Messages in this thread | | | Date | Sun, 27 Nov 2011 10:47:25 -0800 | From | Tejun Heo <> | Subject | Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids |
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Hello, Pavel.
On Fri, Nov 25, 2011 at 02:14:56PM +0400, 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) > > Oleg, Tejun, do you agree with such an approach?
Hmmm... Any attempt to reserve PIDs without full control over the namespace is futile. It can never be complete / reliable. Let's just forget about it. If anyone, including gdb, wants to have fun with CR, let them manage namespace too; otherwise, it's never gonna be reliable.
If you take the above out, setting last_pid is as simple as it gets and good enough. It's essentially few tens of lines of code to add userland interface for setting one pid_t value. Let's restrict manipulation to root for now and see whether finer grained CAP_* makes sense as we go along.
Thanks.
-- tejun
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