Subject | Re: [PATCH RFC v3 2/2] pidns: introduce syscall getvpid | From | Konstantin Khlebnikov <> | Date | Tue, 20 Oct 2015 13:04:23 +0300 |
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On 28.09.2015 19:57, Eric W. Biederman wrote: > Konstantin Khlebnikov <khlebnikov@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. >
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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