Re: [RFC][v4][PATCH 7/7]: Define clone_extended() syscall

[Oren Laadan <orenl-RdfvBDnrOixBDgjK7y7TUQ@public.gmane.org>]

Serge E. Hallyn wrote:
> Quoting Sukadev Bhattiprolu (sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org):
>> Subject: [RFC][v4][PATCH 7/7]: Define clone_extended() syscall
>>
>> Container restart requires that a task 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 patch defines, a new system call, clone_extended() which is like clone(),
>> but takes a new 'pid_set' parameter.  This parameter lets caller choose
>> specific pid numbers for the child process, in the process's active and
>> ancestor pid namespaces. (Descendant pid namespaces in general don't matter
>> since processes don't have pids in them anyway, but see comments in
>> copy_target_pids() regarding CLONE_NEWPID).
>>
>> Unlike clone(), however, clone_extended() needs CAP_SYS_ADMIN, at least for
>> now, to prevent unprivileged processes from misusing this interface.
> 
> It only needs that when specifying pids.
> 
>> While the main motivation for this interface is the need to let a process
>> choose its 'pid numbers', the clone_extended() interface uses 64-bit clone
>> flags.  The 'higher' portion of the clone flags are unused and are only
>> included to preclude yet another version of clone when a new clone flag is
>> needed. 
>>
>> ===== Interface:
>>
>> Compared to clone(), clone_extended() needs to pass in three more pieces
>> of information:
>>
>> 	- additional 32-bit of clone_flags
>> 	- number of pids in the set
>> 	- user buffer containing the list of pids.
>>
>> But since clone() already takes 5 parameters and some (all ?) architectures
>> are restricted to 6 parameters to a system-call, additional data-structures
>> (and copy_from_user()) are needed.
>>
>> The proposed interface for clone_extended() is:
>>
>> 	struct clone_tid_info {
>> 		void *parent_tid; 	/* parent_tid_ptr parameter */
>> 		void *child_tid; 	/* child_tid_ptr parameter */
>> 	};
>>
>> 	struct pid_set {
>> 		int num_pids;
>> 		pid_t *pids;
>> 	};
>>
>> 	int clone_extended(int flags_low, int flags_high, void *child_stack,
>> 			void *unused, struct clone_tid_info *tid_ptrs,
>> 			struct pid_set *pid_setp);
> 
> I was thinking additional flags would be passed in the (renamed)
> struct pid_set.

Yes.

But maybe in (renamed) 'struct clone_info' instead of 'struct pid_set' ?

I vaguely recall a strong preference to not require copy-from-user
during a fast-path clone, because it may hurt performance.

*If* this is the case, then maybe place extra flags among the
"base" args, or at least a CLONE_EXTRA would indicate that more
arguments need to be pulled from user-space ?

Do you intend to get feedback from LKML too ?

Oren.