Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Pavel Emelyanov wrote:
> OK, here's another proposal that seem to suit all of us:
>
> 1. me wants to clone tasks with pids set
> 2. Pedro wants to fork task with not changing pids and w/o root perms
> 3. Oleg and Tejun want to have little intrusion into fork() path
>
> The proposal is to implement the PR_RESERVE_PID prctl which allocates and puts a
> pid on the current. The subsequent fork() uses this pid, this pid survives and keeps
> its bit in the pidmap after detach. The 2nd fork() after the 1st task death thus
> can reuse the same pid again. This basic thing doesn't require root perms at all
> and safe against pid reuse problems. When requesting for pid reservation task may
> specify a pid number it wants to have, but this requires root perms (CAP_SYS_ADMIN).
>
> Pedro, I suppose this will work for your checkpoint feature in gdb, am I right?
>
> Few comments about intrusion:
>
>   * the common path - if (pid !=&init_struct_pid) - on fork is just modified
>   * we have -1 argument to copy_process
>   * one more field on struct pid is OK, since it size doesn't change (32 bit level is
>     anyway not required, it's OK to reduce on down to 16 bits)
>   * no clone flags extension
>   * no new locking - the reserved pid manipulations happen under tasklist_lock and
>     existing common paths do not require more of it
>   * yes, we have +1 member on task_struct :(
>
> Current API problems:
>
>   * Only one fork() with pid at a time. Next call to PR_RESERVE_PID will kill the
>     previous reservation (don't know how to fix)
>   * No way to fork() an init of a pid sub-namespace with desired pid in current
>     (can be fixed for a flag for PR_RESERVE_PID saying that we need a pid for a
>      namespace of a next level)

>   * No way to grab existing pid for reserve (can be fixed, if someone wants this)

We can add flag to sys_wait4(), and stash pid in wait_task_zombie(), right before release_task()
code will looks something like this:

-       if (p != NULL)
+       if (p != NULL) {
+               if ((wo->wo_flags & WCATCHPID) && !current->pid_stash) {
+                       struct pid *pid = task_pid(p);
+
+                       pid->flags |= PID_STASHED;
+                       current->pid_stash = get_pid(pid);
+               }
                 release_task(p);
+       }

And next fork() creates child with the same pid.
So, struct pid will work like boomerang =)