[RFC][v8][PATCH 0/10] Implement clone3() system call

[Daniel Lezcano]

Sukadev Bhattiprolu wrote:
> Daniel Lezcano [daniel.lezcano at free.fr] wrote:
>   
>> Sukadev Bhattiprolu wrote:
>>     
>>> Subject: [RFC][v8][PATCH 0/10] Implement clone3() system call
>>>
>>> To support application checkpoint/restart, a task must have the same pid it
>>> had when it was checkpointed.  When containers are nested, the tasks within
>>> the containers exist in multiple pid namespaces and hence have multiple pids
>>> to specify during restart.
>>>
>>> This patchset implements a new system call, clone3() that lets a process
>>> specify the pids of the child process.
>>>
>>> Patches 1 through 7 are helper patches, needed for choosing a pid for the
>>> child process.
>>>
>>> PATCH 9 defines a prototype of the new system call. PATCH 10 adds some
>>> documentation on the new system call, some/all of which will eventually
>>> go into a man page.
>>>   
>>>       
>> Sorry for jumping so late in the discussion and for having maybe my
>> remarks pointless...
>>
>> If this syscall is only for checkpoint / restart, why this shouldn't be
>> used with a future generic sys_restart syscall ?
>>     
>
> As I tried to explain in PATCH 0/9, the ability to choose a pid is only
> for C/R but we are also trying to clone-flags so we won't need yet
> another variant of clone() fairly soon.
>
>   
>> Otherwise, shouldn't be more convenient to have something usable for
>> everyone, let's say:
>>
>> cloneat(pid_t pid, pid_t desiredpid, ...);
>>
>> Where 'desiredpid' is a hint of for the kernel for the pid to be
>> allocated (zero means the kernel will choose one for us) and the newly
>> allocated task is the son of 'pid'.
>>     
>
> Hmm, so P1 would call cloneat() to create a child P3 _on behalf_ of process
> P2 ?  I did not know we had a requirement for that. Can you explain the
> use-case more ? IOW, why can't P2 create the child P3 by itself ?
>   
I forgot to mention a constraint with the specified pid : P2 has to be 
child of P1.
In other word, you can not specify a pid to clonat which is not your 
descendant (including yourself).
With this constraint I think there is no security issues.

Concerning of forking on behalf of another process, we can consider it 
is up to the caller / programmer to know what it does. If a process in 
the process hierarchy exec'ed a program and we cloneat this process and 
then the program fails because of an "unexpected error", well, we should 
have not done that. A similar example is when the IPC are removed while 
they are used by some other processes.

Here it is a interesting use case:
 * if you created a pid namespace, and, let's say, booted a system 
container where the container init is the "init" process, then with this 
call you can enter the container at any time by doing cloneat() followed 
by an exec of your command. I think that was a requirement when there 
were discussions around "sys_hijack".

Another point. It's another way to extend the exhausted clone  flags as 
the cloneat can be called as a compatibility way, with cloneat(getpid(), 
0, ... )

> Note also that 'desiredpid' must be a list of pids (one for each pid
> namespaces that the child will belong to) and hence we need 'nr_pids'
> to specify the list. Given that we are limited to 6 parameters to the
> syscall, such parameters must be stuffed into 'struct clone_args'.
>
> So we should do something like:
>
> 	sys_clone3(u32 flags_low, pid_t pid, struct clone_args *carg,
> 		pid_t *desired_pids)
>
> or (to match the name and parameters, move 'pid' parameter into clone_args)
>   
Well, hiding multiple clone in one clone call is ... weird. AFAIR, there 
was a debate between kernel or userspace proctree creation but it looks 
like it's done from the kernel with this call.

I don't really see a difference between sys_restart(pid_t pid , int fd, 
long flags) where pid_t is the topmost in the hierarchy, fd is a file 
descriptor to a structure "pid_t * + struct clone_args *" and flags is 
"PROCTREE".

IMHO, it is nicer to recursively restore the process tree for the nested 
pid namespaces, that will be really an userspace process tree creation 
and cloneat will be your friend here :)

>> That looks more consistent with the "<syscall>at" family, 'openat',
>> 'faccessat', 'readlinkat', etc ... and usable for something else than
>> the checkpoint / restart.
>>     
>
> The subtle difference though is that openat() does not open a file on
> behalf of another process and so the 'at' suffix would not apply ?
>   
Yes and no, depending of where you put the cursor. If you consider the 
'at' suffix means a process context, then I agree with you, there is a 
difference because the cloneat will be out of the current process 
context. But if you consider the 'at' suffix as a context in general, 
and openat means "relatively to a file descriptor" and cloneat means 
"relatively to a pid namespace" the 'at' suffix may apply. But I agree 
that we are so used to call the posix "fork", that cloneat sounds scary :)

Thanks
  -- Daniel