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

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

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.

-serge