[CRIU] is CR_STATE_RESTORE_CREDS necessary?

[Pavel Emelyanov]

On 12/10/2015 11:58 PM, Andrey Wagin wrote:
> 2015-12-10 23:33 GMT+03:00 Pavel Emelyanov <xemul at parallels.com>:
>> On 12/10/2015 11:13 PM, Andrey Wagin wrote:
>>> 2015-12-09 20:05 GMT+03:00 Tycho Andersen <tycho.andersen at canonical.com>:
>>>> Hi all,
>>>>
>>>> I'm trying to figure out a way to get rid of this FIXME:
>>>> https://github.com/xemul/criu/blob/master/test/zdtm/live/static/seccomp_filter.c#L103
>>>>
>>>> to do this, we need to call restore_seccomp() before restore_creds(),
>>>> but if we're doing that, we need to suspend seccomp so that it doesn't
>>>> kill the task if the policy has blocked some syscall that
>>>> restore_creds does.
>>>>
>>>> Right now, seccomp is suspended in attach_to_tasks(), because that's
>>>> the last ptrace attach that criu does to the tasks. However, that
>>>> happens after CR_STATE_RESTORE_CREDS, i.e. after the creds are
>>>> restored.
>>>
>>> CR_STATE_RESTORE_CREDS is the last synchronization point. We stop on
>>> __NR_sigreturn with help of ptrace, but it doesn't work if we trace
>>> "criu restore". I use strace to investigate bugs very often.
>>>
>>> strace -fo strace.log -s 256 ./criu restore ...
>>>
>>> I would like to save this ability if it's possible.
>>
>> How do stages help you with strace? I thought that the only reason
>> it works was -- if criu's restore strace fails it just exits and lets
>> restorer blobs self-unload (not completely, but still) from task.
> 
> We can't use ptrace to synchronize tasks.

Yes, that's clear -- we need global final sync point before total
unfreeze and resume.

>>
>>>>
>>>> I could just move the attach_to_tasks call above
>>>> CR_STATE_RESTORE_CREDS, but I think that would deadlock since the main criu
>>>> tasks waits for the tasks to be complete, but they're all in the
>>>> stopped state (because we'd have ptraced them), and the loop that lets
>>>> them run is in parasite_stop_on_syscall much further below.
>>>
>>> We use breakpoints for x86_64. Maybe we can use them for other arch-s.
>>> Or we can attach tasks, suspend seccomp for them and resume them back
>>> and then interrupt them again after CR_STATE_RESTORE_CREDS.
>>>
>>> And there is another reason to have CR_STATE_RESTORE_CREDS. It allows
>>> us to reduce the number of syscalls which we need to trace with
>>> PTRACE_SYSCALL, because it works slow.
>>
>> OK, but why do we need the separate CR_STATE_RESTORE_SIGCHILD?
> 
> We can't fail on CR_STATE_RESTORE_CREDS, because network is already
> unlocked, sbo we should do minimum actions on this stage. It's a
> reason why we don't want to merge CR_STATE_RESTORE_SIGCHILD and
> CR_STATE_RESTORE_CREDS.

So the RESTORE_SIGCHILD is the last sync point till which something can
fail. Is that all?

-- Pavel