[CRIU] is CR_STATE_RESTORE_CREDS necessary?

[Andrey Wagin]

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.

>
>>>
>>> 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.

>
> -- Pavel
>