Re: uninterruptible CLONE_VFORK (Was: oom: Make coredump interruptible)

[Oleg Nesterov <oleg@redhat.com>]

On 06/14, Roland McGrath wrote:
>
> > Hmm. Even without debugger, the parent doesn't react to SIGSTOP.
>
> Yes.  It's been a long time since I thought about the vfork stuff much.
> But I now recall thinking about the SIGSTOP/SIGTSTP issue too.  It does
> seem bad.  OTOH, it has lurked there for many years now without complaints.
>
> Note that supporting stop/fatal signals in the normal way means that the
> call has to return and pass the syscall-exit tracing point first.  This
> means a change in the order of events seen by a debugger.  It also
> complicates the subject of PTRACE_EVENT_VFORK_DONE reports, which today
> happen before syscall-exit or signal stuff is possible.  For proper
> stopping in the normal way, the vfork-wait would be restarted via
> sys_restart_syscall or something.

Yes. I was thinking about this too.

The parent can play with real_blocked or saved_sigmask to block all
signals except STOP and KILL, use TASK_INTERRUPTIBLE for wait, and
just return ERESTART each time it gets the signal (it should clear
child->vfork_done if fatal_signal_pending).

We should also check PF_KTHREAD though, there are in kernel users
of CLONE_VFORK.

> Bu the way that happens ordinarily is
> to get all the way back to user mode and reenter with a normal syscall.
> That doesn't touch the user stack itself, but it sure makes one nervous.

me too. Especially because I do not really know how !x86 machines
implement this all.

We should also verify that the exiting/stopping parent can never write
to its ->mm. For example, exit_mm() does put_user(tsk->clear_child_tid).
Fortunately we can rely on PF_SIGNALED flag in this case.

Oleg.