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SubjectRe: [PATCH linux-cr] nested pid namespaces (v2)
Quoting Oren Laadan (orenl@cs.columbia.edu):
>
>
> Serge E. Hallyn wrote:
> >Support checkpoint and restart of tasks in nested pid namespaces. At
> >Oren's request here is an alternative to my previous implementation. In
> >this one, we keep the original single pids_array to minimize memory
> >allocations. The pids array entries are augmented with a pidns depth
>
> Thanks for adapting the patch.
>
> FWIW, not only minimize memory allocations, but also permit a more
> regular structure of the image data (array of fixed size elements
> followed by an array of vpids), which simplifies the code that needs
> to read/write/access this data.
>
> >(relative to the container init's pidns, and an "rpid" which is the pid
> >in the checkpointer's pidns (or 0 if no valid pid exists). The rpid
> >will be used by userspace to gather more information (like
> >/proc/$$/mountinfo) after the kernel sys_checkpoint. If any tasks are
> >in nested pid namespace, another single array holds all of the vpids.
> >At restart those are used by userspace to determine how to call
> >eclone(). Kernel ignores them.
> >
> >All cr_tests including the new pid_ns testcase pass.
> >
> >Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> >---
>
> [...]

Thanks, Oren - all other input is taken into what I'm about to post,
except:

> >@@ -293,10 +295,15 @@ static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
> > _ckpt_err(ctx, -EPERM, "%(T)Nested net_ns unsupported\n");
> > ret = -EPERM;
> > }
> >- /* no support for >1 private pidns */
> >- if (nsproxy->pid_ns != ctx->root_nsproxy->pid_ns) {
> >- _ckpt_err(ctx, -EPERM, "%(T)Nested pid_ns unsupported\n");
> >- ret = -EPERM;
> >+ /* pidns must be descendent of root_nsproxy */
> >+ pidns = nsproxy->pid_ns;
> >+ while (pidns != ctx->root_nsproxy->pid_ns) {
> >+ if (pidns == &init_pid_ns) {
> >+ ret = -EPERM;
> >+ _ckpt_err(ctx, ret, "%(T)stranger pid_ns\n");
> >+ break;
> >+ }
> >+ pidns = pidns->parent;
>
> Currently we do this while() loop twice - once here and once when
> we collect the vpids. While I doubt if this has any performance
> impact, is there an advantage to doing it also here ? (a violation
> will be observed there too).

With the new logic (ripped verbatim from Louis' email) such a move
would make the checkpoint_vpids() code a bit uglier. I'm about to
resend, please let me know if you still want the code moved.

...

> >diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> >index 0da0d83..6d86240 100644
> >--- a/kernel/nsproxy.c
> >+++ b/kernel/nsproxy.c
> >@@ -364,8 +364,13 @@ static struct nsproxy *do_restore_ns(struct ckpt_ctx *ctx)
> > get_net(net_ns);
> > nsproxy->net_ns = net_ns;
> >- get_pid_ns(current->nsproxy->pid_ns);
> >- nsproxy->pid_ns = current->nsproxy->pid_ns;
> >+ /*
> >+ * The pid_ns will get assigned the first time that we
> >+ * assign the nsproxy to a task. The task had unshared
> >+ * its pid_ns in userspace before calling restart, and
> >+ * we want to keep using that pid_ns.
> >+ */
> >+ nsproxy->pid_ns = NULL;
>
> This doesn't look healthy.
>
> If it is (or will be) possible for another process to look at the
> restarting process, not having a pid-ns may confuse other code in
> the kernel ?

No task will have this nproxy attached before we assign a valid
pid_ns. The NULL pid_ns is only while it is in the objhash but
not attached to a task.

thanks,
-serge


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